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Photocatalytic Transfer Hydrogenation Reactions Using Water as the Proton Source

  • En Zhao
    En Zhao
    Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China
    More by En Zhao
    Orcidhttps://orcid.org/0000-0003-3416-2414
  • , 
  • Wenjun Zhang
    Wenjun Zhang
    Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China
    More by Wenjun Zhang
  • , 
  • Lin Dong
    Lin Dong
    Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China
    More by Lin Dong
  • , 
  • Radek Zbořil*
    Radek Zbořil
    Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research Institute (CATRIN), Palacký University Olomouc, Šlechtitelů 27, Olomouc 783 71, Czech Republic
    Nanotechnology Centre, CEET, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, Ostrava-Poruba 708 00, Czech Republic
    *Email for R.Z.: [email protected]
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    Orcidhttps://orcid.org/0000-0002-3147-2196
  • , and 
  • Zupeng Chen*
    Zupeng Chen
    Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China
    *Email for Z.C.: [email protected]
    More by Zupeng Chen
    Orcidhttps://orcid.org/0000-0002-7351-3240
Cite this: ACS Catal. 2023, 13, 11, 7557–7567
Publication Date (Web):May 21, 2023
https://doi.org/10.1021/acscatal.3c00326

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Transfer hydrogenation using liquid hydrogen carriers as the direct proton sources under mild conditions has received extensive attention in the research area of organic synthesis. The emerging photocatalytic water-donating transfer hydrogenation (PWDTH) is a promising alternative over the conventional hydrogenation technology due to the advantages of being eco-friendly. This paper focuses on the recent advances in the rising and rapidly developing field of PWDTH reactions, devoted to elucidating the mechanism of the hydrogen transfer process and rationalizing the design principles of efficient photocatalysts. Finally, the current challenges and future opportunities are described.

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1. Introduction

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Hydrogenation, as one of the most important pillars of the chemical industry, has been widely used in the petrochemical, coal chemical, fine chemical, and pharmaceutical industries, and it has been assessed that 25% of chemical transformations involve at least one hydrogenation step. (1,2) Traditional thermal hydrogenation is an energy-intensive process that usually requires harsh conditions of high temperature and pressure with a flammable H2 atmosphere (Scheme 1). (3,4) The transfer hydrogenation (TH) reaction, referring to the addition of hydrogen to an organic molecule from a non-H2 source (e.g., formic acid, silane, and borane–ammonia), is a prospective method to access various hydrogenated organics. (3) For instance, Xu et al. reported the TH of alkene by taking ammonia borane as a hydrogen source. (5) Kappe et al. presented a catalyst-free process for the in situ generation of diimide, which was applied to the reduction of alkenes. (6) Zbořil et al. reported the selective hydrogenation of nitroaromatics using hydrazine as a proton source. (7) Moreover, alcohols and acids are also used as proton sources for the hydrogenation of diverse unsaturated bonds. (8−13) Nevertheless, these systems have the disadvantage of the use of non-environmentally friendly proton sources (Scheme 1).

Scheme 1

Scheme 1. Traditional Thermal Hydrogenation Uses Flammable H2 at High Temperature and Pressure and Conventional TH Uses Eco-Unfriendly Hydrogen Sources (e.g., Formic Acid, Hydrazine, and Borane–Ammonia), while Photocatalytic Transfer Hydrogenation Uses H2O as the Green Hydrogen Source at Normal Pressure and Temperature
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In consideration of the ultimate goal of developing eco-friendly and sustainable processes in chemical industry production, water is regarded as the ultimate green source of a hydrogen donor for TH. (14) Unfortunately, the activation of water molecules is extremely challenging under mild conditions due to its intrinsic thermostability (ΔG° = +237 kJ mol–1). (15,16) Tremendous efforts had been devoted to exploring the feasibility of water as a hydrogen source. For example, Stokes et al. showed that the hazardous additive B2(OH)4 could mediate the transfer of H or D atoms from H2O or deuterated D2O to alkenes and alkynes. (17) The explosive magnesium (18) and manganese (19) powders were also employed to activate H–O bonds in water for the hydrogenation of unsaturated bonds. These technologies have provided new opportunities in H–O bond activations, but they have important limitations due to the use of hazardous or toxic species. This encourages the employment of a safe and sustainable method to activate the water molecules.
The development of photocatalytic water splitting has inspired sustainable concepts for the generation of “green hydrogen” within chemical energy storage schemes, enabling the pursuit of carbon neutrality and alleviating the energy crisis associated with the depletion of fossil fuels. It is well-known that H2 is one of the most attractive fuels because it possesses a high energy density and does not produce pollutants during combustion. (20) However, the efficient storage and transport of flammable H2 is challenging and increases the costs for the hydrogenation industry. In this context, the utilization of the active hydrogen intermediates (H*) derived from water splitting for the hydrogenation of unsaturated bonds is intriguing (Scheme 1). Therefore, the photocatalytic water-donating transfer hydrogenation (PWDTH) efficiently integrates the processes of photocatalytic water splitting and hydrogenation, which could activate water under mild conditions, avert the storage and transport of H2, and generate high-value-added organic products in a more sustainable manner. (14−16)
Multiple excellent reviews have provided comprehensive coverage of transfer hydrogenation reactions in thermocatalytic, electrocatalytic, and photocatalytic processes, including the exploration of transition-metal catalyst diversity, the use of green hydrogen donors, and the investigation of TH mechanisms. (2−4,21−23) For instance, Choudhury’s group reviewed the advantages of using biomass-derived alcohols as hydrogen sources to replace nonrenewable H2 gas for the TH of CO2. (21) Xu’s group outlined catalytic TH using formic acid or formate as an alternative to high-pressure hydrogen, despite the inevitable decomposition of FA or formate to the greenhouse gas CO2 during the catalytic process. (22) Unfortunately, there is still no systematic summary concentrating on the photocatalytic transfer hydrogenation using water as the ultimate green proton source under mild conditions. Herein, we present a timely and concise perspective of the recent progress of the photocatalytic water-donating transfer hydrogenation reactions (Scheme 1). We first reveal the mechanism of PWDTH in this perspective. Then, the rational design of photocatalysts (i.e., single-atom catalysts and alloy catalysts) to achieve efficient conversion and high selectivity for PWDTH is emphasized. Furthermore, photocatalytic deuteration using D2O as a deuterium source is discussed. Finally, the challenges and opportunities are addressed to provide intriguing perspectives to further advance the PWDTH reactions.

2. Fundamentals of Photocatalytic Water-Donating Transfer Hydrogenation

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Photocatalytic water splitting into H2 is widely regarded as a prominent approach for storing solar energy as chemical energy. (24) Meanwhile, photocatalysis has evolved over the last decades into a broadly used approach for organic synthesis (e.g., transfer hydrogenation). (25) Inspired by these studies, a seminal work in the field of PWDTH was reported by Knecht’s group in 2014. (26) They developed a palladium nanoparticle decorated Cu2O catalyst (Cu2O/Pd) via a galvanic exchange, which used the in situ generated H2 from photocatalytic water splitting at the Cu2O surface for the subsequent hydrodehalogenation of polychlorinated biphenyls at the Pd nanoparticles (Figure 1a). Figure 1b shows that 3-chlorobiphenyl (PCB2) could be fully dechlorinated to biphenyl within 15 h under visible-light irradiation. However, the work did not track the pathway of hydrogen addition during the PWDTH reaction, which is fundamental to the mechanistic study. Likewise, although the groups of Qiu (27) and Su (28) reported the PWDTH reaction using the in situ generated H* species from water, the presence of additives (i.e., HCOOH and NaHSO4) increases the difficulty of mechanism investigation (Scheme 2a,b). Moreover, Su et al. presented the deuteration of alkenes and alkynes utilizing the in situ generated deuterium species (D*) from D2O splitting. (29) Unfortunately, the presence of CD3OD in the reaction system complicated the identification of the source of active D species (Scheme 2c).

Figure 1

Figure 1. (a) Illustration of fabrication of Cu2O/Pd catalysts for photocatalytic water-donating dehalogenation. (b) Concentration changes of PCB2 and biphenyl product during the photocatalytic water-donating dehalogenation reaction over Cu2O/Pd. Reaction conditions: catalyst (100 mg), substrate (1.25 μmol), H2O (25 mL), CH3OH (25 mL), 450 W Hg lamp, pressure (1 bar), N2 atmosphere. Reproduced with permission from ref (26). Copyright 2014 American Chemical Society.

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Scheme 2

Scheme 2. Illustration of the PWDTH Process in the Presence of Hydrogen-Rich Additivesa
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aReaction conditions: (a) catalyst (10 mg), substrate (0.1 mmol), H2O (1.5 mL), ethyl acetate (2 mL), CH3OH (1.4 mL), additive (HCOOH, 0.1 mL), 20 W blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (b) catalyst (10 mg), substrate (0.1 mmol), H2O (1.5 mL), ethyl acetate (2 mL), CH3OH (1.5 mL), additive (NaHSO4, 0.1 mmol), 20 W blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (c) catalyst (10 mg), substrate (0.1 mmol), D2O (1.5 mL), ethyl acetate (2 mL), CD3OD (1.5 mL), additive (AlCl3, 0.1 mmol), 20 W blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere.

2.1. Mechanism Investigation

To disclose the origin of the hydrogen addition in the PWDTH reaction, Xu et al. employed isotope-labeling experiments, (15) in which styrene was tested as a model compound toward TH. As shown in the mass spectra (Figure 2, top), when H2O was used, the m/z values of 91.1 and 106.1 were assigned to the fragmentation ion [ph-CH2]+ and the molecular ion of ethylbenzene, respectively. These signals shifted to 92.1 and 108.1 when replacing H2O with D2O, suggesting that the proton source for TH originates from water. However, this method cannot identify the pathway of hydrogen addition in actual scenarios. To address the matter, our recent study employed operando nuclear magnetic resonance (NMR) measurements for the first time to track the reaction pathway during the PWDTH reaction. (14) As shown in the 1H NMR spectra (Figure 2, bottom), proton signals from the benzene ring (region a) and ethenyl (regions b–d) of the styrene molecule were observed under dark conditions. Then, the signals of styrene remained untouched after 2 h in the dark, suggesting that the styrene is steady under dark conditions and the unsaturated structure reluctantly exchanges protons with D2O. After in situ illumination for 2 h, the ethenyl signal diminished, and the methyl structure (region e) on ethylbenzene began to appear, verifying the conversion of styrene toward ethylbenzene. Notably, a great deal of D2O was activated under the irradiation conditions and the D of D2O underwent a rapid proton exchange with the H of the benzene structure, broadening the 1H NMR signals of the benzene ring in the region a. To exclude the possibility of indirect hydrogenation of styrene with the generated H2 from water splitting, 1 bar of hydrogen gas was fed into the operando closed NMR tube. The results indicate that the proton addition occurs in both the benzene ring and ethenyl, whereas the proton addition only occurs in the ethenyl structure without H2 introduction. Therefore, the operando NMR experiments unequivocally prove that the proton addition mechanism from styrene to ethylbenzene is a direct hydrogenation reaction utilizing the in situ generated protons from photocatalytic water splitting instead of in situ generated H2 gas. Moreover, the strong proton exchange could efficiently activate water molecules for photocatalytic transfer hydrogenation.

Figure 2

Figure 2. (top) Mass spectra of the liquid product from the PWDTH reaction of styrene. Reproduced with permission from ref (15). Copyright 2020 American Chemical Society. (bottom) Operando 1H NMR spectra of the liquid product from the PWDTH reaction of styrene. Reproduced with permission from ref (14). Copyright 2022 Wiley.

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Traditional thermal hydrogenation with flammable H2 is carried out over transition-metal catalysts, which consists of multiple steps including bond cleavage, rearrangement, and recombination. (22) In terms of the PWDTH reaction, H2 is replaced by H* from photocatalytic water splitting, where the H* could be used in the subsequent hydrogenation step and spill over from the metal sites to evolve into H2. Due to the competition between these two reactions, exploring the subtle relationship between hydrogenation and hydrogen evolution provides insight into the mechanism of the PWDTH reaction. For example, Xiong and co-workers used a TiO2–Pd0.5Pt0.5 hybrid catalyst for the PWDTH reaction and they detected the amount of hydrogen evolution in the presence and absence of the substrate, respectively. (30) Compared to a large amount of hydrogen evolution in the absence of a substrate, the catalyst exhibited little hydrogen evolution activity in the presence of a substrate, indicating that most of H* (ca. 99% H*) was transferred to the hydrogenation reaction. This is due to the adsorption of H* on the Pd surface as a hollow-site configuration with strong binding (discussed in detail in subsequent subsections). Similar phenomena have been found in other PWDTH systems, where either nanoparticle catalysts (15,31) or heterojunctions (32) were used. In our previous work, (14) we developed a palladium single-atom catalyst for the PWDTH reaction, and the performance of the transfer hydrogenation was much improved compared to the above works due to the intrinsic properties of the single-atom catalyst, resulting in superior hydrogen evolution activity and an abundance of catalytically active sites. However, the proportion of the H* used in the PWDTH reaction has declined (ca. 85% H*) compared to the above works, suggesting that single-atom catalysts inevitably waste some of the H* while improving the PWDTH performance. In summary, very few works have been done to investigate the competition between hydrogenation and H2 evolution. Available studies have found that most of the H* is preferentially used in PWDTH reactions, and this is more evident in nanoparticle or heterojunction catalysts (i.e., catalysts with lower water-splitting activity). Although single-atom catalysts could enhance the performance of PWDTH reactions, they also promote hydrogen evolution. The effect of the structure of photocatalysts on the competition between hydrogenation and H2 evolution remains unknown. As more research is conducted, the relationship between them may be clearer.
Researchers have employed various techniques to probe the reaction mechanism of the subsequent hydrogenation process. To explore the detailed processes of the hydrogenation of nitrobenzene to aniline, Xu et al. employed in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). (32) First, nitrobenzene (PhNO2) adsorbed onto the catalytic site and formed a PhN(OH)2 intermediate by two H* transfers, followed by the removal of one molecule of water to form a PhNO intermediate, then by two H* transfers to form a PhNHOH intermediate, followed by the removal of one molecule of water to form a PhN intermediate, and finally by two H* transfer to form aniline (PhNH2), which was desorbed from the catalyst. Zhao and co-workers examined a nickel single-atom catalyst preadsorbed by water under visible-light irradiation through extended X-ray absorption fine structure spectra (EXAFS) combined with density functional theory (DFT) calculations to propose an adsorption configuration of H* (i.e., OH–Ni–N2···N–H). (33) The subsequent semihydrogenation of alkynes then began with the transfer of H* from pyridinic N to the adsorbed substrate.
To investigate the mechanism of hydrogenation of aryl bromides, Ren et al. conducted some controlled experiments, in which the radical inhibitors 2,6-di-tert-butyl-4-methylphenol (BHT) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) were added to the reaction system. (34) The results showed that the presence of BHT or TEMPO resulted in a dramatic decrease in the content of the product aryl compounds. The electron-deficient aryl bromides possessed a higher reactivity under the reaction system. A single electron transfer (SET) mechanism could theoretically explain such an electron effect, as an increase in the electron density of the aromatic ring could hasten the SET from the catalyst to the substrate by enhancing the electron-accepting capacity of the substrate. (35) Fan and co-workers employed fluorescence emission spectroscopy to monitor the PWDTH process of N-sulfonylimines and similarly proposed a SET mechanism (Scheme 3a). (36) Under visible light irradiation, the photocatalyst fac-Ir(ppy)3 is excited to produce Ir3+*, followed by a single electron transfer process with radical A. The resulting Ir2+ is highly reductive (37) and therefore capable of producing deuterium radicals. (38) The deuterium radicals are then added to the N-sulfonylimine, which produces the final products by further quenching of radical D. Lloret-Fillol et al. proposed that the hydrogenation of aromatic ketones could proceed via a single electron transfer–hydrogen atom transfer (SET-HAT) mechanism (Scheme 3b), which was confirmed by radical clock experiments. (39) It is well-known that [Cu(bathocuproine)(Xantphos)](PF6) (PSCu), a photoredox catalyst, is excited under light irradiation and then reductively quenched by an electron donor (ED) to give PSCu–, (40) which reduces the reduction catalyst [Co(OTf)(Py2Tstacn)](OTf) ([Co]) to the intermediate [CoI]. (41) [CoI] is protonated by water to form the putative [CoIII–H], which is readily reduced to give the active [CoII–H] species. Subsequently, for the hydrogenation of ketones and aldehydes, the substrate is converted to a carbonyl radical anion intermediate through a single electron transfer of PSCu–. Ultimately, the intermediate is converted to the product via a hydrogen atom transfer from the [CoII–H] species (homolytic pathway, Scheme 3b, left). Alternatively, a direct nucleophilic attack of a putative [CoII–H] species may be also considered (heterolytic pathway, Scheme 3b, right). By similar methods, their group has proposed that the reduction of aromatic olefins in their reaction system takes place by a HAT mechanism, most likely via [Co–H] intermediates. (42)

Scheme 3

Scheme 3. Proposed Mechanism for the PWDTH Process of (a) N-Sulfonylimines and (b) Aromatic Ketones and Aldehydesa
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aReproduced with permission from refs (36and39). Copyright 2019 Royal Society of Chemistry.

2.2. From Nanoparticle to Single-Atom Catalysts

During the PWDTH reaction, the photogenerated H* from water splitting could be adsorbed to the surface of metal species and subsequently added to various unsaturated organic substrates. Han et al. demonstrated that nitrobenzenes with different groups could be hydrogenated to the corresponding anilines by the H* from photocatalytic water splitting over a TiO2-supported Pd nanoparticle catalyst (Pd/TiO2). (43) Similarly, Xu et al. realized the proton addition of different unsaturated bonds (C═C, C═O, and N═O) over platinum nanoparticle loaded carbon nitride (Pt/CN or PtPd/CN). (15) Unfortunately, these nanoparticle catalysts usually suffer from sluggish conversion efficiency and poor metal utilization because the metal atoms inside the bulk of nanoparticles are inaccessible to the reactants.
Single-atom catalysts (SACs) have recently received extensive research attention due to their maximum metal dispersion and atom-utilization efficiency, which have exhibited excellent catalytic performance in a broad range of applications including Suzuki coupling, selective oxidation, hydroformylation, etc. (44,45) In our recent work, we extended the employment of SACs to photocatalytic transfer hydrogenation for the first time. The reported carbon nitride supported palladium SAC (Pd1-mpg-C3N4) (Figure 3a,b) showed preeminent performance in the PWDTH reaction compared to its nanoparticle counterpart with a similar Pd loading (PdNP-mpg-C3N4). (14) As shown in Figure 3c, a comparison on a metal basis manifested the highest ethylbenzene formation rate (22.73 molethylbenzene molPd–1 h–1) for the 0.54 wt % SAC sample. However, the rate of ethylbenzene formation decreased linearly to 2.02 molethylbenzene molPd–1 h–1 when the Pd content was further increased to 2.76 wt %, which is 11-fold lower than that of the optimized Pd1-mpg-C3N4. The inhibited reaction rate could be attributed to the formation of clusters or nanoparticles in samples with a Pd loading higher than 0.8 wt %, which emphasizes the advantages of the specific electronic properties (cationic form) of atomically dispersed sites over conventional zerovalent nanometals toward the PWDTH reaction.

Figure 3

Figure 3. (a) Aberration-corrected high-angle annular dark-field scanning transmission electron microscopic (AC-HAADF-STEM) image and (b) Fourier transform of Pd K-edge extended X-ray absorption fine structure spectra (EXAFS) spectra of Pd1-mpg-C3N4. Pd foil and PdO were applied for comparison purposes. (c) Metal-specific reaction rate toward ethylbenzene formation as a function of palladium content for the PWDTH reaction of styrene. Reaction conditions: catalyst (10 mg), substrate (0.1 mmol), H2O (2 mL), 1,4-dioxane (3 mL), triethanolamine (0.5 mL), 40 W blue light (λ = 427 nm), temperature (308 K), pressure (1 bar), N2 atmosphere. Reproduced with permission from ref (14). Copyright 2022 Wiley.

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The PWDTH reactions are a complex class of multistage catalytic reactions. In the process, the water molecule is first split to produce the H*, which is then transferred to hydrogenated active sites where the substrate is adsorbed, and finally, the organic unsaturated bond is added. In the first step (i.e., photocatalytic water splitting), the rate of the H* production determines the rate of the subsequent hydrogenation step. In other words, the O–H bond dissociation of water molecules may be involved in the rate-limiting step. (33) Over the past decades, researchers have been dedicated to boosting the activity of photocatalytic water splitting. (46) Loading of a cocatalyst has attracted a great deal of attention due to its significant advantages in terms of enhanced light absorption, photogenerated charge migration, and surface catalytic reactions (e.g., hydrogenation, oxidation, and coupling). (47,48) The photocatalytic water splitting activity of non-noble-metal-loaded catalysts, however, is currently far from being on par with noble-metal-loaded catalysts. As a result, the majority of studies on the PWDTH have relied on noble-metal-loaded catalysts (e.g., PdNP-CN, PtNP-CN, PdNP/TiO2, and Cu2O/PdNP). (15,26,34,43,49) SACs have the potential to greatly contribute to the rational design of targeted catalysts for preferred catalytic performance by acting as the perfect platforms to investigate catalytic mechanisms at the molecular or atomic level and to comprehend the relationship between structure and catalytic performance, (50−52) as confirmed in our previous work. (14) Meanwhile, non-noble-metal catalysts should be intelligently constructed to achieve efficient PWDTH in order to reduce excessive dependence on noble-metal catalysts. Zhao’s group designed a Ni2+–N4 site built on carbon nitride for photocatalyzed semihydrogenation of alkynes using water as the hydrogen source without alkane production. (33) Moreover, Xu and co-workers developed a TiO2/Ce2S3 S-scheme heterojunction photocatalyst for aniline production by nitrobenzene hydrogenation with water as the proton source. (32) This is an innovative work, free from noble-metal catalysts, but the scope of heterojunction photocatalysts for various PWDTH reactions needs to be explored. In general, the design of the photocatalysts must meet the requirements for excellent water-splitting activity and the presence of hydrogenation active sites (normally metal species, e.g., VIII subgroup metals). In addition, several articles have confirmed that the specific electronic properties (cationic form) of atomically dispersed sites are perhaps more suitable for the PWDTH reactions. (14,33)

2.3. Photocatalytic Selective Transfer Hydrogenation

Selective hydrogenation is a class of highly important organic transformations for both petrochemical and fine chemical industries. (2) The critical aspect of selective hydrogenation relies on the fabrication of efficient and selective catalysts. Xiong et al. reported a TiO2–PdxPt1–x hybrid catalyst toward photocatalytic selective 2-methyl-3-butyn-2-ol (MBY) semihydrogenation to 2-methyl-3-buten-2-ol (MBE) using water as a proton source (Scheme 4a). (30) As shown in Figure 4a, the PdxPt1–x alloys vastly surmount the limitations in catalytic activity and semihydrogenation selectivity compared with pure Pd and Pt. To reveal the mechanism, the authors studied the hydrogen evolution performance, which is the competing reaction that consumes the active H* species (Figure 4b). A significantly improved performance of photocatalytic hydrogen evolution was observed in the absence of MBY in all samples. When MBY was introduced, almost all samples were inactive for the H2 evolution process except for pure Pt, indicating that H* was mostly used in the semihydrogenation of MBY. In terms of pure Pt, H* species are weakly bound to Pt sites in either top-site or hollow-site configurations with barely any surface diffusion barrier. Therefore, H* could readily spill over the surface and subsequently evolve into H2, limiting the conversion of MBY. In contrast, the adsorbed H* strongly binds to the Pd surface by a hollow-site configuration. Although pure Pd could retain a high semihydrogenation selectivity, the slow H* diffusion and desorption would be a bottleneck in improving the conversion of MBY. Having this in mind, the authors envisaged that alloying Pd with Pt would offer Pd–Pt hollow sites for H* adsorption, in which the M–H binding could be adjusted to a suitable range for appropriate diffusion and desorption of H*. As expected, the selectivity of the TiO2–Pd0.5Pt0.5 sample remained high when the conversion approached 100% (Figure 4a). This work emphasized the importance of lattice engineering design on photocatalytic selective hydrogenation using water as a proton source.

Scheme 4

Scheme 4. Illustration of Photocatalytic Selective Transfer Hydrogenation Using H2O as an H Sourcea
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aReaction conditions: (a) catalyst (10 mg), substrate (0.5 mmol), H2O (7.5 mL), CH3OH (2.5 mL), 300 W Xe lamp (λ < 400 nm), pressure (1 bar), Ar atmosphere; (b) catalyst (0.002 μmol), sensitizer (0.1 μmol [Ru(bpy)3]2+ or 2.5 mg mpg-CN), substrate (1 atm C2H2, ≥99.5 vol %), aqueous bicarbonate buffer (2 mL, pH 8.4), sodium ascorbate (0.2 mmol), blue light (λ = 450 nm), temperature (298 K); (c) catalyst (30 mg), substrate (1 atm CO2), H2O (0.1 mL), 300 W Xe lamp.

Figure 4

Figure 4. (a) The catalytic performance of selective 2-methyl-3-butyn-2-ol semihydrogenation to 2-methyl-3-buten-2-ol (MBY) using water as a proton source with various TiO2–PdxPt1–x photocatalysts. Reaction conditions are as indicated in Scheme 4a. (b) The amount of hydrogen evolution in the presence of MBY or the absence of MBY. Reproduced with permission from ref (30). Copyright 2017 Wiley.

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Selective hydrogenation of acetylene to ethylene in the presence of an excess amount of ethylene is an important process for the petrochemical industry since a ppm level of acetylene poisons the ethylene polymerization. (53) Weiss et al. reported a PWDTH system that reduces acetylene to ethylene with ≥99% selectivity under both noncompetitive (no ethylene cofeeding) and competitive (ethylene cofeeding) conditions. (54) Importantly, nearly 100% conversion was achieved in the latter case, which is more industrially relevant (Scheme 4b). This system used a molecular catalyst based on [meso-tetra(4-sulfonatophenyl)porphyrinato]cobalt(III) (CoTPPS) operating under ambient conditions and sensitized by either tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate ([Ru(bpy)3]2+) or mesoporous graphitic carbon nitride (mpg-C3N4) under visible light. Moreover, Xiong et al. realized 99% selective CO2 methanation in a pure water system by introducing Pt single atoms to defective carbon nitride (Pt@Def-CN) (Scheme 4c). (55) Despite the low catalytic activity (6.3 μmolCH4 gcat–1 h–1), the photoreduction of CO2 into a high-value-added product in pure water has been demonstrated.

2.4. D2O as a Deuterium Source

Deuterium labeling is essential in organic synthesis and the pharmaceutical industry. Nevertheless, the state-of-the-art C–H/C–D exchange using noble-metal catalysts or strong bases/acids has the disadvantages of poor functional group tolerance and inferior selectivity. Loh et al. proposed an innovative strategy for controllable deuteration of halogenated compounds by photocatalytic D2O splitting under mild conditions using porous CdSe nanosheets as the catalyst (Scheme 5a). (56) A variety of organic iodides including aryl, alkyl, and alkynyl iodides could be deuterated with satisfactory functional group tolerance (Scheme 5b). This approach is also competent to deuterate less reductively labile C–Br, C–Cl, and even C–F bonds in the presence of activating groups on the benzene ring (Scheme 5c–e). Moreover, the authors have developed a D-labeled tool kit containing deuterated aryl halogen, boric acid, alkyne, alkene, etc., which are valuable D-containing building blocks for drugs or advanced materials (Scheme 5f). This work provided an attractive and promising strategy for controllable deuteration.

Scheme 5

Scheme 5. (a) Illustration of Photocatalytic Deuteration of Halogenated Compounds using D2O as a D Source, Scope of photocatalytic (b) C–I, (c) C–Br, (d) C–Cl, and (e) C–F to C–D Transformation, and (f) D-Labeled Toolbox from Photocatalytic C–I to C–D Transformation for Medical Intermediatesa
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aReaction conditions: catalyst (5 mg), substrate (0.1 mmol), D2O (1.5 mL), CH3CN (2.5 mL), Na2SO3 (1 mmol), 150 W Xe lamp, temperature (298 K), pressure (1 bar), Ar atmosphere. Reproduced with permission from ref (56). Copyright 2017 Nature Publishing Group.

Shortly thereafter, Wu et al. developed the deuteration of aromatic alcohols with D2O as a D source (70–99% D incorporation) under visible-light irradiation using CdSe quantum dots as the photocatalyst (Scheme 6a). (57) Zhao et al. realized the synthesis of deuterium-labeled alkenes with distinguished levels of D incorporation (99%) over nickel nanoparticles supported on carbon nitride (Ni/C3N4) (Scheme 6b). (58) Also, Sun et al. reported a metal-free defective ultrathin ZnIn2S4 nanosheet (D-ZIS) catalyst for photocatalytic deuteration of carbonyls utilizing the in situ generated D protons from D2O splitting with decent deuteration incorporation ratios (Scheme 6c). (59) These strategies provided promising insights into the sustainable and eco-friendly organic synthesis of value-added deuterated chemicals. In addition, all of the above deuteration studies have also attempted deuteration synthesis of pharmaceutical intermediates (Figure 5), pointing to the potential of the PWDTH technique for the pharmaceutical industry. Although they all only attempted the reaction of one pharmaceutical intermediate, the implications for the development of a green and safe pharmaceutical industry could be significant.

Scheme 6

Scheme 6. Illustration of Photocatalytic Deuteration Using D2O as a D Sourcea
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aReaction conditions: (a) catalyst (20 μM), substrate (0.2 mmol), D2O (5 mmol), CH3CN (2 mL), triethylamine (0.4 mmol), blue light (λ = 450 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (b) catalyst (3 mg), substrate (0.03 mmol), D2O (1.2 mL), CH3CN (4.8 mL), triethanolamine (0.6 mmol), blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (c) catalyst (5 mg), substrate (0.1 mmol), D2O (1.8 mL), CH3CN (0.2 mL), Na2SO3 (2 mmol), white LED light (λ > 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere.

Figure 5

Figure 5. Deuterated synthesis of pharmaceutical intermediates through the PWDTH technique: (a) methyl nicotinate-2-d, (56) (b) isopropyl 2-(4-((4-chlorophenyl)(hydroxy)methyl-d)phenoxy)-2-methylpropanoate, (57) (c) methyl pent-4-enoate-4,5,5-d3, (58) and (d) isopropyl 2-(4-((4-chlorophenyl)(hydroxy-d)methyl-d)phenoxy)-2-methylpropanoate. (59)

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3. Conclusion and Outlook

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Although photocatalytic transfer hydrogenation has found widespread use over the last decades, using water as the direct proton source remains underdeveloped. The innovative photocatalytic water-donating transfer hydrogenation enables the utilization of active H* species from water splitting for hydrogenation under mild conditions. This strategy is attractive due to the safe, green, eco-friendly, and sustainable reaction conditions. This perspective highlights the reaction pathways and mechanisms, which provide insights into the construction of catalytic systems and the design of efficient photocatalysts, and discusses catalyst design, leading to guidance for tailoring the catalytic activity and selectivity of different heterogeneous catalysts. While some progress has been made in the field of PWDTH, it is in its infancy and many unsolved problems and challenges remain, as outlined below.
(1)

It is crucial to enhance the performance of the PWDTH while avoiding the evolution of H* from water splitting. As far as current studies are concerned, most of the H* is prioritized for the PWDTH reaction, which is more pronounced in nanoparticle or heterojunction catalysts. Whereas for SACs, they might boost the performance of the PWDTH reaction and hydrogen evolution simultaneously. Thus, the competitive relationship between hydrogenation and hydrogen evolution has to be explored. Meanwhile, the regulation of the supported metal species (i.e., nanoparticles, clusters, single atoms, or synergistic effect of different species) should be better studied, which might lead to unexpected results in the PWDTH reactions.

(2)

At present, noble-metal catalysts still dominate in PWDTH reactions. Due to the scarcity and expense of noble metals, it is vital to develop efficient non-noble-metal catalysts such as those of Ni and Co for PWDTH reactions.

(3)

Selective hydrogenation reactions have a wide range of applications in fine chemicals and pharmaceuticals as well as petrochemicals. However, photocatalytic selective hydrogenation using water as the hydrogen source remains to be developed: for example, selective hydrogenation of α,β-unsaturated carbonyl compounds. The rational design of highly selective and versatile photocatalysts by precise modulation of the local coordination environment would be a promising direction.

(4)

The majority of existing studies of PWDTH reactions employ additional hole sacrificial agents (e.g., methanol, triethanolamine, triethylamine, and Na2SO3) to promote the generation of protons, which produce worthless substances at the oxidation side. (30) A promising strategy is to substitute the hole sacrificial agents with other valuable organics, which could produce value-added reductive and oxidative products at the same time.

(5)

In the PWDTH process, the water molecule is first split to produce the H*, which is then transferred to catalytically active sites for the hydrogenation of organics or spills over to evolve into H2. Nevertheless, the specific reaction pathways and mechanisms of PWDTH reactions remain complex and ambiguous. The understanding of reaction mechanisms requires more work, such as monitoring the reaction process at the molecular level by utilizing advanced in situ/operando characterization techniques and understanding the elementary reaction mechanism through theoretical calculations.

(6)

As the most common and nonpolluting solvent, water also has enormous potential for use in other types of reactions. It has been demonstrated that water can participate in reactions as an oxidizing or reducing agent, such as hydrogenolysis (60) and couplings. (61,62) This provides directions for the development of water involvement in reactions.

In conclusion, although the field is still in its nascent stage and has many challenges, discoveries will propel the field by solving these issues with an increasing understanding of their organocatalytic and photochemical reactivity, which will have a significant impact on the energy transition to a zero-carbon economy.

Author Information

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  • Corresponding Authors
    • Radek Zbořil - Regional Centre of Advanced Technologies and Materials, Czech Advanced Technology and Research Institute (CATRIN), Palacký University Olomouc, Šlechtitelů 27, Olomouc 783 71, Czech Republic;  Nanotechnology Centre, CEET, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, Ostrava-Poruba 708 00, Czech Republic;  Orcidhttps://orcid.org/0000-0002-3147-2196;  Email: [email protected]
    • Zupeng Chen - Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China;  Orcidhttps://orcid.org/0000-0002-7351-3240;  Email: [email protected]
  • Authors
    • En Zhao - Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China;  Orcidhttps://orcid.org/0000-0003-3416-2414
    • Wenjun Zhang - Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China
    • Lin Dong - Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Longpan Road 159, Nanjing 210037, People’s Republic of China
  • Author Contributions

    E.Z. wrote the draft of the manuscript. W.Z., L.D., R.Z., and Z.C. revised the manuscript. R.Z. and Z.C. conceived and supervised the whole project.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the National Natural Science Foundation of China (22202105, 22205113, 22002043), the Natural Science Foundation of Jiangsu Province (BK20210608, BK20210626), the Natural Science Foundation of Jiangsu Higher Education Institutions of China (21KJA150003, 21KJB150027), and the China Postdoctoral Science Foundation (2022M711645).

References

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    Selective hydrogenation of alkynes to alkenes and reductive amination are industrially important reactions to synthesize a variety of fine and bulk chems. We report herein on a green and convenient approach for Z-alkenes and secondary amines using gold catalyst and formic acid (FA) as a green reductant. Furthermore, we highlight that the key to successfully obtain high reaction rates is to use an appropriate amine, which acts cooperatively with the gold surface, to activate formic acid. Studies with deuterium-labeled hydrogen donors gave insights that the decompn. of Au-formate species is involved in the rate-detg. step. Moreover, various valuable secondary amines could be synthesized from readily available nitro and carbonyl compds. This new strategy provides a cleaner, safer, more efficient and selective way to catalyze the synthesis of Z-alkenes and valuable amines.
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  13. 13
    Kusy, R.; Grela, K. E- and Z-Selective Transfer Semihydrogenation of Alkynes Catalyzed by Standard Ruthenium Olefin Metathesis Catalysts. Org. Lett. 2016, 18 (23), 6196– 6199,  DOI: 10.1021/acs.orglett.6b03254
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    13
    E- and Z-Selective Transfer Semihydrogenation of Alkynes Catalyzed by Standard Ruthenium Olefin Metathesis Catalysts
    Kusy, Rafal; Grela, Karol
    Organic Letters (2016), 18 (23), 6196-6199CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    In the presence of the first and second generation Hoveyda-Grubbs catalysts, internal alkynes and internal aryl alkynes underwent chemoselective and diastereoselective transfer semihydrogenation using formic acid as a hydrogen donor with NaH in THF to yield (Z)- or (E)-alkenes and aryl alkenes, resp. This catalytic system is distinguished by its selectivity and compatibility with many functional groups (halogens, cyano, nitro, sulfide, alkenes). The second generation Hoveyda-Grubbs catalyst was used in the tandem ring closing metathesis and (E)-selective semihydrogenation of the aryl alkyne 4-PhC≡CC6H4CH(OCH2CH:CH2)CH2CH:CH2 to yield the styrylphenyldihydropyran I. The structure of I was detd. by X-ray crystallog.
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  14. 14
    Zhao, E.; Li, M.; Xu, B.; Wang, X. L.; Jing, Y.; Ma, D.; Mitchell, S.; Pérez-Ramírez, J.; Chen, Z. Transfer Hydrogenation with a Carbon-Nitride-Supported Palladium Single-Atom Photocatalyst and Water as a Proton Source. Angew. Chem., Int. Ed. 2022, 61 (40), e202207410  DOI: 10.1002/anie.202207410
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    14
    Transfer Hydrogenation with a Carbon-Nitride-Supported Palladium Single-Atom Photocatalyst and Water as a Proton Source
    Zhao, En; Li, Manman; Xu, Beibei; Wang, Xue-Lu; Jing, Yu; Ma, Ding; Mitchell, Sharon; Perez-Ramirez, Javier; Chen, Zupeng
    Angewandte Chemie, International Edition (2022), 61 (40), e202207410CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Solar-driven transfer hydrogenation of unsatd. bonds has received considerable attention in the research area of sustainable org. synthesis; however, water, the ultimate green source of hydrogen, has rarely been investigated due to the high barrier assocd. with splitting of water mols. We report a carbon-nitride-supported palladium single-atom heterogeneous catalyst with unparalleled performance in photocatalytic water-donating transfer hydrogenation compared to its nanoparticle counterparts. Isotopic-labeling expts. and operando NMR measurements confirm the direct hydrogenation mechanism using in situ-generated protons from water splitting under visible-light irradn. D. functional theory calcns. attribute the high activity to lower barriers for hydrogenation, facilitated desorption of ethylbenzene, and facile hydrogen replenishment from water on the at. palladium sites.
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  15. 15
    Han, C.; Du, L.; Konarova, M.; Qi, D.-C.; Phillips, D. L.; Xu, J. Beyond Hydrogen Evolution: Solar-Driven, Water-Donating Transfer Hydrogenation over Platinum/Carbon Nitride. ACS Catal. 2020, 10 (16), 9227– 9235,  DOI: 10.1021/acscatal.0c01932
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    15
    Beyond Hydrogen Evolution: Solar-Driven, Water-Donating Transfer Hydrogenation over Platinum/Carbon Nitride
    Han, Chenhui; Du, Lili; Konarova, Muxina; Qi, Dong-Chen; Phillips, David Lee; Xu, Jingsan
    ACS Catalysis (2020), 10 (16), 9227-9235CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Hydrogen-rich org. mols. such as alcs. are widely used as hydrogen donors in transfer hydrogenation. Nevertheless, water as a more abundant and ecofriendly hydrogen source has hardly been used due to the high difficulty in splitting water mols. Herein, we designed a photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was extd. from water under light illumination and then in situ added to different unsatd. bonds (C=C, C=O, N=O) for chem. synthesis. Platinum-loaded carbon nitride (Pt/CN) was used as the model catalyst for this cascade reaction, which is beyond its normal applications for water splitting. This approach was highly accessible to efficiency optimization, either by modifying CN for extended light absorption and enhanced charge transfer or by alloying Pt with another metal for better catalytic activities. Remarkably, a quantum efficiency of up to 21.8% was achieved for nitrobenzene hydrogenation under 380 nm irradn., which is 3 times higher than that obtained in the single water-splitting reaction, indicating that the PWDTH can be more rewarding than hydrogen evolution for solar energy harvesting. Deep insights into the underlying mechanism were provided by detailed measurements and interpretations of femtosecond transient absorption spectra, action spectra (quantum efficiency as a function of excitation wavelength), and reaction kinetic profiles under varied conditions including the variation of light intensities, temps., and water isotopes. The mild reaction conditions, simple processing, and broad substituent group tolerance endow this approach with a high potential toward a general solar-to-chem. conversion technique.
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  16. 16
    Zhang, D.; Ren, P.; Liu, W.; Li, Y.; Salli, S.; Han, F.; Qiao, W.; Liu, Y.; Fan, Y.; Cui, Y.; Shen, Y.; Richards, E.; Wen, X.; Rummeli, M. H.; Li, Y.; Besenbacher, F.; Niemantsverdriet, H.; Lim, T.; Su, R. Photocatalytic Abstraction of Hydrogen Atoms from Water Using Hydroxylated Graphitic Carbon Nitride for Hydrogenative Coupling Reactions. Angew. Chem., Int. Ed. 2022, 61 (24), e202204256  DOI: 10.1002/anie.202204256
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    16
    Photocatalytic Abstraction of Hydrogen Atoms from Water Using Hydroxylated Graphitic Carbon Nitride for Hydrogenative Coupling Reactions
    Zhang, Dongsheng; Ren, Pengju; Liu, Wuwen; Li, Yaru; Salli, Sofia; Han, Feiyu; Qiao, Wei; Liu, Yu; Fan, Yingzhu; Cui, Yi; Shen, Yanbin; Richards, Emma; Wen, Xiaodong; Rummeli, Mark H.; Li, Yongwang; Besenbacher, Flemming; Niemantsverdriet, Hans; Lim, Tingbin; Su, Ren
    Angewandte Chemie, International Edition (2022), 61 (24), e202204256CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Employing pure water, the ultimate green source of hydrogen donor to initiate chem. reactions that involve a hydrogen atom transfer (HAT) step is fascinating but challenging due to its large H-O bond dissocn. energy (BDEH-O=5.1 eV). Many approaches have been explored to stimulate water for hydrogenative reactions, but the efficiency and productivity still require significant enhancement. Here, we show that the surface hydroxylated graphitic carbon nitride (gCN-OH) only requires 2.25 eV to activate H-O bonds in water, enabling abstraction of hydrogen atoms via dehydrogenation of pure water into hydrogen peroxide under visible light irradn. The gCN-OH presents a stable catalytic performance for hydrogenative N-N coupling, pinacol-type coupling and dehalogenative C-C coupling, all with high yield and efficiency, even under solar radiation, featuring extensive impacts in using renewable energy for a cleaner process in dye, electronic, and pharmaceutical industries.
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    Cummings, S. P.; Le, T.-N.; Fernandez, G. E.; Quiambao, L. G.; Stokes, B. J. Tetrahydroxydiboron-Mediated Palladium-Catalyzed Transfer Hydrogenation and Deuteriation of Alkenes and Alkynes Using Water as the Stoichiometric H or D Atom Donor. J. Am. Chem. Soc. 2016, 138 (19), 6107– 6110,  DOI: 10.1021/jacs.6b02132
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    17
    Tetrahydroxydiboron-Mediated Palladium-Catalyzed Transfer Hydrogenation and Deuteriation of Alkenes and Alkynes Using Water as the Stoichiometric H or D Atom Donor
    Cummings, Steven P.; Le, Thanh-Ngoc; Fernandez, Gilberto E.; Quiambao, Lorenzo G.; Stokes, Benjamin J.
    Journal of the American Chemical Society (2016), 138 (19), 6107-6110CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    There are few examples of catalytic transfer hydrogenations of simple alkenes and alkynes that use water as a stoichiometric H or D atom donor. We have found that diboron reagents efficiently mediate the transfer of H or D atoms from water directly onto unsatd. C-C bonds using a palladium catalyst. This reaction is conducted on a broad variety of alkenes and alkynes at ambient temp., and boric acid is the sole byproduct. Mechanistic expts. suggest that this reaction is made possible by a hydrogen atom transfer from water that generates a Pd-hydride intermediate. Importantly, complete deuterium incorporation from stoichiometric D2O has also been achieved.
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    Sharma, P.; Sasson, Y. Sustainable Visible Light Assisted In-Situ Hydrogenation via a Magnesium-Water System Catalyzed by a Pd-g-C3N4 Photocatalyst. Green Chem. 2019, 21 (2), 261– 268,  DOI: 10.1039/C8GC02221F
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    18
    Sustainable visible light assisted in situ hydrogenation via a magnesium-water system catalyzed by a Pd-g-C3N4 photocatalyst
    Sharma, Priti; Sasson, Yoel
    Green Chemistry (2019), 21 (2), 261-268CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    A non-hazardous and relatively mild protocol was formulated for an effectual hydrogen generation process via a "magnesium-activated water" system with a Pd-g-C3N4 photocatalyst under visible light at room temp. Water functions photochem. as a hydrogen donor without any external source with the Pd-g-C3N4 photocatalyst. The synthesized Pd-g-C3N4 photocatalyst is highly efficient under visible light for the selective redn. of a wide range of unsatd. derivs. and nitro compds. to afford excellent yields (>99%). The photocatalyst Pd-g-C3N4 could be easily recovered and reused for several runs without any deactivation during the photochem. hydrogen transfer reaction process.
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  19. 19
    Zhao, C.-Q.; Chen, Y.-G.; Qiu, H.; Wei, L.; Fang, P.; Mei, T.-S. Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed Semihydrogenation of Alkynes. Org. Lett. 2019, 21 (5), 1412– 1416,  DOI: 10.1021/acs.orglett.9b00148
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    19
    Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed Semihydrogenation of Alkynes
    Zhao, Chuan-Qi; Chen, Yue-Gang; Qiu, Hui; Wei, Lei; Fang, Ping; Mei, Tian-Sheng
    Organic Letters (2019), 21 (5), 1412-1416CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    Palladium-catalyzed transfer semihydrogenation of alkynes using H2O as the hydrogen source and Mn as the reducing reagent is developed, affording cis- and trans-alkenes selectively under mild conditions. In addn., this method provides an efficient way to access various cis-1,2-dideuterioalkenes and trans-1,2-dideuterioalkenes by using D2O instead of H2O.
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    Wang, Q.; Domen, K. Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies. Chem. Rev. 2020, 120 (2), 919– 985,  DOI: 10.1021/acs.chemrev.9b00201
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    20
    Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies
    Wang, Qian; Domen, Kazunari
    Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 919-985CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps, absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, charge sepn. and migration of these photoexcited carriers, and surface chem. reactions based on these carriers. In this review, the focus is on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chem. and materials science to design and prep. photocatalysts for overall water splitting.
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  21. 21
    Kumar, A.; Bhardwaj, R.; Mandal, S. K.; Choudhury, J. Transfer Hydrogenation of CO2 and CO2 Derivatives using Alcohols as Hydride Sources: Boosting an H2-Free Alternative Strategy. ACS Catal. 2022, 12 (15), 8886– 8903,  DOI: 10.1021/acscatal.2c01982
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    21
    Transfer Hydrogenation of CO2 and CO2 Derivatives using Alcohols as Hydride Sources: Boosting an H2-Free Alternative Strategy
    Kumar, Abhishek; Bhardwaj, Ritu; Mandal, Sanajit Kumar; Choudhury, Joyanta
    ACS Catalysis (2022), 12 (15), 8886-8903CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Review. Numerous strategies have been developed for the redn. of highly challenging CO2 gas and its conversion into useful feedstock chems. Among all of the developed protocols, the traditional approach where H2 gas is used as a reductant has been dominantly exploited. During the past decade, enormous efforts have been made in tackling the challenge by keeping sustainability as a major goal. As an alternative option, the adoption of a "transfer hydrogenation" strategy has received attention for the CO2 redn. process. The utilization of biomass-derived alcs. as hydride donors promises to make the process viable and advantageous over the hydrogenation process. The survival of homogeneous transition-metal-based catalysts used in these processes under the harsh reaction conditions (elevated temp. and highly basic reaction medium) is a considerable challenge. Hence, the development of efficient and robust homogeneous catalysts for the CO2-transfer hydrogenation process is highly important. In this Perspective, we highlight the overall evolution of the transfer hydrogenation strategy for the redn. of CO2 gas (and its derivs.) to hydrogen-rich useful products achieved during the past decade. The role of tuning the ligand backbone to make the process kinetically more favorable is discussed in detail. The available reports in the field emphasized the advantages of using biomass-derived alcs. as hydride donors in place of nonrenewable H2 gas. Potential benefits and opportunities of the CO2-transfer hydrogenation process over the traditional hydrogenation are critically presented to encourage further intense research in the field.
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  22. 22
    Nie, R.; Tao, Y.; Nie, Y.; Lu, T.; Wang, J.; Zhang, Y.; Lu, X.; Xu, C. C. Recent Advances in Catalytic Transfer Hydrogenation with Formic Acid over Heterogeneous Transition Metal Catalysts. ACS Catal. 2021, 11 (3), 1071– 1095,  DOI: 10.1021/acscatal.0c04939
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    22
    Recent Advances in Catalytic Transfer Hydrogenation with Formic Acid over Heterogeneous Transition Metal Catalysts
    Nie, Renfeng; Tao, Yuewen; Nie, Yunqing; Lu, Tianliang; Wang, Jianshe; Zhang, Yongsheng; Lu, Xiuyang; Xu, Chunbao Charles
    ACS Catalysis (2021), 11 (3), 1071-1095CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A review. Recent progress in catalytic transfer hydrogenation (CTH) via heterogeneous hydrogen transfer from FA were summarized. Transformations of biomass-derived platform chems. (e.g., arom. units, C5 and C6 sugars, furans, glycerol, fatty acids, levulinic acid (LA)), nitrogen-contg. compds. (e.g., nitroarenes, quinolines), and organochlorinated compds. via transfer hydrogenation, hydrogenolysis, and hydrodechlorination (HDC) were outlined. Synthesis strategies of the heterogeneous metal catalysts (e.g., metal and support type, metal-support interaction, single-atom, alloy effect, and confinement effect) and optimization of the reaction conditions (e.g., temp., solvents, additives, and FA dosages) for enhancing the catalytic activity and regulating the product distribution were presented. Structure-activity relationships based on both dehydrogenation and hydrogenation of metal catalysts as well as the mechanistic interpretation of CTH with FA were also highlighted. Finally, current challenges and outlook for the future development of the field were discussed.
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  23. 23
    Lau, S.; Gasperini, D.; Webster, R. L. Amine-Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective. Angew. Chem., Int. Ed. 2021, 60 (26), 14272– 14294,  DOI: 10.1002/anie.202010835
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    23
    Amine-Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective
    Lau, Samantha; Gasperini, Danila; Webster, Ruth L.
    Angewandte Chemie, International Edition (2021), 60 (26), 14272-14294CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Transfer hydrogenation (TH) has historically been dominated by Meerwein-Ponndorf-Verley (MPV) reactions. However, with growing interest in amine-boranes, not least ammonia-borane (H3N·BH3), as potential hydrogen storage materials, these compds. have also started to emerge as an alternative reagent in TH reactions. In this Review we discuss TH chem. using H3N·BH3 and their analogs (amine-boranes and metal amidoboranes) as sacrificial hydrogen donors. Three distinct pathways were considered: (1) classical TH, (2) nonclassical TH, and (3) hydrogenation. Simple exptl. mechanistic probes can be employed to distinguish which pathway is operating and computational anal. can corroborate or discount mechanisms. We find that the pathway in operation can be perturbed by changing the temp., solvent, amine-borane, or even the substrate used in the system, and subsequently assignment of the mechanism can become nontrivial.
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    Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D. J.; Higashi, M.; Kong, D.; Abe, R.; Tang, J. Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems. Chem. Rev. 2018, 118 (10), 5201– 5241,  DOI: 10.1021/acs.chemrev.7b00286
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    24
    Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems
    Wang, Yiou; Suzuki, Hajime; Xie, Jijia; Tomita, Osamu; Martin, David James; Higashi, Masanobu; Kong, Dan; Abe, Ryu; Tang, Junwang
    Chemical Reviews (Washington, DC, United States) (2018), 118 (10), 5201-5241CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review is given. Visible light-driven water splitting using cheap and robust photocatalysts is one of the most exciting ways to produce clean and renewable energy for future generations. Cutting edge research within the field focuses on so-called Z-scheme systems, which are inspired by the photosystem II-photosystem I (PSII/PSI) coupling from natural photosynthesis. A Z-scheme system comprises 2 photocatalysts and generates 2 sets of charge carriers, splitting water into its constituent parts, H and O, at sep. locations. This is not only more efficient than using a single photocatalyst, but practically it could also be safer. Researchers within the field are constantly aiming to bring systems toward industrial level efficiencies by maximizing light absorption of the materials, engineering more stable redox couples, and also searching for new hydrogen and oxygen evolution cocatalysts. This review provides an in-depth survey of relevant Z-schemes from past to present, with particular focus on mechanistic breakthroughs, and highlights current state of the art systems which are at the forefront of the field.
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    Marzo, L.; Pagire, S. K.; Reiser, O.; Konig, B. Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?. Angew. Chem., Int. Ed. 2018, 57 (32), 10034– 10072,  DOI: 10.1002/anie.201709766
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    25
    Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?
    Marzo, Leyre; Pagire, Santosh K.; Reiser, Oliver; Koenig, Burkhard
    Angewandte Chemie, International Edition (2018), 57 (32), 10034-10072CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review on visible-light photocatalysis has evolved over the last decade into a widely used method in org. synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloaddns., ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temp., and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in org. synthesis. The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chem. and to open up new avenues by accessing reactive species hitherto unknown, esp. by merging photocatalysis with organo- or metal catalysis.
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  26. 26
    Zahran, E. M.; Bedford, N. M.; Nguyen, M. A.; Chang, Y. J.; Guiton, B. S.; Naik, R. R.; Bachas, L. G.; Knecht, M. R. Light-Activated Tandem Catalysis Driven by Multicomponent Nanomaterials. J. Am. Chem. Soc. 2014, 136 (1), 32– 35,  DOI: 10.1021/ja410465s
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    26
    Light-Activated Tandem Catalysis Driven by Multicomponent Nanomaterials
    Zahran, Elsayed M.; Bedford, Nicholas M.; Nguyen, Michelle A.; Chang, Yao-Jen; Guiton, Beth S.; Naik, Rajesh R.; Bachas, Leonidas G.; Knecht, Marc R.
    Journal of the American Chemical Society (2014), 136 (1), 32-35CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Transitioning energy-intensive and environmentally intensive processes toward sustainable conditions is necessary in light of the current global condition. To this end, photocatalytic processes represent new approaches for H2 generation; however, their application toward tandem catalytic reactivity remains challenging. Here, we demonstrate that metal oxide materials decorated with noble metal nanoparticles advance visible light photocatalytic activity toward new reactions not typically driven by light. For this, Pd nanoparticles were deposited onto Cu2O cubes to generate a composite structure. Once characterized, their hydrodehalogenation activity was studied via the reductive dechlorination of polychlorinated biphenyls. To this end, tandem catalytic reactivity was obsd. with H2 generation via H2O redn. at the Cu2O surface, followed by dehalogenation at the Pd using the in situ generated H2. Such results present methods to achieve sustainable catalytic technologies by advancing photocatalytic approaches toward new reaction systems.
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    Fan, X.; Yao, Y.; Xu, Y.; Yu, L.; Qiu, C. Visible-Light-Driven Photocatalytic Hydrogenation of Olefins Using Water as the H Source. ChemCatChem. 2019, 11 (11), 2596– 2599,  DOI: 10.1002/cctc.201900262
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    27
    Visible-light-driven photocatalytic hydrogenation of olefins using water as the H source
    Fan, Xin; Yao, Yanling; Xu, Yangsen; Yu, Lei; Qiu, Chuntian
    ChemCatChem (2019), 11 (11), 2596-2599CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In this work, a highly efficient PCN-KCl (KCl-modified polymeric carbon nitride) nanosheet photocatalyst was synthesized with the assistance of KCl. The as-prepd. PCN-KCl catalyst shows a more than 30-fold enhancement in the photocatalytic activity for H2 evolution from water compared to the pristine PCN. More importantly, when PCN-KCl was composited with a second catalyst (Pd nanoparticles), the simultaneous prodn. and utilization of active H species for alkenes hydrogenation was achieved by visible light irradn. under ambient conditions.
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    Xu, Y.; Qiu, C.; Fan, X.; Xiao, Y.; Zhang, G.; Yu, K.; Ju, H.; Ling, X.; Zhu, Y.; Su, C. K+-Induced Crystallization of Polymeric Carbon Nitride to Boost Its Photocatalytic Activity for H2 Evolution and Hydrogenation of Alkenes. Appl. Catal., B 2020, 268, 118457,  DOI: 10.1016/j.apcatb.2019.118457
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    K+-induced crystallization of polymeric carbon nitride to boost its photocatalytic activity for H2 evolution and hydrogenation of alkenes
    Xu, Yangsen; Qiu, Chuntian; Fan, Xin; Xiao, Yonghao; Zhang, Guoqiang; Yu, Kunyi; Ju, Huanxin; Ling, Xiang; Zhu, Yongfa; Su, Chenliang
    Applied Catalysis, B: Environmental (2020), 268 (), 118457CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Cryst. semiconductors with ordered long-range structure and minimized phase defect are capable of efficient sepn. and diffusion of photoexcited charge carriers, which is crucial for achieving high photocatalytic performances. Here, a new strategy is presented using KCl as structure inducer, where potassium ions (K+) act as a smart "binder" for re-ordering the structure of amorphous polymer carbon nitride (PCN) to furnish K+ implanted cryst. PCN (KPCN). The XPS depth profiling with Ar+ cluster ion sputtering illustrated that the element K is uniformly distributed in bulk of KPCN. The microstructure evolution of KPCN under elevated temp. was identified using in situ Fourier-transform IR spectroscopy. This cryst. structure endows the ordered electronic transmission channels in KPCN, thus enhanced the efficiency of hot charge carriers sepn. and migration, as well as visible light capture. Therefore, the re-ordered KPCN displays nearly 20 times enhancement toward photocatalytic hydrogen evolution, and high activity in water-splitting-based alkenes hydrogenation using the in-situ photo-generated H-species from water as sustainable H-source. The present work highlights a green and reliable strategy to remodel the structure of PCN by K+ thus dramatically boosting the photocatalytic activity for hydrogen evolution as well as water-splitting-based photosynthesis of high value-added fine chems.
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    Qiu, C.; Xu, Y.; Fan, X.; Xu, D.; Tandiana, R.; Ling, X.; Jiang, Y.; Liu, C.; Yu, L.; Chen, W.; Su, C. Highly Crystalline K-Intercalated Polymeric Carbon Nitride for Visible-Light Photocatalytic Alkenes and Alkynes Deuterations. Adv. Sci. 2019, 6 (1), 1801403,  DOI: 10.1002/advs.201801403
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    Highly Crystalline K-Intercalated Polymeric Carbon Nitride for Visible-Light Photocatalytic Alkenes and Alkynes Deuterations
    Qiu Chuntian; Xu Yangsen; Fan Xin; Tandiana Rika; Ling Xiang; Jiang Yanan; Liu Cuibo; Su Chenliang; Qiu Chuntian; Su Chenliang; Fan Xin; Yu Lei; Xu Dong; Chen Wei
    Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2019), 6 (1), 1801403 ISSN:2198-3844.
    In addition to the significance of photocatalytic hydrogen evolution, the utilization of the in situ generated H/D (deuterium) active species from water splitting for artificial photosynthesis of high value-added chemicals is very attractive and promising. Herein, photocatalytic water splitting technology is utilized to generate D-active species (i.e., Dad) that can be stabilized on anchored 2nd metal catalyst and are readily for tandem controllable deuterations of carbon-carbon multibonds to produce high value-added D-labeled chemicals/pharmaceuticals. A highly crystalline K cations intercalated polymeric carbon nitride (KPCN), rationally designed, and fabricated by a solid-template induced growth, is served as an ultraefficient photocatalyst, which shows a greater than 18-fold enhancement in the photocatalytic hydrogen evolution over the bulk PCN. The photocatalytic in situ generated D-species by superior KPCN are utilized for selective deuteration of a variety of alkenes and alkynes by anchored 2nd catalyst, Pd nanoparticles, to produce the corresponding D-labeled chemicals and pharmaceuticals with high yields and D-incorporation. This work highlights the great potential of developing photocatalytic water splitting technology for artificial photosynthesis of value-added chemicals instead of H2 evolution.
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    Li, M.; Zhang, N.; Long, R.; Ye, W.; Wang, C.; Xiong, Y. PdPt Alloy Nanocatalysts Supported on TiO2: Maneuvering Metal-Hydrogen Interactions for Light-Driven and Water-Donating Selective Alkyne Semihydrogenation. Small 2017, 13 (23), 1604173,  DOI: 10.1002/smll.201604173
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    Yao, F.; Dai, L.; Bi, J.; Xue, W.; Deng, J.; Fang, C.; Zhang, L.; Zhao, H.; Zhang, W.; Xiong, P.; Fu, Y.; Sun, J.; Zhu, J. Loofah-like Carbon Nitride Sponge towards the Highly-Efficient Photocatalytic Transfer Hydrogenation of Nitrophenols with Water as the Hydrogen Source. Chem. Eng. J. 2022, 444, 136430,  DOI: 10.1016/j.cej.2022.136430
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    Loofah-like carbon nitride sponge towards the highly-efficient photocatalytic transfer hydrogenation of nitrophenols with water as the hydrogen source
    Yao, Fanglei; Dai, Liming; Bi, Jiabao; Xue, Wenkang; Deng, Jingyao; Fang, Chenchen; Zhang, Litong; Zhao, Hongan; Zhang, Wenyao; Xiong, Pan; Fu, Yongsheng; Sun, Jingwen; Zhu, Junwu
    Chemical Engineering Journal (Amsterdam, Netherlands) (2022), 444 (), 136430CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    The vigorous development of photocatalytic water splitting technol. has laid the foundation for the photocatalytic transfer hydrogenation of org. substrates to produce the high value-added chems. using water as hydrogen source. Nevertheless, the high dissocn. energy of the O-H bond impedes its academic progress and the practical applications. Herein, we synthesize a 3D hierarchical porous loofah-like carbon nitride sponge (LCN) with ultrathin thickness via the supramol. pre-organization coupling with the oxidn. etching process, in which the heterogeneous oxygen atoms and the nitrogen vacancies are in-situ engineered. On top of the adorable photocatalytic H2 evolution (4812μmol h-1 g-1), LCN assocd. with Pt cocatalyst reveals a conversion rate of 96.5% towards the hydrogenation of 4-nitrophenol, substantially superior to the ref. expt. (8.3%). Further based on the isotope-labeling tests and the d. functional theory calcns., the photo-generated H0 from water is clarified to be the direct reducing agent, tactfully skipping the hydrogen extn. step in the traditional path. This work provides a green and sustainable methodol. to transfer the solar energy to the valuable fine chems., as well as highlights the importance of the 3D hierarchical porous structure to the catalytic activity.
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    Xu, F.; Meng, K.; Cao, S.; Jiang, C.; Chen, T.; Xu, J.; Yu, J. Step-by-Step Mechanism Insights into the TiO2/Ce2S3 S-Scheme Photocatalyst for Enhanced Aniline Production with Water as a Proton Source. ACS Catal. 2022, 12 (1), 164– 172,  DOI: 10.1021/acscatal.1c04903
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    Step-by-Step Mechanism Insights into the TiO2/Ce2S3 S-Scheme Photocatalyst for Enhanced Aniline Production with Water as a Proton Source
    Xu, Feiyan; Meng, Kai; Cao, Shuang; Jiang, Chenhui; Chen, Tao; Xu, Jingsan; Yu, Jiaguo
    ACS Catalysis (2022), 12 (1), 164-172CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Exploring heterostructured photocatalysts for the photocatalytic hydrogenation reaction with water as a proton source and investigating the corresponding intrinsic step-by-step mechanism are of great interest. Here, we develop an S-scheme heterojunction through theor. design and carried out solvothermal growth of Ce2S3 nanoparticles (NPs) onto electrospun TiO2 nanofibers. The low-dimensional (0D/1D) heterostructure unveils enhanced photocatalytic activity for aniline prodn. by nitrobenzene hydrogenation with water as a proton source. D. functional theory (DFT) calcns. indicate the electrons transfer from Ce2S3 to TiO2 upon hybridization due to their Fermi level difference and creates an internal elec. field at the interface, driving the sepn. of the photoexcited charge carriers, which is authenticated by in situ XPS along with femtosecond transient absorption spectroscopy. The step-by-step reaction mechanism of the photocatalytic nitrobenzene hydrogenation to yield aniline is revealed by in situ diffuse reflectance IR Fourier transform spectroscopy, assocd. with DFT computational prediction.
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    Jia, T.; Meng, D.; Duan, R.; Ji, H.; Sheng, H.; Chen, C.; Li, J.; Song, W.; Zhao, J. Single-Atom Nickel on Carbon Nitride Photocatalyst Achieves Semihydrogenation of Alkynes with Water Protons via Monovalent Nickel. Angew. Chem., Int. Ed. 2023, 62 (9), e202216511  DOI: 10.1002/anie.202216511
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    Single-Atom Nickel on Carbon Nitride Photocatalyst Achieves Semihydrogenation of Alkynes with Water Protons via Monovalent Nickel
    Jia, Tongtong; Meng, Di; Duan, Ran; Ji, Hongwei; Sheng, Hua; Chen, Chuncheng; Li, Jikun; Song, Wenjing; Zhao, Jincai
    Angewandte Chemie, International Edition (2023), 62 (9), e202216511CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Prospects in light-driven water activation have prompted rapid progress in hydrogenation reactions. A Ni2+-N4 site built on carbon nitride for catalyzed semihydrogenation of alkynes RCCR1 (R = 2-bromophenyl, cyclohexyl, thiophen-2-yl, etc.; R1 = H, Ph, thiophen-2-yl, etc.), with water supplying protons, powered by visible-light irradn. was described. Importantly, the photocatalytic approach developed here enabled access to diverse deuterated alkenes RC(D)=C(D)R2 (R2 = H, D) in D2O with excellent deuterium incorporation. Under visible-light irradn., evolution of a four-coordinate Ni2+ species into a three-coordinate Ni+ species was spectroscopically identified. In combination with theor. calcns., the photo-evolved Ni+ is posited as HO-Ni+-N2 with an uncoordinated, protonated pyridine nitrogen, formed by coupled Ni2+ redn. and water dissocn. The paired Ni-N prompts hydrogen liberation from water, and it renders desorption of alkene preferred over further hydrogenation to alkane, ensuring excellent semihydrogenation selectivity.
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    Guo, Y.; An, W.; Tian, X.; Xie, L.; Ren, Y.-L. Coupling Photocatalytic overall Water Splitting with Hydrogenation of Organic Molecules: a Strategy for Using Water as a Hydrogen Source and an Electron Donor to Enable Hydrogenation. Green Chem. 2022, 24 (23), 9211– 9219,  DOI: 10.1039/D2GC02427F
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    Coupling photocatalytic overall water splitting with hydrogenation of organic molecules: a strategy for using water as a hydrogen source and an electron donor to enable hydrogenation
    Guo, Yinggang; An, Wankai; Tian, Xinzhe; Xie, Lixia; Ren, Yun-Lai
    Green Chemistry (2022), 24 (23), 9211-9219CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    It is fascinating to use water as a hydrogen source to enable the hydrogenation of org. mols. in green chem. Nevertheless, current light-driven strategies suffer from an expense of reductants for proton redn. due to a high difficulty in overall water splitting. Herein, we have overcome this challenge and report that light-induced overall water splitting is coupled with hydrogenation of aryl bromides by cooperative catalysis between recyclable Pd/g-C3N4 and Fe species, which opens up a photocatalytic avenue to use water as both an electron donor and a hydrogen source to enable hydrogenation of aryl bromides, avoiding the use of addnl. reductants. Moreover, mild conditions, recyclable catalyst systems and the use of visible-light as the energy source make this process greener. The present method also allowed various high value-added deuterated arenes to be effectively synthesized. This work will guide chemists to use water as both an electron donor and a hydrogen source to develop green procedures for the hydrogenation of various org. compds.
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    Chmiel, A. F.; Williams, O. P.; Chernowsky, C. P.; Yeung, C. S.; Wickens, Z. K. Non-Innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides. J. Am. Chem. Soc. 2021, 143 (29), 10882– 10889,  DOI: 10.1021/jacs.1c05988
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    Non-innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides
    Chmiel, Alyah F.; Williams, Oliver P.; Chernowsky, Colleen P.; Yeung, Charles S.; Wickens, Zachary K.
    Journal of the American Chemical Society (2021), 143 (29), 10882-10889CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We describe a photocatalytic system that elicits potent photoreductant activity from conventional photocatalysts by leveraging radical anion intermediates generated in situ. The combination of an isophthalonitrile photocatalyst and sodium formate promotes diverse aryl radical coupling reactions from abundant but difficult to reduce aryl chloride substrates. Mechanistic studies reveal two parallel pathways for substrate redn. both enabled by a key terminal reductant byproduct, carbon dioxide radical anion.
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    Zhang, X.; Chen, J.; Gao, Y.; Li, K.; Zhou, Y.; Sun, W.; Fan, B. Photocatalyzed Transfer Hydrogenation and Deuteriation of Cyclic N-Sulfonylimines. Org. Chem. Front. 2019, 6 (14), 2410– 2414,  DOI: 10.1039/C9QO00231F
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    Photocatalyzed transfer hydrogenation and deuteration of cyclic N-sulfonylimines
    Zhang, Xuexin; Chen, Jingchao; Gao, Yang; Li, Kangkui; Zhou, Yongyun; Sun, Weiqing; Fan, Baomin
    Organic Chemistry Frontiers (2019), 6 (14), 2410-2414CODEN: OCFRA8; ISSN:2052-4129. (Royal Society of Chemistry)
    The photocatalyzed transfer hydrogenation/deuteration of cyclic N-sulfonylimines was accomplished using water/deuterium oxide as hydrogen/deuterium sources to afford cyclic sultams/deuterium-labeled cyclic sultams I [R = Ph, 2-MeOC6H4, 4-FC6H4, etc.; X = H, D] in mild reaction conditions.
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    Dai, P.; Ma, J.; Huang, W.; Chen, W.; Wu, N.; Wu, S.; Li, Y.; Cheng, X.; Tan, R. Photoredox C-F Quaternary Annulation Catalyzed by a Strongly Reducing Iridium Species. ACS Catal. 2018, 8 (2), 802– 806,  DOI: 10.1021/acscatal.7b03089
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    Photoredox C-F Quaternary Annulation Catalyzed by a Strongly Reducing Iridium Species
    Dai, Peng; Ma, Junyu; Huang, Wenhao; Chen, Wenxin; Wu, Na; Wu, Shengfu; Li, Ying; Cheng, Xu; Tan, Renxiang
    ACS Catalysis (2018), 8 (2), 802-806CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Authors report a fac-Ir(ppy)3*-IrII-IrIII photocatalytic cycle involving t-BuOK as the terminal reductant in a visible-light-induced sp2 C-F quaternary annulation reaction that proceeds in yields up to 98%. Because of the high activity of the IrII(ppy)3 catalyst, even at a loading of 50 ppm, the annulation reaction was able to compete with an uncatalyzed nucleophilic arom. substitution reaction. The annulation reaction was stereoconvergent, and an annulated product was synthesized with complete retention of enantiomeric excess.
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    Cuerva, J. M.; Campana, A. G.; Justicia, J.; Rosales, A.; Oller-Lopez, J. L.; Robles, R.; Cardenas, D. J.; Bunuel, E.; Oltra, J. E. Water: the Ideal Hydrogen-Atom Source in Free-Radical Chemistry Mediated by Ti(III) and Other Single-Electron-Transfer Metals?. Angew. Chem., Int. Ed. 2006, 45 (33), 5522– 5526,  DOI: 10.1002/anie.200600831
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    Water: the ideal hydrogen-atom source in free-radical chemistry mediated by TiIII and other single-electron-transfer metals?
    Cuerva, Juan M.; Campana, Araceli G.; Justicia, Jose; Rosales, Antonio; Oller-Lopez, Juan L.; Robles, Rafael; Cardenas, Diego J.; Bunuel, Elena; Oltra, J. Enrique
    Angewandte Chemie, International Edition (2006), 45 (33), 5522-5526CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Hydrogen-atom transfer from water to free radicals can be mediated by aqua complexes of titanium(III). Asym. epoxidn. in combination with [Cp2TiCl]/H2O-mediated reductive epoxide opening can be viewed as an alternative with complementary stereoselectivity to the hydroboration-epoxidn. method for the enantioselective synthesis of anti-Markovnikov alcs. from alkenes.
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    Call, A.; Casadevall, C.; Acuna-Pares, F.; Casitas, A.; Lloret-Fillol, J. Dual Cobalt-Copper Light-Driven Catalytic Reduction of Aldehydes and Aromatic Ketones in Aqueous Media. Chem. Sci. 2017, 8 (7), 4739– 4749,  DOI: 10.1039/C7SC01276D
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    Dual cobalt-copper light-driven catalytic reduction of aldehydes and aromatic ketones in aqueous media
    Call, Arnau; Casadevall, Carla; Acuna-Pares, Ferran; Casitas, Alicia; Lloret-Fillol, Julio
    Chemical Science (2017), 8 (7), 4739-4749CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    An efficient, general, fast, and robust light-driven methodol. based on earth-abundant elements to reduce aryl ketones, and both aryl and aliph. aldehydes (up to 1400 TON) has been presented. The catalytic system consists of a robust and well-defined aminopyridyl cobalt complex active for photocatalytic water redn. and the [Cu(bathocuproine)(Xantphos)](PF6) photoredox catalyst. The dual cobalt-copper system uses visible light as the driving-force and H2O and an electron donor (Et3N or iPr2EtN) as the hydride source. The catalytic system operates in aq. mixts. (80-60% water) with high selectivity towards the redn. of org. substrates (>2000) vs. water redn., and tolerates O2. Remarkably, the catalytic system also shows unique selectivity for the redn. of acetophenone in the presence of aliph. aldehydes. The catalytic system provides a simple and convenient method to obtain α,β-deuterated alcs. Both the obsd. reactivity and the DFT modeling support a common cobalt hydride intermediate. The DFT modeled energy profile for the [Co-H] nucleophilic attack to acetophenone and water rationalizes the competence of [CoII-H] to reduce acetophenone in the presence of water. Mechanistic studies suggest alternative mechanisms depending on the redox potential of the substrate. These results show the potential of the water redn. catalyst to develop light-driven selective org. transformations and fine solar chems.
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    Fischer, S.; Hollmann, D.; Tschierlei, S.; Karnahl, M.; Rockstroh, N.; Barsch, E.; Schwarzbach, P.; Luo, S.-P.; Junge, H.; Beller, M.; Lochbrunner, S.; Ludwig, R.; Brückner, A. Death and Rebirth: Photocatalytic Hydrogen Production by a Self-Organizing Copper-Iron System. ACS Catal. 2014, 4 (6), 1845– 1849,  DOI: 10.1021/cs500387e
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    Death and rebirth: Photocatalytic hydrogen production by a self-organizing copper-iron system
    Fischer, Steffen; Hollmann, Dirk; Tschierlei, Stefanie; Karnahl, Michael; Rockstroh, Nils; Barsch, Enrico; Schwarzbach, Patrick; Luo, Shu-Ping; Junge, Henrik; Beller, Matthias; Lochbrunner, Stefan; Ludwig, Ralf; Brueckner, Angelika
    ACS Catalysis (2014), 4 (6), 1845-1849CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    This study provides detailed mechanistic insights into light-driven hydrogen prodn. using an abundant copper-iron system. It focuses on the role of the heteroleptic copper photosensitizer [Cu(P ̂ P)(N ̂ N)]+, which can be oxidized or reduced after photoexcitation. By means of IR, EPR, and UV/vis spectroscopy as well as computational studies and spectroelectrochem., the possibility of both mechanisms was confirmed. UV/vis spectroscopy revealed the reorganization of the original heteroleptic photosensitizer during catalysis toward a homoleptic [Cu(N ̂ N)2]+ species. Operando FTIR spectroscopy showed the formation of a catalytic diiron intermediate, which resembles well-known hydrogenase active site models.
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    Call, A.; Codola, Z.; Acuna-Pares, F.; Lloret-Fillol, J. Photo- and Electrocatalytic H2 Production by New First-Row Transition-Metal Complexes Based on an Aminopyridine Pentadentate Ligand. Chem. - Eur. J. 2014, 20 (20), 6171– 6183,  DOI: 10.1002/chem.201303317
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    Photo- and Electrocatalytic H2 Production by New First-Row Transition-Metal Complexes Based on an Aminopyridine Pentadentate Ligand
    Call, Arnau; Codola, Zoel; Acuna-Pares, Ferran; Lloret-Fillol, Julio
    Chemistry - A European Journal (2014), 20 (20), 6171-6183CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The synthesis and characterization of the pentadentate ligand 1,4-di(picolyl)-7-(p-toluenesulfonyl)-1,4,7-triazacyclononane (Py2Tstacn) and their metal complexes [M(CF3SO3)(Py2Tstacn)][CF3SO3], (M = Fe (1Fe), Co (1Co) and Ni (1Ni)) are reported. Complex 1Co presents excellent H2 photoprodn. catalytic activity when using [Ir(ppy)2(bpy)]PF6 (PSIr) as photosensitizer (PS) and Et3N as electron donor, but 1Ni and 1Fe result in a low activity and a complete lack of it, resp. However, all three complexes have excellent electrocatalytic proton redn. activity in MeCN, when using HO2CCF3 (TFA) as a proton source with moderate overpotentials for 1Co (0.59 V vs. SCE) and 1Ni (0.56 V vs. SCE) and higher for 1Fe (0.87 V vs. SCE). Under conditions of MeCN/H2O/Et3N (3:7:0.2), 1Co (5 μΜ), with PSIr (100 μΜ) and irradiating at 447 nm gives a turnover no. (TON) of 690 (nH2/n1Co) and initial turnover frequency (TOF) (TON×t-1) of 703 h-1 for H2 prodn. It should be noted that 1Co retains 25% of the catalytic activity for photoprodn. of H2 in the presence of O2. The inexistence of a lag time for H2 evolution and the absence of nanoparticles during the first 30 min of the reaction suggest that the main catalytic activity obsd. is derived from a mol. system. Kinetic studies show that the reaction is -0.7 order in catalyst, and time-dependent diffraction light scattering (DLS) expts. indicate formation of metal aggregates and then nanoparticles, leading to catalyst deactivation. By a combination of exptl. and computational studies the lack of activity in photochem. H2O redn. by 1Fe can be attributed to the 1FeII/I redox couple, which is significantly lower than the PSIrIII/II, while for 1Ni the pKa value (-0.4) is too small in comparison with the pH (11.9) imposed using Et3N as electron donor.
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    Casadevall, C.; Pascual, D.; Aragon, J.; Call, A.; Casitas, A.; Casademont-Reig, I.; Lloret-Fillol, J. Light-Driven Reduction of Aromatic Olefins in Aqueous Media Catalysed by Aminopyridine Cobalt Complexes. Chem. Sci. 2022, 13 (15), 4270– 4282,  DOI: 10.1039/D1SC06608K
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    Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes
    Casadevall, Carla; Pascual, David; Aragon, Jordi; Call, Arnau; Casitas, Alicia; Casademont-Reig, Irene; Lloret-Fillol, Julio
    Chemical Science (2022), 13 (15), 4270-4282CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    A catalytic system based on earth-abundant elements that efficiently hydrogenates aryl olefins using visible light as the driving-force and H2O as the sole hydrogen atom source was reported. The catalytic system involved a robust and well-defined aminopyridine cobalt complex and a heteroleptic Cu photoredox catalyst. The system showed the redn. of styrene in aq. media with a remarkable selectivity (>20000) vs. water redn. (WR). Reactivity and mechanistic studies supported the formation of a [Co-H] intermediate, which reacted with the olefin via a hydrogen atom transfer (HAT). Synthetically useful deuterium-labeled compds. can be straightforwardly obtained by replacing H2O with D2O. Moreover, the dual photocatalytic system and the photocatalytic conditions can be rationally designed to tune the selectivity for aryl olefin vs. aryl ketone redn.; not only by changing the structural and electronic properties of the cobalt catalysts, but also by modifying the redn. properties of the photoredox catalyst.
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    Zhou, B.; Song, J.; Zhou, H.; Wu, T.; Han, B. Using the Hydrogen and Oxygen in Water Directly for Hydrogenation Reactions and Glucose Oxidation by Photocatalysis. Chem. Sci. 2016, 7 (1), 463– 468,  DOI: 10.1039/C5SC03178H
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    Using the hydrogen and oxygen in water directly for hydrogenation reactions and glucose oxidation by photocatalysis
    Zhou, Baowen; Song, Jinliang; Zhou, Huacong; Wu, Tianbin; Han, Buxing
    Chemical Science (2016), 7 (1), 463-468CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Direct utilization of the abundant hydrogen and oxygen in water for org. reactions is very attractive and challenging in chem. Herein, we report the first work on the utilization of the hydrogen in water for the hydrogenation of various org. compds. to form valuable chems. and the oxygen for the oxidn. of glucose, simultaneously by photocatalysis. It was discovered that various unsatd. compds. could be efficiently hydrogenated with high conversion and selectivity by the hydrogen from water splitting and glucose reforming over Pd/TiO2 under UV irradn. (350 nm). At the same time, glucose was oxidated by the hydroxyl radicals from water splitting and the holes caused by UV irradn. to form biomass-derived chems., such as arabinose, erythrose, formic acid, and hydroxyacetic acid. Thus, the hydrogen and oxygen were used ideally. This work presents a new and sustainable strategy for hydrogenation and biomass conversion by using the hydrogen and oxygen in water.
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    Kaiser, S. K.; Chen, Z.; Faust Akl, D.; Mitchell, S.; Pérez-Ramírez, J. Single-Atom Catalysts across the Periodic Table. Chem. Rev. 2020, 120 (21), 11703– 11809,  DOI: 10.1021/acs.chemrev.0c00576
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    Single-Atom Catalysts across the Periodic Table
    Kaiser, Selina K.; Chen, Zupeng; Faust Akl, Dario; Mitchell, Sharon; Perez-Ramirez, Javier
    Chemical Reviews (Washington, DC, United States) (2020), 120 (21), 11703-11809CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Isolated atoms featuring unique reactivity are at the heart of enzymic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, detg. the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of mol. processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and assocd. properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.
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    Shi, Y.; Zhou, Y.; Lou, Y.; Chen, Z.; Xiong, H.; Zhu, Y. Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations. Adv. Sci. 2022, 9 (24), 2201520,  DOI: 10.1002/advs.202201520
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    Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations
    Shi, Yujie; Zhou, Yuwei; Lou, Yang; Chen, Zupeng; Xiong, Haifeng; Zhu, Yongfa
    Advanced Science (Weinheim, Germany) (2022), 9 (24), 2201520CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly detd. by the uniformity of active sites. However, the non-identical no. of metal atoms, geometric shape, and morphol. of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examd. In particular, their unique behavior in boosting the catalytic performance in various chem. transformations is discussed, including selective hydrogenation, selective oxidn., Suzuki coupling, and other catalytic reactions. In addn., the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.
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    Wang, Q.; Domen, K. Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies. Chem. Rev. 2020, 120 (2), 919– 985,  DOI: 10.1021/acs.chemrev.9b00201
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    46
    Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies
    Wang, Qian; Domen, Kazunari
    Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 919-985CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps, absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, charge sepn. and migration of these photoexcited carriers, and surface chem. reactions based on these carriers. In this review, the focus is on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chem. and materials science to design and prep. photocatalysts for overall water splitting.
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    Li, X.; Bi, W.; Zhang, L.; Tao, S.; Chu, W.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y. Single-Atom Pt as Co-Catalyst for Enhanced Photocatalytic H2 Evolution. Adv. Mater. 2016, 28 (12), 2427– 2431,  DOI: 10.1002/adma.201505281
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    Single-Atom Pt as Co-Catalyst for Enhanced Photocatalytic H2 Evolution
    Li, Xiaogang; Bi, Wentuan; Zhang, Lei; Tao, Shi; Chu, Wangsheng; Zhang, Qun; Luo, Yi; Wu, Changzheng; Xie, Yi
    Advanced Materials (Weinheim, Germany) (2016), 28 (12), 2427-2431CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Isolated single Pt atoms as a new form of co-catalyst were developed, by embedding them in sub-nanoporosity in 2D g-C3N4, which maximizes the atom efficiency of the noble metal and represents a new, highly efficient photocatalytic system for H2 evolution. The cooperation of the single-atom co-catalyst in g-C3N4 provides a new strategy to modulate the electronic structure, resulting in a longer lifetime of photogenerated electrons due to the isolated single Pt atoms induced, intrinsic change of the surface trap states. Single-atom Pt co-catalyst eventually leads to tremendously enhanced photocatalytic H2 generation performance, 8.6 times higher than that of Pt nanoparticles on the per Pt atom basis, and nearly 50 times of that for bare g-C3N4 . It is believed that the single-atom co-catalyst strategy will provide a promising way to reduce the high cost of noble metals and pave a new avenue for the development of highly efficient co-catalysts.
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    Dong, P.; Wang, Y.; Zhang, A.; Cheng, T.; Xi, X.; Zhang, J. Platinum Single Atoms Anchored on a Covalent Organic Framework: Boosting Active Sites for Photocatalytic Hydrogen Evolution. ACS Catal. 2021, 11 (21), 13266– 13279,  DOI: 10.1021/acscatal.1c03441
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    48
    Platinum Single Atoms Anchored on a Covalent Organic Framework: Boosting Active Sites for Photocatalytic Hydrogen Evolution
    Dong, Pengyu; Wang, Yan; Zhang, Aicaijun; Cheng, Ting; Xi, Xinguo; Zhang, Jinlong
    ACS Catalysis (2021), 11 (21), 13266-13279CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    It is of great importance to explore and achieve a more effective approach toward the controllable synthesis of single-atom-based photocatalysts with high metal content and long-term durability. Herein, single-atom platinum (Pt) with high loading content anchored on the pore walls of two-dimensional β-ketoenamine-linked covalent org. frameworks (TpPa-1-COF) is presented. Aided by advanced characterization techniques of aberration-cor. high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy, it has been demonstrated that atomically dispersed Pt is formed on the TpPa-1-COF support through a six-coordinated C3N-Pt-Cl2 species. The optimized Pt1@TpPa-1 catalyst exhibits a high photocatalytic H2 evolution rate of 719μmol g-1 h-1 under visible-light irradn., a high actual Pt loading content of 0.72 wt %, and a large turnover frequency (TOF) of 19.5 h-1, with activity equiv. to 3.9 and 48 times higher than those of Pt nanoparticles/TpPa-1 and bare TpPa-1, resp. The improved photocatalytic performance for H2 evolution is ascribed to the effective photogenerated charge sepn. and migration and well-dispersed active sites of single-atom Pt. Moreover, d. functional theory (DFT) calcns. further reveal the role of Pt single atoms in the enhanced photocatalytic activity for H2 evolution. Overall, this work provides some inspiration for designing single-atom-based photocatalysts with outstanding stability and efficiency using COFs as the support.
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  49. 49
    Ling, X.; Xu, Y.; Wu, S.; Liu, M.; Yang, P.; Qiu, C.; Zhang, G.; Zhou, H.; Su, C. A Visible-Light-Photocatalytic Water-Splitting Strategy for Sustainable Hydrogenation/Deuteration of Aryl Chlorides. Sci. China-Chem. 2020, 63 (3), 386– 392,  DOI: 10.1007/s11426-019-9672-8
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    A visible-light-photocatalytic water-splitting strategy for sustainable hydrogenation/deuteration of aryl chlorides
    Ling, Xiang; Xu, Yangsen; Wu, Shaoping; Liu, Mofan; Yang, Peng; Qiu, Chuntian; Zhang, Guoqiang; Zhou, Hongwei; Su, Chenliang
    Science China: Chemistry (2020), 63 (3), 386-392CODEN: SCCCCS; ISSN:1869-1870. (Science China Press)
    Abstr.: Hydrogenation/deuteration of carbon chloride (C-Cl) bonds is of high significance but remains a remarkable challenge in synthetic chem., esp. using safe and inexpensive hydrogen donors. In this article, a visible-light-photocatalytic water splitting hydrogenation technol. (WSHT) is proposed to in-situ generate active H-species (i.e., Had) for controllable hydrogenation of aryl chlorides instead of using flammable H2. When applying heavy water-splitting systems, we could selectively install deuterium at the C-Cl position of aryl chlorides under mild conditions for the sustainable synthesis of high-valued added deuterated chems. Sub-micrometer Pd nanosheets (Pd NSs) decorated crystallined polymeric carbon nitrides (CPCN) is developed as the bifunctional photocatalyst, whereas Pd NSs not only serve as a cocatalyst of CPCN to generate and stabilize H (D)-species but also play a significant role in the sequential activation and hydrogenation/deuteration of C-Cl bonds. This article highlights a photocatalytic-WSHT for controllable hydrogenation/deuteration of low-cost aryl chlorides, providing a promising way for the photosynthesis of high-valued added chems. instead of the hydrogen evolution.
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    Ji, S.; Chen, Y.; Wang, X.; Zhang, Z.; Wang, D.; Li, Y. Chemical Synthesis of Single Atomic Site Catalysts. Chem. Rev. 2020, 120 (21), 11900– 11955,  DOI: 10.1021/acs.chemrev.9b00818
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    Chemical Synthesis of Single Atomic Site Catalysts
    Ji, Shufang; Chen, Yuanjun; Wang, Xiaolu; Zhang, Zedong; Wang, Dingsheng; Li, Yadong
    Chemical Reviews (Washington, DC, United States) (2020), 120 (21), 11900-11955CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chem. synthesis. The recent emergence of single at. site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the max. efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.
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    Mo, Q.; Zhang, L.; Li, S.; Song, H.; Fan, Y.; Su, C. Y. Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2022, 144 (49), 22747– 22758,  DOI: 10.1021/jacs.2c10801
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    51
    Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction
    Mo, Qijie; Zhang, Li; Li, Sihong; Song, Haili; Fan, Yanan; Su, Cheng-Yong
    Journal of the American Chemical Society (2022), 144 (49), 22747-22758CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    As a type of heterogeneous catalyst expected for the max. atom efficiency, a series of single-atom catalysts (SACs) contg. spatially isolated metal single atoms (M-SAs) have been successfully prepd. by confining M-SAs in the pore-nanospaces of porphyrinic metal-org. frameworks (MOFs). The prepd. MOF composites of M-SAs@Pd-PCN-222-NH2 (M = Pt, Ir, Au, and Ru) display exceptionally high and persistent efficiency in the photocatalytic hydrogen evolution reaction with a turnover no. (TON) of up to 21713 in 32 h and a beginning/lasting turnover frequency (TOF) larger than 1200/600 h-1 based on M-SAs under visible light irradn. (λ ≥ 420 nm). The photo-/electrochem. property studies and d. functional theory calcns. disclose that the close proximity of the catalytically active Pt-SAs to the Pd-porphyrin photosensitizers with the confinement and stabilization effect by chem. binding could accelerate electron-hole sepn. and charge transfer in pore-nanospaces, thus promoting the catalytic H2 evolution reaction with lasting effectiveness.
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    Jin, X.; Wang, R.; Zhang, L.; Si, R.; Shen, M.; Wang, M.; Tian, J.; Shi, J. Electron Configuration Modulation of Nickel Single Atoms for Elevated Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2020, 59 (17), 6827– 6831,  DOI: 10.1002/anie.201914565
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    Electron Configuration Modulation of Nickel Single Atoms for Elevated Photocatalytic Hydrogen Evolution
    Jin, Xixiong; Wang, Rongyan; Zhang, Lingxia; Si, Rui; Shen, Meng; Wang, Min; Tian, Jianjian; Shi, Jianlin
    Angewandte Chemie, International Edition (2020), 59 (17), 6827-6831CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The emerging metal single-atom catalyst has aroused extensive attention in multiple fields, such as clean energy, environmental protection, and biomedicine. Unfortunately, though it has been shown to be highly active, the origins of the activity of the single-atom sites remain unrevealed to date owing to the lack of deep insight on electronic level. Now, partially oxidized Ni single-atom sites were constructed in polymeric carbon nitride (CN), which elevates the photocatalytic performance by over 30-fold. The 3d orbital of the partially oxidized Ni single-atom sites is filled with unpaired d-electrons, which are ready to be excited under irradn. Such an electron configuration results in elevated light response, cond., charge sepn., and mobility of the photocatalyst concurrently, thus largely augmenting the photocatalytic performance.
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    Sholl, D. S.; Lively, R. P. Seven Chemical Separations to Change the World. Nature 2016, 532 (7600), 435– 437,  DOI: 10.1038/532435a
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    Seven chemical separations to change the world
    Sholl David S; Lively Ryan P
    Nature (2016), 532 (7600), 435-7 ISSN:.
    There is no expanded citation for this reference.
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    Arcudi, F.; Đorđevic, L.; Schweitzer, N.; Stupp, S. I.; Weiss, E. A. Selective Visible-Light Photocatalysis of Acetylene to Ethylene Using a Cobalt Molecular Catalyst and Water as a Proton Source. Nat. Chem. 2022, 14 (9), 1007– 1012,  DOI: 10.1038/s41557-022-00966-5
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    Selective visible-light photocatalysis of acetylene to ethylene using a cobalt molecular catalyst and water as a proton source
    Arcudi, Francesca; Dordjevic, Luka; Schweitzer, Neil; Stupp, Samuel I.; Weiss, Emily A.
    Nature Chemistry (2022), 14 (9), 1007-1012CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
    The prodn. of polymers from ethylene requires the ethylene feed to be sufficiently purified of acetylene contaminant. Accomplishing this task by thermally hydrogenating acetylene requires a high temp., an external feed of H2 gas and noble-metal catalysts. It is not only expensive and energy-intensive, but also prone to overhydrogenating to ethane. Here we report a photocatalytic system that reduces acetylene to ethylene with ≥99% selectivity under both non-competitive (no ethylene co-feed) and competitive (ethylene co-feed) conditions, and near 100% conversion under the latter industrially relevant conditions. Our system uses a mol. catalyst based on earth-abundant cobalt operating under ambient conditions and sensitized by either [Ru(bpy)3]2+ or an inexpensive org. semiconductor (metal-free mesoporous graphitic carbon nitride) under visible light. These features and the use of water as a proton source offer advantages over current hydrogenation technologies with respect to selectivity and sustainability.
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    Shi, X.; Huang, Y.; Bo, Y.; Duan, D.; Wang, Z.; Cao, J.; Zhu, G.; Ho, W.; Wang, L.; Huang, T.; Xiong, Y. Highly Selective Photocatalytic CO2 Methanation with Water Vapor on Single-Atom Platinum-Decorated Defective Carbon Nitride. Angew. Chem., Int. Ed. 2022, 61 (27), e202203063  DOI: 10.1002/anie.202203063
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    Highly Selective Photocatalytic CO2 Methanation with Water Vapor on Single-Atom Platinum-Decorated Defective Carbon Nitride
    Shi, Xianjin; Huang, Yu; Bo, Yanan; Duan, Delong; Wang, Zhenyu; Cao, Junji; Zhu, Gangqiang; Ho, Wingkei; Wang, Liqin; Huang, Tingting; Xiong, Yujie
    Angewandte Chemie, International Edition (2022), 61 (27), e202203063CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Solar-driven CO2 methanation with water is an important route to simultaneously address carbon neutrality and produce fuels. It is challenging to achieve high selectivity in CO2 methanation due to competing reactions. Nonetheless, aspects of the catalyst design can be controlled with meaningful effects on the catalytic outcomes. We report highly selective CO2 methanation with water vapor using a photocatalyst that integrates polymeric carbon nitride (CN) with single Pt atoms. As revealed by exptl. characterization and theor. simulations, the widely explored Pt-CN catalyst is adapted for selective CO2 methanation with our rationally designed synthetic method. The synthesis creates defects in CN along with formation of hydroxyl groups proximal to the coordinated Pt atoms. The photocatalyst exhibits high activity and carbon selectivity (99%) for CH4 prodn. in photocatalytic CO2 redn. with pure water. This work provides at. scale insight into the design of photocatalysts for selective CO2 methanation.
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    Liu, C.; Chen, Z.; Su, C.; Zhao, X.; Gao, Q.; Ning, G. H.; Zhu, H.; Tang, W.; Leng, K.; Fu, W.; Tian, B.; Peng, X.; Li, J.; Xu, Q. H.; Zhou, W.; Loh, K. P. Controllable Deuteration of Halogenated Compounds by Photocatalytic D2O Splitting. Nat. Commun. 2018, 9 (1), 80,  DOI: 10.1038/s41467-017-02551-8
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    Controllable deuteration of halogenated compounds by photocatalytic D2O splitting
    Liu Cuibo; Su Chenliang; Gao Qiang; Tian Bingbing; Li Jing; Xu Qing-Hua; Loh Kian Ping; Liu Cuibo; Chen Zhongxin; Su Chenliang; Zhao Xiaoxu; Gao Qiang; Ning Guo-Hong; Zhu Hai; Leng Kai; Fu Wei; Tian Bingbing; Peng Xinwen; Li Jing; Xu Qing-Hua; Loh Kian Ping; Chen Zhongxin; Zhao Xiaoxu; Tang Wei; Zhou Wu
    Nature communications (2018), 9 (1), 80 ISSN:.
    Deuterium labeling is of great value in organic synthesis and the pharmaceutical industry. However, the state-of-the-art C-H/C-D exchange using noble metal catalysts or strong bases/acids suffers from poor functional group tolerances, poor selectivity and lack of scope for generating molecular complexity. Herein, we demonstrate the deuteration of halides using heavy water as the deuteration reagent and porous CdSe nanosheets as the catalyst. The deuteration mechanism involves the generation of highly active carbon and deuterium radicals via photoinduced electron transfer from CdSe to the substrates, followed by tandem radicals coupling process, which is mechanistically distinct from the traditional methods involving deuterium cations or anions. Our deuteration strategy shows better selectivity and functional group tolerances than current C-H/C-D exchange methods. Extending the synthetic scope, deuterated boronic acids, halides, alkynes, and aldehydes can be used as synthons in Suzuki coupling, Click reaction, C-H bond insertion reaction etc. for the synthesis of complex deuterated molecules.
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  • Abstract

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    Scheme 1

    Scheme 1. Traditional Thermal Hydrogenation Uses Flammable H2 at High Temperature and Pressure and Conventional TH Uses Eco-Unfriendly Hydrogen Sources (e.g., Formic Acid, Hydrazine, and Borane–Ammonia), while Photocatalytic Transfer Hydrogenation Uses H2O as the Green Hydrogen Source at Normal Pressure and Temperature
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    Figure 1

    Figure 1. (a) Illustration of fabrication of Cu2O/Pd catalysts for photocatalytic water-donating dehalogenation. (b) Concentration changes of PCB2 and biphenyl product during the photocatalytic water-donating dehalogenation reaction over Cu2O/Pd. Reaction conditions: catalyst (100 mg), substrate (1.25 μmol), H2O (25 mL), CH3OH (25 mL), 450 W Hg lamp, pressure (1 bar), N2 atmosphere. Reproduced with permission from ref (26). Copyright 2014 American Chemical Society.

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    Scheme 2

    Scheme 2. Illustration of the PWDTH Process in the Presence of Hydrogen-Rich Additivesa
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    aReaction conditions: (a) catalyst (10 mg), substrate (0.1 mmol), H2O (1.5 mL), ethyl acetate (2 mL), CH3OH (1.4 mL), additive (HCOOH, 0.1 mL), 20 W blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (b) catalyst (10 mg), substrate (0.1 mmol), H2O (1.5 mL), ethyl acetate (2 mL), CH3OH (1.5 mL), additive (NaHSO4, 0.1 mmol), 20 W blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (c) catalyst (10 mg), substrate (0.1 mmol), D2O (1.5 mL), ethyl acetate (2 mL), CD3OD (1.5 mL), additive (AlCl3, 0.1 mmol), 20 W blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere.

    Figure 2

    Figure 2. (top) Mass spectra of the liquid product from the PWDTH reaction of styrene. Reproduced with permission from ref (15). Copyright 2020 American Chemical Society. (bottom) Operando 1H NMR spectra of the liquid product from the PWDTH reaction of styrene. Reproduced with permission from ref (14). Copyright 2022 Wiley.

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    Scheme 3

    Scheme 3. Proposed Mechanism for the PWDTH Process of (a) N-Sulfonylimines and (b) Aromatic Ketones and Aldehydesa
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    aReproduced with permission from refs (36and39). Copyright 2019 Royal Society of Chemistry.

    Figure 3

    Figure 3. (a) Aberration-corrected high-angle annular dark-field scanning transmission electron microscopic (AC-HAADF-STEM) image and (b) Fourier transform of Pd K-edge extended X-ray absorption fine structure spectra (EXAFS) spectra of Pd1-mpg-C3N4. Pd foil and PdO were applied for comparison purposes. (c) Metal-specific reaction rate toward ethylbenzene formation as a function of palladium content for the PWDTH reaction of styrene. Reaction conditions: catalyst (10 mg), substrate (0.1 mmol), H2O (2 mL), 1,4-dioxane (3 mL), triethanolamine (0.5 mL), 40 W blue light (λ = 427 nm), temperature (308 K), pressure (1 bar), N2 atmosphere. Reproduced with permission from ref (14). Copyright 2022 Wiley.

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    Scheme 4

    Scheme 4. Illustration of Photocatalytic Selective Transfer Hydrogenation Using H2O as an H Sourcea
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    aReaction conditions: (a) catalyst (10 mg), substrate (0.5 mmol), H2O (7.5 mL), CH3OH (2.5 mL), 300 W Xe lamp (λ < 400 nm), pressure (1 bar), Ar atmosphere; (b) catalyst (0.002 μmol), sensitizer (0.1 μmol [Ru(bpy)3]2+ or 2.5 mg mpg-CN), substrate (1 atm C2H2, ≥99.5 vol %), aqueous bicarbonate buffer (2 mL, pH 8.4), sodium ascorbate (0.2 mmol), blue light (λ = 450 nm), temperature (298 K); (c) catalyst (30 mg), substrate (1 atm CO2), H2O (0.1 mL), 300 W Xe lamp.

    Figure 4

    Figure 4. (a) The catalytic performance of selective 2-methyl-3-butyn-2-ol semihydrogenation to 2-methyl-3-buten-2-ol (MBY) using water as a proton source with various TiO2–PdxPt1–x photocatalysts. Reaction conditions are as indicated in Scheme 4a. (b) The amount of hydrogen evolution in the presence of MBY or the absence of MBY. Reproduced with permission from ref (30). Copyright 2017 Wiley.

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    Scheme 5

    Scheme 5. (a) Illustration of Photocatalytic Deuteration of Halogenated Compounds using D2O as a D Source, Scope of photocatalytic (b) C–I, (c) C–Br, (d) C–Cl, and (e) C–F to C–D Transformation, and (f) D-Labeled Toolbox from Photocatalytic C–I to C–D Transformation for Medical Intermediatesa
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    aReaction conditions: catalyst (5 mg), substrate (0.1 mmol), D2O (1.5 mL), CH3CN (2.5 mL), Na2SO3 (1 mmol), 150 W Xe lamp, temperature (298 K), pressure (1 bar), Ar atmosphere. Reproduced with permission from ref (56). Copyright 2017 Nature Publishing Group.

    Scheme 6

    Scheme 6. Illustration of Photocatalytic Deuteration Using D2O as a D Sourcea
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    aReaction conditions: (a) catalyst (20 μM), substrate (0.2 mmol), D2O (5 mmol), CH3CN (2 mL), triethylamine (0.4 mmol), blue light (λ = 450 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (b) catalyst (3 mg), substrate (0.03 mmol), D2O (1.2 mL), CH3CN (4.8 mL), triethanolamine (0.6 mmol), blue light (λ = 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere; (c) catalyst (5 mg), substrate (0.1 mmol), D2O (1.8 mL), CH3CN (0.2 mL), Na2SO3 (2 mmol), white LED light (λ > 420 nm), temperature (298 K), pressure (1 bar), Ar atmosphere.

    Figure 5

    Figure 5. Deuterated synthesis of pharmaceutical intermediates through the PWDTH technique: (a) methyl nicotinate-2-d, (56) (b) isopropyl 2-(4-((4-chlorophenyl)(hydroxy)methyl-d)phenoxy)-2-methylpropanoate, (57) (c) methyl pent-4-enoate-4,5,5-d3, (58) and (d) isopropyl 2-(4-((4-chlorophenyl)(hydroxy-d)methyl-d)phenoxy)-2-methylpropanoate. (59)

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  • References

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      Cheruvathoor Poulose, Aby; Zoppellaro, Giorgio; Konidakis, Ioannis; Serpetzoglou, Efthymis; Stratakis, Emmanuel; Tomanec, Ondrej; Beller, Matthias; Bakandritsos, Aristides; Zboril, Radek
      Nature Nanotechnology (2022), 17 (5), 485-492CODEN: NNAABX; ISSN:1748-3387. (Nature Portfolio)
      Redn. of nitroaroms. to the corresponding amines is a key process in the fine and bulk chems. industry to produce polymers, pharmaceuticals, agrochems. and dyes. However, their effective and selective redn. requires high temps. and pressurized hydrogen and involves noble metal-based catalysts. Here we report on an earth-abundant, plasmonic nano-photocatalyst, with an excellent reaction rate toward the selective hydrogenation of nitroaroms. With solar light as the only energy input, the chalcopyrite catalyst operates through the combined action of hot holes and photothermal effects. Ultrafast laser transient absorption and light-induced ESR spectroscopies have unveiled the energy matching of the hot holes in the valence band of the catalyst with the frontier orbitals of the hydrogen and electron donor, via a transient coordination intermediate. Consequently, the reusable and sustainable copper-iron-sulfide (CuFeS2) catalyst delivers previously unattainable turnover frequencies, even in large-scale reactions, while the cost-normalized prodn. rate stands an order of magnitude above the state of the art.
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    8. 8
      Huang, Z.; Wang, Y.; Leng, X.; Huang, Z. An Amine-Assisted Ionic Monohydride Mechanism Enables Selective Alkyne cis-Semihydrogenation with Ethanol: From Elementary Steps to Catalysis. J. Am. Chem. Soc. 2021, 143 (12), 4824– 4836,  DOI: 10.1021/jacs.1c01472
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      8
      An Amine-Assisted Ionic Monohydride Mechanism Enables Selective Alkyne cis-Semihydrogenation with Ethanol: From Elementary Steps to Catalysis
      Huang, Zhidao; Wang, Yulei; Leng, Xuebing; Huang, Zheng
      Journal of the American Chemical Society (2021), 143 (12), 4824-4836CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The selective synthesis of Z-alkenes in alkyne semihydrogenation relies on the reactivity difference of the catalysts toward the starting materials and the products. Here we report Z-selective semihydrogenation of alkynes with ethanol via a coordination-induced ionic monohydride mechanism. The EtOH-coordination-driven Cl- dissocn. in a pincer Ir(III) hydridochloride complex (NCP)IrHCl (1) forms a cationic monohydride, [(NCP)IrH(EtOH)]+Cl-, that reacts selectively with alkynes over the corresponding Z-alkenes, thereby overcoming competing thermodynamically dominant alkene Z-E isomerization and overredn. The challenge for establishing a catalytic cycle, however, lies in the alcoholysis step; the reaction of the alkyne insertion product (NCP)IrCl(vinyl) with EtOH does occur, but very slowly. Surprisingly, the alcoholysis does not proceed via direct protonolysis of the Ir-C(vinyl) bond. Instead, mechanistic data are consistent with an anion-involved alcoholysis pathway involving ionization of (NCP)IrCl(vinyl) via EtOH-for-Cl substitution and reversible protonation of Cl- ion with an Ir(III)-bound EtOH, followed by β-H elimination of the ethoxy ligand and C(vinyl)-H reductive elimination. The use of an amine is key to the monohydride mechanism by promoting the alcoholysis. The 1-amine-EtOH catalytic system exhibits an unprecedented level of substrate scope, generality, and compatibility, as demonstrated by Z-selective redn. of all alkyne classes, including challenging enynes and complex polyfunctionalized mols. Comparison with a cationic monohydride complex bearing a noncoordinating BArF- ion elucidates the beneficial role of the Cl- ion in controlling the stereoselectivity, and comparison between 1-amine-EtOH and 1-NaOtBu-EtOH underscores the fact that this base variable, albeit in catalytic amts., leads to different mechanisms and consequently different stereoselectivity.
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    9. 9
      Su, K.; Wang, Y.; Zhang, C.; Gao, Z.; Han, J.; Wang, F. Tuning the Pt Species on Nb2O5 by Support-Induced Modification in the Photocatalytic Transfer Hydrogenation of Phenylacetylene. Appl. Catal., B 2021, 298, 120554,  DOI: 10.1016/j.apcatb.2021.120554
      [Crossref], [CAS], Google Scholar
      9
      Tuning the Pt species on Nb2O5 by support-induced modification in the photocatalytic transfer hydrogenation of phenylacetylene
      Su, Kaiyi; Wang, Yehong; Zhang, Chaofeng; Gao, Zhuyan; Han, Jianyu; Wang, Feng
      Applied Catalysis, B: Environmental (2021), 298 (), 120554CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Herein, we reported the tuning of dominant products in the photocatalytic hydrogen transfer reaction over Pt/Nb2O5 photocatalysts, using a support-induced modification strategy. In the photocatalytic conversion of phenylacetylene with ethanol as a probe reaction, the dominant product was styrene over Pt/2D Nb2O5 nanoplates; in contrast, H2 was the main product over the Pt/bulk Nb2O5. Compared to the metallic Pt species over bulk Nb2O5, the pos. charged Pt (Ptδ+) species over 2D Nb2O5 nanoplates contributed to the redn. of phenylacetylene, which was revealed by CO adsorbed Fourier transform IR spectra. Furthermore, the present Ptδ+ species were due to the formation of Ptδ+-O-Nb structure via oxygen transfer of 2D Nb2O5 nanoplates, which was verified by the results of X-ray photoelectron spectra and H2 temp.-programmed redn. This work sheds light on the design and application of Nb-based catalysts in the transfer hydrogenation of orgs. in photocatalysis.
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    10. 10
      Hao, C. H.; Guo, X. N.; Pan, Y. T.; Chen, S.; Jiao, Z. F.; Yang, H.; Guo, X. Y. Visible-Light-Driven Selective Photocatalytic Hydrogenation of Cinnamaldehyde over Au/SiC Catalysts. J. Am. Chem. Soc. 2016, 138 (30), 9361– 9364,  DOI: 10.1021/jacs.6b04175
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      10
      Visible-Light-Driven Selective Photocatalytic Hydrogenation of Cinnamaldehyde over Au/SiC Catalysts
      Hao, Cai-Hong; Guo, Xiao-Ning; Pan, Yung-Tin; Chen, Shuai; Jiao, Zhi-Feng; Yang, Hong; Guo, Xiang-Yun
      Journal of the American Chemical Society (2016), 138 (30), 9361-9364CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Highly selective hydrogenation of cinnamaldehyde to cinnamyl alc. with 2-propanol was achieved using SiC-supported Au nanoparticles as photocatalyst. The hydrogenation reached a turnover frequency as high as 487 h-1 with 100% selectivity for the prodn. of alc. under visible light irradn. at 20 °C. This high performance is attributed to a synergistic effect of localized surface plasmon resonance of Au NPs and charge transfer across the SiC/Au interface. The charged metal surface facilitates the oxidn. of 2-propanol to form acetone, while the electron and steric effects at the interface favor the preferred end-adsorption of α,β-unsatd. aldehydes for their selective conversion to unsatd. alcs. We show that this Au/SiC photocatalyst is capable of hydrogenating a large variety of α,β-unsatd. aldehydes to their corresponding unsatd. alcs. with high conversion and selectivity.
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    11. 11
      Hu, Y.; Huang, W.; Wang, H.; He, Q.; Zhou, Y.; Yang, P.; Li, Y.; Li, Y. Metal-Free Photocatalytic Hydrogenation Using Covalent Triazine Polymers. Angew. Chem., Int. Ed. 2020, 59 (34), 14378– 14382,  DOI: 10.1002/anie.202006618
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      11
      Metal-Free Photocatalytic Hydrogenation Using Covalent Triazine Polymers
      Hu, Yongpan; Huang, Wei; Wang, Hongshuai; He, Qing; Zhou, Yuan; Yang, Ping; Li, Youyong; Li, Yanguang
      Angewandte Chemie, International Edition (2020), 59 (34), 14378-14382CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Photocatalytic hydrogenation of biomass-derived org. mols. transforms solar energy into high-energy-d. chem. bonds. Reported herein is the prepn. of a thiophene-contg. covalent triazine polymer as a photocatalyst, with unique donor-acceptor units, for the metal-free photocatalytic hydrogenation of unsatd. org. mols. Under visible-light illumination, the polymeric photocatalyst enables the transformation of maleic acid into succinic acid with a prodn. rate of about 2 mmol g-1 h-1, and furfural into furfuryl alc. with a prodn. rate of about 0.5 mmol g-1 h-1. Great catalyst stability and recyclability are also measured. Given the structural diversity of polymeric photocatalysts and their readily tunable optical and electronic properties, metal-free photocatalytic hydrogenation represents a highly promising approach for solar energy conversion.
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    12. 12
      Fiorio, J. L.; Araújo, T. P.; Barbosa, E. C. M.; Quiroz, J.; Camargo, P. H. C.; Rudolph, M.; Hashmi, A. S. K.; Rossi, L. M. Gold-Amine Cooperative Catalysis for Reductions and Reductive Aminations Using Formic Acid as Hydrogen Source. Appl. Catal., B 2020, 267, 118728,  DOI: 10.1016/j.apcatb.2020.118728
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      12
      Gold-amine cooperative catalysis for reductions and reductive aminations using formic acid as hydrogen source
      Fiorio, Jhonatan L.; Araujo, Thaylan P.; Barbosa, Eduardo C. M.; Quiroz, Jhon; Camargo, Pedro H. C.; Rudolph, Matthias; Hashmi, A. Stephen K.; Rossi, Liane M.
      Applied Catalysis, B: Environmental (2020), 267 (), 118728CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Selective hydrogenation of alkynes to alkenes and reductive amination are industrially important reactions to synthesize a variety of fine and bulk chems. We report herein on a green and convenient approach for Z-alkenes and secondary amines using gold catalyst and formic acid (FA) as a green reductant. Furthermore, we highlight that the key to successfully obtain high reaction rates is to use an appropriate amine, which acts cooperatively with the gold surface, to activate formic acid. Studies with deuterium-labeled hydrogen donors gave insights that the decompn. of Au-formate species is involved in the rate-detg. step. Moreover, various valuable secondary amines could be synthesized from readily available nitro and carbonyl compds. This new strategy provides a cleaner, safer, more efficient and selective way to catalyze the synthesis of Z-alkenes and valuable amines.
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    13. 13
      Kusy, R.; Grela, K. E- and Z-Selective Transfer Semihydrogenation of Alkynes Catalyzed by Standard Ruthenium Olefin Metathesis Catalysts. Org. Lett. 2016, 18 (23), 6196– 6199,  DOI: 10.1021/acs.orglett.6b03254
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      13
      E- and Z-Selective Transfer Semihydrogenation of Alkynes Catalyzed by Standard Ruthenium Olefin Metathesis Catalysts
      Kusy, Rafal; Grela, Karol
      Organic Letters (2016), 18 (23), 6196-6199CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
      In the presence of the first and second generation Hoveyda-Grubbs catalysts, internal alkynes and internal aryl alkynes underwent chemoselective and diastereoselective transfer semihydrogenation using formic acid as a hydrogen donor with NaH in THF to yield (Z)- or (E)-alkenes and aryl alkenes, resp. This catalytic system is distinguished by its selectivity and compatibility with many functional groups (halogens, cyano, nitro, sulfide, alkenes). The second generation Hoveyda-Grubbs catalyst was used in the tandem ring closing metathesis and (E)-selective semihydrogenation of the aryl alkyne 4-PhC≡CC6H4CH(OCH2CH:CH2)CH2CH:CH2 to yield the styrylphenyldihydropyran I. The structure of I was detd. by X-ray crystallog.
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    14. 14
      Zhao, E.; Li, M.; Xu, B.; Wang, X. L.; Jing, Y.; Ma, D.; Mitchell, S.; Pérez-Ramírez, J.; Chen, Z. Transfer Hydrogenation with a Carbon-Nitride-Supported Palladium Single-Atom Photocatalyst and Water as a Proton Source. Angew. Chem., Int. Ed. 2022, 61 (40), e202207410  DOI: 10.1002/anie.202207410
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      14
      Transfer Hydrogenation with a Carbon-Nitride-Supported Palladium Single-Atom Photocatalyst and Water as a Proton Source
      Zhao, En; Li, Manman; Xu, Beibei; Wang, Xue-Lu; Jing, Yu; Ma, Ding; Mitchell, Sharon; Perez-Ramirez, Javier; Chen, Zupeng
      Angewandte Chemie, International Edition (2022), 61 (40), e202207410CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Solar-driven transfer hydrogenation of unsatd. bonds has received considerable attention in the research area of sustainable org. synthesis; however, water, the ultimate green source of hydrogen, has rarely been investigated due to the high barrier assocd. with splitting of water mols. We report a carbon-nitride-supported palladium single-atom heterogeneous catalyst with unparalleled performance in photocatalytic water-donating transfer hydrogenation compared to its nanoparticle counterparts. Isotopic-labeling expts. and operando NMR measurements confirm the direct hydrogenation mechanism using in situ-generated protons from water splitting under visible-light irradn. D. functional theory calcns. attribute the high activity to lower barriers for hydrogenation, facilitated desorption of ethylbenzene, and facile hydrogen replenishment from water on the at. palladium sites.
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    15. 15
      Han, C.; Du, L.; Konarova, M.; Qi, D.-C.; Phillips, D. L.; Xu, J. Beyond Hydrogen Evolution: Solar-Driven, Water-Donating Transfer Hydrogenation over Platinum/Carbon Nitride. ACS Catal. 2020, 10 (16), 9227– 9235,  DOI: 10.1021/acscatal.0c01932
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      15
      Beyond Hydrogen Evolution: Solar-Driven, Water-Donating Transfer Hydrogenation over Platinum/Carbon Nitride
      Han, Chenhui; Du, Lili; Konarova, Muxina; Qi, Dong-Chen; Phillips, David Lee; Xu, Jingsan
      ACS Catalysis (2020), 10 (16), 9227-9235CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Hydrogen-rich org. mols. such as alcs. are widely used as hydrogen donors in transfer hydrogenation. Nevertheless, water as a more abundant and ecofriendly hydrogen source has hardly been used due to the high difficulty in splitting water mols. Herein, we designed a photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was extd. from water under light illumination and then in situ added to different unsatd. bonds (C=C, C=O, N=O) for chem. synthesis. Platinum-loaded carbon nitride (Pt/CN) was used as the model catalyst for this cascade reaction, which is beyond its normal applications for water splitting. This approach was highly accessible to efficiency optimization, either by modifying CN for extended light absorption and enhanced charge transfer or by alloying Pt with another metal for better catalytic activities. Remarkably, a quantum efficiency of up to 21.8% was achieved for nitrobenzene hydrogenation under 380 nm irradn., which is 3 times higher than that obtained in the single water-splitting reaction, indicating that the PWDTH can be more rewarding than hydrogen evolution for solar energy harvesting. Deep insights into the underlying mechanism were provided by detailed measurements and interpretations of femtosecond transient absorption spectra, action spectra (quantum efficiency as a function of excitation wavelength), and reaction kinetic profiles under varied conditions including the variation of light intensities, temps., and water isotopes. The mild reaction conditions, simple processing, and broad substituent group tolerance endow this approach with a high potential toward a general solar-to-chem. conversion technique.
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    16. 16
      Zhang, D.; Ren, P.; Liu, W.; Li, Y.; Salli, S.; Han, F.; Qiao, W.; Liu, Y.; Fan, Y.; Cui, Y.; Shen, Y.; Richards, E.; Wen, X.; Rummeli, M. H.; Li, Y.; Besenbacher, F.; Niemantsverdriet, H.; Lim, T.; Su, R. Photocatalytic Abstraction of Hydrogen Atoms from Water Using Hydroxylated Graphitic Carbon Nitride for Hydrogenative Coupling Reactions. Angew. Chem., Int. Ed. 2022, 61 (24), e202204256  DOI: 10.1002/anie.202204256
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      16
      Photocatalytic Abstraction of Hydrogen Atoms from Water Using Hydroxylated Graphitic Carbon Nitride for Hydrogenative Coupling Reactions
      Zhang, Dongsheng; Ren, Pengju; Liu, Wuwen; Li, Yaru; Salli, Sofia; Han, Feiyu; Qiao, Wei; Liu, Yu; Fan, Yingzhu; Cui, Yi; Shen, Yanbin; Richards, Emma; Wen, Xiaodong; Rummeli, Mark H.; Li, Yongwang; Besenbacher, Flemming; Niemantsverdriet, Hans; Lim, Tingbin; Su, Ren
      Angewandte Chemie, International Edition (2022), 61 (24), e202204256CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Employing pure water, the ultimate green source of hydrogen donor to initiate chem. reactions that involve a hydrogen atom transfer (HAT) step is fascinating but challenging due to its large H-O bond dissocn. energy (BDEH-O=5.1 eV). Many approaches have been explored to stimulate water for hydrogenative reactions, but the efficiency and productivity still require significant enhancement. Here, we show that the surface hydroxylated graphitic carbon nitride (gCN-OH) only requires 2.25 eV to activate H-O bonds in water, enabling abstraction of hydrogen atoms via dehydrogenation of pure water into hydrogen peroxide under visible light irradn. The gCN-OH presents a stable catalytic performance for hydrogenative N-N coupling, pinacol-type coupling and dehalogenative C-C coupling, all with high yield and efficiency, even under solar radiation, featuring extensive impacts in using renewable energy for a cleaner process in dye, electronic, and pharmaceutical industries.
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    17. 17
      Cummings, S. P.; Le, T.-N.; Fernandez, G. E.; Quiambao, L. G.; Stokes, B. J. Tetrahydroxydiboron-Mediated Palladium-Catalyzed Transfer Hydrogenation and Deuteriation of Alkenes and Alkynes Using Water as the Stoichiometric H or D Atom Donor. J. Am. Chem. Soc. 2016, 138 (19), 6107– 6110,  DOI: 10.1021/jacs.6b02132
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      17
      Tetrahydroxydiboron-Mediated Palladium-Catalyzed Transfer Hydrogenation and Deuteriation of Alkenes and Alkynes Using Water as the Stoichiometric H or D Atom Donor
      Cummings, Steven P.; Le, Thanh-Ngoc; Fernandez, Gilberto E.; Quiambao, Lorenzo G.; Stokes, Benjamin J.
      Journal of the American Chemical Society (2016), 138 (19), 6107-6110CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      There are few examples of catalytic transfer hydrogenations of simple alkenes and alkynes that use water as a stoichiometric H or D atom donor. We have found that diboron reagents efficiently mediate the transfer of H or D atoms from water directly onto unsatd. C-C bonds using a palladium catalyst. This reaction is conducted on a broad variety of alkenes and alkynes at ambient temp., and boric acid is the sole byproduct. Mechanistic expts. suggest that this reaction is made possible by a hydrogen atom transfer from water that generates a Pd-hydride intermediate. Importantly, complete deuterium incorporation from stoichiometric D2O has also been achieved.
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    18. 18
      Sharma, P.; Sasson, Y. Sustainable Visible Light Assisted In-Situ Hydrogenation via a Magnesium-Water System Catalyzed by a Pd-g-C3N4 Photocatalyst. Green Chem. 2019, 21 (2), 261– 268,  DOI: 10.1039/C8GC02221F
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      18
      Sustainable visible light assisted in situ hydrogenation via a magnesium-water system catalyzed by a Pd-g-C3N4 photocatalyst
      Sharma, Priti; Sasson, Yoel
      Green Chemistry (2019), 21 (2), 261-268CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      A non-hazardous and relatively mild protocol was formulated for an effectual hydrogen generation process via a "magnesium-activated water" system with a Pd-g-C3N4 photocatalyst under visible light at room temp. Water functions photochem. as a hydrogen donor without any external source with the Pd-g-C3N4 photocatalyst. The synthesized Pd-g-C3N4 photocatalyst is highly efficient under visible light for the selective redn. of a wide range of unsatd. derivs. and nitro compds. to afford excellent yields (>99%). The photocatalyst Pd-g-C3N4 could be easily recovered and reused for several runs without any deactivation during the photochem. hydrogen transfer reaction process.
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    19. 19
      Zhao, C.-Q.; Chen, Y.-G.; Qiu, H.; Wei, L.; Fang, P.; Mei, T.-S. Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed Semihydrogenation of Alkynes. Org. Lett. 2019, 21 (5), 1412– 1416,  DOI: 10.1021/acs.orglett.9b00148
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      19
      Water as a Hydrogenating Agent: Stereodivergent Pd-Catalyzed Semihydrogenation of Alkynes
      Zhao, Chuan-Qi; Chen, Yue-Gang; Qiu, Hui; Wei, Lei; Fang, Ping; Mei, Tian-Sheng
      Organic Letters (2019), 21 (5), 1412-1416CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
      Palladium-catalyzed transfer semihydrogenation of alkynes using H2O as the hydrogen source and Mn as the reducing reagent is developed, affording cis- and trans-alkenes selectively under mild conditions. In addn., this method provides an efficient way to access various cis-1,2-dideuterioalkenes and trans-1,2-dideuterioalkenes by using D2O instead of H2O.
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    20. 20
      Wang, Q.; Domen, K. Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies. Chem. Rev. 2020, 120 (2), 919– 985,  DOI: 10.1021/acs.chemrev.9b00201
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      20
      Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies
      Wang, Qian; Domen, Kazunari
      Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 919-985CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps, absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, charge sepn. and migration of these photoexcited carriers, and surface chem. reactions based on these carriers. In this review, the focus is on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chem. and materials science to design and prep. photocatalysts for overall water splitting.
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    21. 21
      Kumar, A.; Bhardwaj, R.; Mandal, S. K.; Choudhury, J. Transfer Hydrogenation of CO2 and CO2 Derivatives using Alcohols as Hydride Sources: Boosting an H2-Free Alternative Strategy. ACS Catal. 2022, 12 (15), 8886– 8903,  DOI: 10.1021/acscatal.2c01982
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      21
      Transfer Hydrogenation of CO2 and CO2 Derivatives using Alcohols as Hydride Sources: Boosting an H2-Free Alternative Strategy
      Kumar, Abhishek; Bhardwaj, Ritu; Mandal, Sanajit Kumar; Choudhury, Joyanta
      ACS Catalysis (2022), 12 (15), 8886-8903CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Review. Numerous strategies have been developed for the redn. of highly challenging CO2 gas and its conversion into useful feedstock chems. Among all of the developed protocols, the traditional approach where H2 gas is used as a reductant has been dominantly exploited. During the past decade, enormous efforts have been made in tackling the challenge by keeping sustainability as a major goal. As an alternative option, the adoption of a "transfer hydrogenation" strategy has received attention for the CO2 redn. process. The utilization of biomass-derived alcs. as hydride donors promises to make the process viable and advantageous over the hydrogenation process. The survival of homogeneous transition-metal-based catalysts used in these processes under the harsh reaction conditions (elevated temp. and highly basic reaction medium) is a considerable challenge. Hence, the development of efficient and robust homogeneous catalysts for the CO2-transfer hydrogenation process is highly important. In this Perspective, we highlight the overall evolution of the transfer hydrogenation strategy for the redn. of CO2 gas (and its derivs.) to hydrogen-rich useful products achieved during the past decade. The role of tuning the ligand backbone to make the process kinetically more favorable is discussed in detail. The available reports in the field emphasized the advantages of using biomass-derived alcs. as hydride donors in place of nonrenewable H2 gas. Potential benefits and opportunities of the CO2-transfer hydrogenation process over the traditional hydrogenation are critically presented to encourage further intense research in the field.
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    22. 22
      Nie, R.; Tao, Y.; Nie, Y.; Lu, T.; Wang, J.; Zhang, Y.; Lu, X.; Xu, C. C. Recent Advances in Catalytic Transfer Hydrogenation with Formic Acid over Heterogeneous Transition Metal Catalysts. ACS Catal. 2021, 11 (3), 1071– 1095,  DOI: 10.1021/acscatal.0c04939
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      22
      Recent Advances in Catalytic Transfer Hydrogenation with Formic Acid over Heterogeneous Transition Metal Catalysts
      Nie, Renfeng; Tao, Yuewen; Nie, Yunqing; Lu, Tianliang; Wang, Jianshe; Zhang, Yongsheng; Lu, Xiuyang; Xu, Chunbao Charles
      ACS Catalysis (2021), 11 (3), 1071-1095CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. Recent progress in catalytic transfer hydrogenation (CTH) via heterogeneous hydrogen transfer from FA were summarized. Transformations of biomass-derived platform chems. (e.g., arom. units, C5 and C6 sugars, furans, glycerol, fatty acids, levulinic acid (LA)), nitrogen-contg. compds. (e.g., nitroarenes, quinolines), and organochlorinated compds. via transfer hydrogenation, hydrogenolysis, and hydrodechlorination (HDC) were outlined. Synthesis strategies of the heterogeneous metal catalysts (e.g., metal and support type, metal-support interaction, single-atom, alloy effect, and confinement effect) and optimization of the reaction conditions (e.g., temp., solvents, additives, and FA dosages) for enhancing the catalytic activity and regulating the product distribution were presented. Structure-activity relationships based on both dehydrogenation and hydrogenation of metal catalysts as well as the mechanistic interpretation of CTH with FA were also highlighted. Finally, current challenges and outlook for the future development of the field were discussed.
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    23. 23
      Lau, S.; Gasperini, D.; Webster, R. L. Amine-Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective. Angew. Chem., Int. Ed. 2021, 60 (26), 14272– 14294,  DOI: 10.1002/anie.202010835
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      23
      Amine-Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective
      Lau, Samantha; Gasperini, Danila; Webster, Ruth L.
      Angewandte Chemie, International Edition (2021), 60 (26), 14272-14294CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Transfer hydrogenation (TH) has historically been dominated by Meerwein-Ponndorf-Verley (MPV) reactions. However, with growing interest in amine-boranes, not least ammonia-borane (H3N·BH3), as potential hydrogen storage materials, these compds. have also started to emerge as an alternative reagent in TH reactions. In this Review we discuss TH chem. using H3N·BH3 and their analogs (amine-boranes and metal amidoboranes) as sacrificial hydrogen donors. Three distinct pathways were considered: (1) classical TH, (2) nonclassical TH, and (3) hydrogenation. Simple exptl. mechanistic probes can be employed to distinguish which pathway is operating and computational anal. can corroborate or discount mechanisms. We find that the pathway in operation can be perturbed by changing the temp., solvent, amine-borane, or even the substrate used in the system, and subsequently assignment of the mechanism can become nontrivial.
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    24. 24
      Wang, Y.; Suzuki, H.; Xie, J.; Tomita, O.; Martin, D. J.; Higashi, M.; Kong, D.; Abe, R.; Tang, J. Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems. Chem. Rev. 2018, 118 (10), 5201– 5241,  DOI: 10.1021/acs.chemrev.7b00286
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      24
      Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems
      Wang, Yiou; Suzuki, Hajime; Xie, Jijia; Tomita, Osamu; Martin, David James; Higashi, Masanobu; Kong, Dan; Abe, Ryu; Tang, Junwang
      Chemical Reviews (Washington, DC, United States) (2018), 118 (10), 5201-5241CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review is given. Visible light-driven water splitting using cheap and robust photocatalysts is one of the most exciting ways to produce clean and renewable energy for future generations. Cutting edge research within the field focuses on so-called Z-scheme systems, which are inspired by the photosystem II-photosystem I (PSII/PSI) coupling from natural photosynthesis. A Z-scheme system comprises 2 photocatalysts and generates 2 sets of charge carriers, splitting water into its constituent parts, H and O, at sep. locations. This is not only more efficient than using a single photocatalyst, but practically it could also be safer. Researchers within the field are constantly aiming to bring systems toward industrial level efficiencies by maximizing light absorption of the materials, engineering more stable redox couples, and also searching for new hydrogen and oxygen evolution cocatalysts. This review provides an in-depth survey of relevant Z-schemes from past to present, with particular focus on mechanistic breakthroughs, and highlights current state of the art systems which are at the forefront of the field.
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    25. 25
      Marzo, L.; Pagire, S. K.; Reiser, O.; Konig, B. Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?. Angew. Chem., Int. Ed. 2018, 57 (32), 10034– 10072,  DOI: 10.1002/anie.201709766
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      25
      Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?
      Marzo, Leyre; Pagire, Santosh K.; Reiser, Oliver; Koenig, Burkhard
      Angewandte Chemie, International Edition (2018), 57 (32), 10034-10072CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review on visible-light photocatalysis has evolved over the last decade into a widely used method in org. synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloaddns., ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temp., and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in org. synthesis. The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chem. and to open up new avenues by accessing reactive species hitherto unknown, esp. by merging photocatalysis with organo- or metal catalysis.
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    26. 26
      Zahran, E. M.; Bedford, N. M.; Nguyen, M. A.; Chang, Y. J.; Guiton, B. S.; Naik, R. R.; Bachas, L. G.; Knecht, M. R. Light-Activated Tandem Catalysis Driven by Multicomponent Nanomaterials. J. Am. Chem. Soc. 2014, 136 (1), 32– 35,  DOI: 10.1021/ja410465s
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      26
      Light-Activated Tandem Catalysis Driven by Multicomponent Nanomaterials
      Zahran, Elsayed M.; Bedford, Nicholas M.; Nguyen, Michelle A.; Chang, Yao-Jen; Guiton, Beth S.; Naik, Rajesh R.; Bachas, Leonidas G.; Knecht, Marc R.
      Journal of the American Chemical Society (2014), 136 (1), 32-35CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Transitioning energy-intensive and environmentally intensive processes toward sustainable conditions is necessary in light of the current global condition. To this end, photocatalytic processes represent new approaches for H2 generation; however, their application toward tandem catalytic reactivity remains challenging. Here, we demonstrate that metal oxide materials decorated with noble metal nanoparticles advance visible light photocatalytic activity toward new reactions not typically driven by light. For this, Pd nanoparticles were deposited onto Cu2O cubes to generate a composite structure. Once characterized, their hydrodehalogenation activity was studied via the reductive dechlorination of polychlorinated biphenyls. To this end, tandem catalytic reactivity was obsd. with H2 generation via H2O redn. at the Cu2O surface, followed by dehalogenation at the Pd using the in situ generated H2. Such results present methods to achieve sustainable catalytic technologies by advancing photocatalytic approaches toward new reaction systems.
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      Fan, X.; Yao, Y.; Xu, Y.; Yu, L.; Qiu, C. Visible-Light-Driven Photocatalytic Hydrogenation of Olefins Using Water as the H Source. ChemCatChem. 2019, 11 (11), 2596– 2599,  DOI: 10.1002/cctc.201900262
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      27
      Visible-light-driven photocatalytic hydrogenation of olefins using water as the H source
      Fan, Xin; Yao, Yanling; Xu, Yangsen; Yu, Lei; Qiu, Chuntian
      ChemCatChem (2019), 11 (11), 2596-2599CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this work, a highly efficient PCN-KCl (KCl-modified polymeric carbon nitride) nanosheet photocatalyst was synthesized with the assistance of KCl. The as-prepd. PCN-KCl catalyst shows a more than 30-fold enhancement in the photocatalytic activity for H2 evolution from water compared to the pristine PCN. More importantly, when PCN-KCl was composited with a second catalyst (Pd nanoparticles), the simultaneous prodn. and utilization of active H species for alkenes hydrogenation was achieved by visible light irradn. under ambient conditions.
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      Xu, Y.; Qiu, C.; Fan, X.; Xiao, Y.; Zhang, G.; Yu, K.; Ju, H.; Ling, X.; Zhu, Y.; Su, C. K+-Induced Crystallization of Polymeric Carbon Nitride to Boost Its Photocatalytic Activity for H2 Evolution and Hydrogenation of Alkenes. Appl. Catal., B 2020, 268, 118457,  DOI: 10.1016/j.apcatb.2019.118457
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      K+-induced crystallization of polymeric carbon nitride to boost its photocatalytic activity for H2 evolution and hydrogenation of alkenes
      Xu, Yangsen; Qiu, Chuntian; Fan, Xin; Xiao, Yonghao; Zhang, Guoqiang; Yu, Kunyi; Ju, Huanxin; Ling, Xiang; Zhu, Yongfa; Su, Chenliang
      Applied Catalysis, B: Environmental (2020), 268 (), 118457CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Cryst. semiconductors with ordered long-range structure and minimized phase defect are capable of efficient sepn. and diffusion of photoexcited charge carriers, which is crucial for achieving high photocatalytic performances. Here, a new strategy is presented using KCl as structure inducer, where potassium ions (K+) act as a smart "binder" for re-ordering the structure of amorphous polymer carbon nitride (PCN) to furnish K+ implanted cryst. PCN (KPCN). The XPS depth profiling with Ar+ cluster ion sputtering illustrated that the element K is uniformly distributed in bulk of KPCN. The microstructure evolution of KPCN under elevated temp. was identified using in situ Fourier-transform IR spectroscopy. This cryst. structure endows the ordered electronic transmission channels in KPCN, thus enhanced the efficiency of hot charge carriers sepn. and migration, as well as visible light capture. Therefore, the re-ordered KPCN displays nearly 20 times enhancement toward photocatalytic hydrogen evolution, and high activity in water-splitting-based alkenes hydrogenation using the in-situ photo-generated H-species from water as sustainable H-source. The present work highlights a green and reliable strategy to remodel the structure of PCN by K+ thus dramatically boosting the photocatalytic activity for hydrogen evolution as well as water-splitting-based photosynthesis of high value-added fine chems.
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      Qiu, C.; Xu, Y.; Fan, X.; Xu, D.; Tandiana, R.; Ling, X.; Jiang, Y.; Liu, C.; Yu, L.; Chen, W.; Su, C. Highly Crystalline K-Intercalated Polymeric Carbon Nitride for Visible-Light Photocatalytic Alkenes and Alkynes Deuterations. Adv. Sci. 2019, 6 (1), 1801403,  DOI: 10.1002/advs.201801403
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      Highly Crystalline K-Intercalated Polymeric Carbon Nitride for Visible-Light Photocatalytic Alkenes and Alkynes Deuterations
      Qiu Chuntian; Xu Yangsen; Fan Xin; Tandiana Rika; Ling Xiang; Jiang Yanan; Liu Cuibo; Su Chenliang; Qiu Chuntian; Su Chenliang; Fan Xin; Yu Lei; Xu Dong; Chen Wei
      Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2019), 6 (1), 1801403 ISSN:2198-3844.
      In addition to the significance of photocatalytic hydrogen evolution, the utilization of the in situ generated H/D (deuterium) active species from water splitting for artificial photosynthesis of high value-added chemicals is very attractive and promising. Herein, photocatalytic water splitting technology is utilized to generate D-active species (i.e., Dad) that can be stabilized on anchored 2nd metal catalyst and are readily for tandem controllable deuterations of carbon-carbon multibonds to produce high value-added D-labeled chemicals/pharmaceuticals. A highly crystalline K cations intercalated polymeric carbon nitride (KPCN), rationally designed, and fabricated by a solid-template induced growth, is served as an ultraefficient photocatalyst, which shows a greater than 18-fold enhancement in the photocatalytic hydrogen evolution over the bulk PCN. The photocatalytic in situ generated D-species by superior KPCN are utilized for selective deuteration of a variety of alkenes and alkynes by anchored 2nd catalyst, Pd nanoparticles, to produce the corresponding D-labeled chemicals and pharmaceuticals with high yields and D-incorporation. This work highlights the great potential of developing photocatalytic water splitting technology for artificial photosynthesis of value-added chemicals instead of H2 evolution.
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      Li, M.; Zhang, N.; Long, R.; Ye, W.; Wang, C.; Xiong, Y. PdPt Alloy Nanocatalysts Supported on TiO2: Maneuvering Metal-Hydrogen Interactions for Light-Driven and Water-Donating Selective Alkyne Semihydrogenation. Small 2017, 13 (23), 1604173,  DOI: 10.1002/smll.201604173
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      Yao, F.; Dai, L.; Bi, J.; Xue, W.; Deng, J.; Fang, C.; Zhang, L.; Zhao, H.; Zhang, W.; Xiong, P.; Fu, Y.; Sun, J.; Zhu, J. Loofah-like Carbon Nitride Sponge towards the Highly-Efficient Photocatalytic Transfer Hydrogenation of Nitrophenols with Water as the Hydrogen Source. Chem. Eng. J. 2022, 444, 136430,  DOI: 10.1016/j.cej.2022.136430
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      Loofah-like carbon nitride sponge towards the highly-efficient photocatalytic transfer hydrogenation of nitrophenols with water as the hydrogen source
      Yao, Fanglei; Dai, Liming; Bi, Jiabao; Xue, Wenkang; Deng, Jingyao; Fang, Chenchen; Zhang, Litong; Zhao, Hongan; Zhang, Wenyao; Xiong, Pan; Fu, Yongsheng; Sun, Jingwen; Zhu, Junwu
      Chemical Engineering Journal (Amsterdam, Netherlands) (2022), 444 (), 136430CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      The vigorous development of photocatalytic water splitting technol. has laid the foundation for the photocatalytic transfer hydrogenation of org. substrates to produce the high value-added chems. using water as hydrogen source. Nevertheless, the high dissocn. energy of the O-H bond impedes its academic progress and the practical applications. Herein, we synthesize a 3D hierarchical porous loofah-like carbon nitride sponge (LCN) with ultrathin thickness via the supramol. pre-organization coupling with the oxidn. etching process, in which the heterogeneous oxygen atoms and the nitrogen vacancies are in-situ engineered. On top of the adorable photocatalytic H2 evolution (4812μmol h-1 g-1), LCN assocd. with Pt cocatalyst reveals a conversion rate of 96.5% towards the hydrogenation of 4-nitrophenol, substantially superior to the ref. expt. (8.3%). Further based on the isotope-labeling tests and the d. functional theory calcns., the photo-generated H0 from water is clarified to be the direct reducing agent, tactfully skipping the hydrogen extn. step in the traditional path. This work provides a green and sustainable methodol. to transfer the solar energy to the valuable fine chems., as well as highlights the importance of the 3D hierarchical porous structure to the catalytic activity.
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      Xu, F.; Meng, K.; Cao, S.; Jiang, C.; Chen, T.; Xu, J.; Yu, J. Step-by-Step Mechanism Insights into the TiO2/Ce2S3 S-Scheme Photocatalyst for Enhanced Aniline Production with Water as a Proton Source. ACS Catal. 2022, 12 (1), 164– 172,  DOI: 10.1021/acscatal.1c04903
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      Step-by-Step Mechanism Insights into the TiO2/Ce2S3 S-Scheme Photocatalyst for Enhanced Aniline Production with Water as a Proton Source
      Xu, Feiyan; Meng, Kai; Cao, Shuang; Jiang, Chenhui; Chen, Tao; Xu, Jingsan; Yu, Jiaguo
      ACS Catalysis (2022), 12 (1), 164-172CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Exploring heterostructured photocatalysts for the photocatalytic hydrogenation reaction with water as a proton source and investigating the corresponding intrinsic step-by-step mechanism are of great interest. Here, we develop an S-scheme heterojunction through theor. design and carried out solvothermal growth of Ce2S3 nanoparticles (NPs) onto electrospun TiO2 nanofibers. The low-dimensional (0D/1D) heterostructure unveils enhanced photocatalytic activity for aniline prodn. by nitrobenzene hydrogenation with water as a proton source. D. functional theory (DFT) calcns. indicate the electrons transfer from Ce2S3 to TiO2 upon hybridization due to their Fermi level difference and creates an internal elec. field at the interface, driving the sepn. of the photoexcited charge carriers, which is authenticated by in situ XPS along with femtosecond transient absorption spectroscopy. The step-by-step reaction mechanism of the photocatalytic nitrobenzene hydrogenation to yield aniline is revealed by in situ diffuse reflectance IR Fourier transform spectroscopy, assocd. with DFT computational prediction.
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      Jia, T.; Meng, D.; Duan, R.; Ji, H.; Sheng, H.; Chen, C.; Li, J.; Song, W.; Zhao, J. Single-Atom Nickel on Carbon Nitride Photocatalyst Achieves Semihydrogenation of Alkynes with Water Protons via Monovalent Nickel. Angew. Chem., Int. Ed. 2023, 62 (9), e202216511  DOI: 10.1002/anie.202216511
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      Single-Atom Nickel on Carbon Nitride Photocatalyst Achieves Semihydrogenation of Alkynes with Water Protons via Monovalent Nickel
      Jia, Tongtong; Meng, Di; Duan, Ran; Ji, Hongwei; Sheng, Hua; Chen, Chuncheng; Li, Jikun; Song, Wenjing; Zhao, Jincai
      Angewandte Chemie, International Edition (2023), 62 (9), e202216511CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Prospects in light-driven water activation have prompted rapid progress in hydrogenation reactions. A Ni2+-N4 site built on carbon nitride for catalyzed semihydrogenation of alkynes RCCR1 (R = 2-bromophenyl, cyclohexyl, thiophen-2-yl, etc.; R1 = H, Ph, thiophen-2-yl, etc.), with water supplying protons, powered by visible-light irradn. was described. Importantly, the photocatalytic approach developed here enabled access to diverse deuterated alkenes RC(D)=C(D)R2 (R2 = H, D) in D2O with excellent deuterium incorporation. Under visible-light irradn., evolution of a four-coordinate Ni2+ species into a three-coordinate Ni+ species was spectroscopically identified. In combination with theor. calcns., the photo-evolved Ni+ is posited as HO-Ni+-N2 with an uncoordinated, protonated pyridine nitrogen, formed by coupled Ni2+ redn. and water dissocn. The paired Ni-N prompts hydrogen liberation from water, and it renders desorption of alkene preferred over further hydrogenation to alkane, ensuring excellent semihydrogenation selectivity.
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      Guo, Y.; An, W.; Tian, X.; Xie, L.; Ren, Y.-L. Coupling Photocatalytic overall Water Splitting with Hydrogenation of Organic Molecules: a Strategy for Using Water as a Hydrogen Source and an Electron Donor to Enable Hydrogenation. Green Chem. 2022, 24 (23), 9211– 9219,  DOI: 10.1039/D2GC02427F
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      Coupling photocatalytic overall water splitting with hydrogenation of organic molecules: a strategy for using water as a hydrogen source and an electron donor to enable hydrogenation
      Guo, Yinggang; An, Wankai; Tian, Xinzhe; Xie, Lixia; Ren, Yun-Lai
      Green Chemistry (2022), 24 (23), 9211-9219CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      It is fascinating to use water as a hydrogen source to enable the hydrogenation of org. mols. in green chem. Nevertheless, current light-driven strategies suffer from an expense of reductants for proton redn. due to a high difficulty in overall water splitting. Herein, we have overcome this challenge and report that light-induced overall water splitting is coupled with hydrogenation of aryl bromides by cooperative catalysis between recyclable Pd/g-C3N4 and Fe species, which opens up a photocatalytic avenue to use water as both an electron donor and a hydrogen source to enable hydrogenation of aryl bromides, avoiding the use of addnl. reductants. Moreover, mild conditions, recyclable catalyst systems and the use of visible-light as the energy source make this process greener. The present method also allowed various high value-added deuterated arenes to be effectively synthesized. This work will guide chemists to use water as both an electron donor and a hydrogen source to develop green procedures for the hydrogenation of various org. compds.
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      Chmiel, A. F.; Williams, O. P.; Chernowsky, C. P.; Yeung, C. S.; Wickens, Z. K. Non-Innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides. J. Am. Chem. Soc. 2021, 143 (29), 10882– 10889,  DOI: 10.1021/jacs.1c05988
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      35
      Non-innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides
      Chmiel, Alyah F.; Williams, Oliver P.; Chernowsky, Colleen P.; Yeung, Charles S.; Wickens, Zachary K.
      Journal of the American Chemical Society (2021), 143 (29), 10882-10889CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We describe a photocatalytic system that elicits potent photoreductant activity from conventional photocatalysts by leveraging radical anion intermediates generated in situ. The combination of an isophthalonitrile photocatalyst and sodium formate promotes diverse aryl radical coupling reactions from abundant but difficult to reduce aryl chloride substrates. Mechanistic studies reveal two parallel pathways for substrate redn. both enabled by a key terminal reductant byproduct, carbon dioxide radical anion.
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      Zhang, X.; Chen, J.; Gao, Y.; Li, K.; Zhou, Y.; Sun, W.; Fan, B. Photocatalyzed Transfer Hydrogenation and Deuteriation of Cyclic N-Sulfonylimines. Org. Chem. Front. 2019, 6 (14), 2410– 2414,  DOI: 10.1039/C9QO00231F
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      Photocatalyzed transfer hydrogenation and deuteration of cyclic N-sulfonylimines
      Zhang, Xuexin; Chen, Jingchao; Gao, Yang; Li, Kangkui; Zhou, Yongyun; Sun, Weiqing; Fan, Baomin
      Organic Chemistry Frontiers (2019), 6 (14), 2410-2414CODEN: OCFRA8; ISSN:2052-4129. (Royal Society of Chemistry)
      The photocatalyzed transfer hydrogenation/deuteration of cyclic N-sulfonylimines was accomplished using water/deuterium oxide as hydrogen/deuterium sources to afford cyclic sultams/deuterium-labeled cyclic sultams I [R = Ph, 2-MeOC6H4, 4-FC6H4, etc.; X = H, D] in mild reaction conditions.
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    37. 37
      Dai, P.; Ma, J.; Huang, W.; Chen, W.; Wu, N.; Wu, S.; Li, Y.; Cheng, X.; Tan, R. Photoredox C-F Quaternary Annulation Catalyzed by a Strongly Reducing Iridium Species. ACS Catal. 2018, 8 (2), 802– 806,  DOI: 10.1021/acscatal.7b03089
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      Photoredox C-F Quaternary Annulation Catalyzed by a Strongly Reducing Iridium Species
      Dai, Peng; Ma, Junyu; Huang, Wenhao; Chen, Wenxin; Wu, Na; Wu, Shengfu; Li, Ying; Cheng, Xu; Tan, Renxiang
      ACS Catalysis (2018), 8 (2), 802-806CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Authors report a fac-Ir(ppy)3*-IrII-IrIII photocatalytic cycle involving t-BuOK as the terminal reductant in a visible-light-induced sp2 C-F quaternary annulation reaction that proceeds in yields up to 98%. Because of the high activity of the IrII(ppy)3 catalyst, even at a loading of 50 ppm, the annulation reaction was able to compete with an uncatalyzed nucleophilic arom. substitution reaction. The annulation reaction was stereoconvergent, and an annulated product was synthesized with complete retention of enantiomeric excess.
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      Cuerva, J. M.; Campana, A. G.; Justicia, J.; Rosales, A.; Oller-Lopez, J. L.; Robles, R.; Cardenas, D. J.; Bunuel, E.; Oltra, J. E. Water: the Ideal Hydrogen-Atom Source in Free-Radical Chemistry Mediated by Ti(III) and Other Single-Electron-Transfer Metals?. Angew. Chem., Int. Ed. 2006, 45 (33), 5522– 5526,  DOI: 10.1002/anie.200600831
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      Water: the ideal hydrogen-atom source in free-radical chemistry mediated by TiIII and other single-electron-transfer metals?
      Cuerva, Juan M.; Campana, Araceli G.; Justicia, Jose; Rosales, Antonio; Oller-Lopez, Juan L.; Robles, Rafael; Cardenas, Diego J.; Bunuel, Elena; Oltra, J. Enrique
      Angewandte Chemie, International Edition (2006), 45 (33), 5522-5526CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Hydrogen-atom transfer from water to free radicals can be mediated by aqua complexes of titanium(III). Asym. epoxidn. in combination with [Cp2TiCl]/H2O-mediated reductive epoxide opening can be viewed as an alternative with complementary stereoselectivity to the hydroboration-epoxidn. method for the enantioselective synthesis of anti-Markovnikov alcs. from alkenes.
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      Call, A.; Casadevall, C.; Acuna-Pares, F.; Casitas, A.; Lloret-Fillol, J. Dual Cobalt-Copper Light-Driven Catalytic Reduction of Aldehydes and Aromatic Ketones in Aqueous Media. Chem. Sci. 2017, 8 (7), 4739– 4749,  DOI: 10.1039/C7SC01276D
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      Dual cobalt-copper light-driven catalytic reduction of aldehydes and aromatic ketones in aqueous media
      Call, Arnau; Casadevall, Carla; Acuna-Pares, Ferran; Casitas, Alicia; Lloret-Fillol, Julio
      Chemical Science (2017), 8 (7), 4739-4749CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      An efficient, general, fast, and robust light-driven methodol. based on earth-abundant elements to reduce aryl ketones, and both aryl and aliph. aldehydes (up to 1400 TON) has been presented. The catalytic system consists of a robust and well-defined aminopyridyl cobalt complex active for photocatalytic water redn. and the [Cu(bathocuproine)(Xantphos)](PF6) photoredox catalyst. The dual cobalt-copper system uses visible light as the driving-force and H2O and an electron donor (Et3N or iPr2EtN) as the hydride source. The catalytic system operates in aq. mixts. (80-60% water) with high selectivity towards the redn. of org. substrates (>2000) vs. water redn., and tolerates O2. Remarkably, the catalytic system also shows unique selectivity for the redn. of acetophenone in the presence of aliph. aldehydes. The catalytic system provides a simple and convenient method to obtain α,β-deuterated alcs. Both the obsd. reactivity and the DFT modeling support a common cobalt hydride intermediate. The DFT modeled energy profile for the [Co-H] nucleophilic attack to acetophenone and water rationalizes the competence of [CoII-H] to reduce acetophenone in the presence of water. Mechanistic studies suggest alternative mechanisms depending on the redox potential of the substrate. These results show the potential of the water redn. catalyst to develop light-driven selective org. transformations and fine solar chems.
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    40. 40
      Fischer, S.; Hollmann, D.; Tschierlei, S.; Karnahl, M.; Rockstroh, N.; Barsch, E.; Schwarzbach, P.; Luo, S.-P.; Junge, H.; Beller, M.; Lochbrunner, S.; Ludwig, R.; Brückner, A. Death and Rebirth: Photocatalytic Hydrogen Production by a Self-Organizing Copper-Iron System. ACS Catal. 2014, 4 (6), 1845– 1849,  DOI: 10.1021/cs500387e
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      Death and rebirth: Photocatalytic hydrogen production by a self-organizing copper-iron system
      Fischer, Steffen; Hollmann, Dirk; Tschierlei, Stefanie; Karnahl, Michael; Rockstroh, Nils; Barsch, Enrico; Schwarzbach, Patrick; Luo, Shu-Ping; Junge, Henrik; Beller, Matthias; Lochbrunner, Stefan; Ludwig, Ralf; Brueckner, Angelika
      ACS Catalysis (2014), 4 (6), 1845-1849CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      This study provides detailed mechanistic insights into light-driven hydrogen prodn. using an abundant copper-iron system. It focuses on the role of the heteroleptic copper photosensitizer [Cu(P ̂ P)(N ̂ N)]+, which can be oxidized or reduced after photoexcitation. By means of IR, EPR, and UV/vis spectroscopy as well as computational studies and spectroelectrochem., the possibility of both mechanisms was confirmed. UV/vis spectroscopy revealed the reorganization of the original heteroleptic photosensitizer during catalysis toward a homoleptic [Cu(N ̂ N)2]+ species. Operando FTIR spectroscopy showed the formation of a catalytic diiron intermediate, which resembles well-known hydrogenase active site models.
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      Call, A.; Codola, Z.; Acuna-Pares, F.; Lloret-Fillol, J. Photo- and Electrocatalytic H2 Production by New First-Row Transition-Metal Complexes Based on an Aminopyridine Pentadentate Ligand. Chem. - Eur. J. 2014, 20 (20), 6171– 6183,  DOI: 10.1002/chem.201303317
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      Photo- and Electrocatalytic H2 Production by New First-Row Transition-Metal Complexes Based on an Aminopyridine Pentadentate Ligand
      Call, Arnau; Codola, Zoel; Acuna-Pares, Ferran; Lloret-Fillol, Julio
      Chemistry - A European Journal (2014), 20 (20), 6171-6183CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The synthesis and characterization of the pentadentate ligand 1,4-di(picolyl)-7-(p-toluenesulfonyl)-1,4,7-triazacyclononane (Py2Tstacn) and their metal complexes [M(CF3SO3)(Py2Tstacn)][CF3SO3], (M = Fe (1Fe), Co (1Co) and Ni (1Ni)) are reported. Complex 1Co presents excellent H2 photoprodn. catalytic activity when using [Ir(ppy)2(bpy)]PF6 (PSIr) as photosensitizer (PS) and Et3N as electron donor, but 1Ni and 1Fe result in a low activity and a complete lack of it, resp. However, all three complexes have excellent electrocatalytic proton redn. activity in MeCN, when using HO2CCF3 (TFA) as a proton source with moderate overpotentials for 1Co (0.59 V vs. SCE) and 1Ni (0.56 V vs. SCE) and higher for 1Fe (0.87 V vs. SCE). Under conditions of MeCN/H2O/Et3N (3:7:0.2), 1Co (5 μΜ), with PSIr (100 μΜ) and irradiating at 447 nm gives a turnover no. (TON) of 690 (nH2/n1Co) and initial turnover frequency (TOF) (TON×t-1) of 703 h-1 for H2 prodn. It should be noted that 1Co retains 25% of the catalytic activity for photoprodn. of H2 in the presence of O2. The inexistence of a lag time for H2 evolution and the absence of nanoparticles during the first 30 min of the reaction suggest that the main catalytic activity obsd. is derived from a mol. system. Kinetic studies show that the reaction is -0.7 order in catalyst, and time-dependent diffraction light scattering (DLS) expts. indicate formation of metal aggregates and then nanoparticles, leading to catalyst deactivation. By a combination of exptl. and computational studies the lack of activity in photochem. H2O redn. by 1Fe can be attributed to the 1FeII/I redox couple, which is significantly lower than the PSIrIII/II, while for 1Ni the pKa value (-0.4) is too small in comparison with the pH (11.9) imposed using Et3N as electron donor.
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      Casadevall, C.; Pascual, D.; Aragon, J.; Call, A.; Casitas, A.; Casademont-Reig, I.; Lloret-Fillol, J. Light-Driven Reduction of Aromatic Olefins in Aqueous Media Catalysed by Aminopyridine Cobalt Complexes. Chem. Sci. 2022, 13 (15), 4270– 4282,  DOI: 10.1039/D1SC06608K
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      Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes
      Casadevall, Carla; Pascual, David; Aragon, Jordi; Call, Arnau; Casitas, Alicia; Casademont-Reig, Irene; Lloret-Fillol, Julio
      Chemical Science (2022), 13 (15), 4270-4282CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      A catalytic system based on earth-abundant elements that efficiently hydrogenates aryl olefins using visible light as the driving-force and H2O as the sole hydrogen atom source was reported. The catalytic system involved a robust and well-defined aminopyridine cobalt complex and a heteroleptic Cu photoredox catalyst. The system showed the redn. of styrene in aq. media with a remarkable selectivity (>20000) vs. water redn. (WR). Reactivity and mechanistic studies supported the formation of a [Co-H] intermediate, which reacted with the olefin via a hydrogen atom transfer (HAT). Synthetically useful deuterium-labeled compds. can be straightforwardly obtained by replacing H2O with D2O. Moreover, the dual photocatalytic system and the photocatalytic conditions can be rationally designed to tune the selectivity for aryl olefin vs. aryl ketone redn.; not only by changing the structural and electronic properties of the cobalt catalysts, but also by modifying the redn. properties of the photoredox catalyst.
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      Zhou, B.; Song, J.; Zhou, H.; Wu, T.; Han, B. Using the Hydrogen and Oxygen in Water Directly for Hydrogenation Reactions and Glucose Oxidation by Photocatalysis. Chem. Sci. 2016, 7 (1), 463– 468,  DOI: 10.1039/C5SC03178H
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      Using the hydrogen and oxygen in water directly for hydrogenation reactions and glucose oxidation by photocatalysis
      Zhou, Baowen; Song, Jinliang; Zhou, Huacong; Wu, Tianbin; Han, Buxing
      Chemical Science (2016), 7 (1), 463-468CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Direct utilization of the abundant hydrogen and oxygen in water for org. reactions is very attractive and challenging in chem. Herein, we report the first work on the utilization of the hydrogen in water for the hydrogenation of various org. compds. to form valuable chems. and the oxygen for the oxidn. of glucose, simultaneously by photocatalysis. It was discovered that various unsatd. compds. could be efficiently hydrogenated with high conversion and selectivity by the hydrogen from water splitting and glucose reforming over Pd/TiO2 under UV irradn. (350 nm). At the same time, glucose was oxidated by the hydroxyl radicals from water splitting and the holes caused by UV irradn. to form biomass-derived chems., such as arabinose, erythrose, formic acid, and hydroxyacetic acid. Thus, the hydrogen and oxygen were used ideally. This work presents a new and sustainable strategy for hydrogenation and biomass conversion by using the hydrogen and oxygen in water.
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      Kaiser, S. K.; Chen, Z.; Faust Akl, D.; Mitchell, S.; Pérez-Ramírez, J. Single-Atom Catalysts across the Periodic Table. Chem. Rev. 2020, 120 (21), 11703– 11809,  DOI: 10.1021/acs.chemrev.0c00576
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      Single-Atom Catalysts across the Periodic Table
      Kaiser, Selina K.; Chen, Zupeng; Faust Akl, Dario; Mitchell, Sharon; Perez-Ramirez, Javier
      Chemical Reviews (Washington, DC, United States) (2020), 120 (21), 11703-11809CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Isolated atoms featuring unique reactivity are at the heart of enzymic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, detg. the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of mol. processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and assocd. properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.
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      Shi, Y.; Zhou, Y.; Lou, Y.; Chen, Z.; Xiong, H.; Zhu, Y. Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations. Adv. Sci. 2022, 9 (24), 2201520,  DOI: 10.1002/advs.202201520
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      Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations
      Shi, Yujie; Zhou, Yuwei; Lou, Yang; Chen, Zupeng; Xiong, Haifeng; Zhu, Yongfa
      Advanced Science (Weinheim, Germany) (2022), 9 (24), 2201520CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly detd. by the uniformity of active sites. However, the non-identical no. of metal atoms, geometric shape, and morphol. of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examd. In particular, their unique behavior in boosting the catalytic performance in various chem. transformations is discussed, including selective hydrogenation, selective oxidn., Suzuki coupling, and other catalytic reactions. In addn., the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.
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      Wang, Q.; Domen, K. Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies. Chem. Rev. 2020, 120 (2), 919– 985,  DOI: 10.1021/acs.chemrev.9b00201
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      46
      Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies
      Wang, Qian; Domen, Kazunari
      Chemical Reviews (Washington, DC, United States) (2020), 120 (2), 919-985CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps, absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, charge sepn. and migration of these photoexcited carriers, and surface chem. reactions based on these carriers. In this review, the focus is on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chem. and materials science to design and prep. photocatalysts for overall water splitting.
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      Li, X.; Bi, W.; Zhang, L.; Tao, S.; Chu, W.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y. Single-Atom Pt as Co-Catalyst for Enhanced Photocatalytic H2 Evolution. Adv. Mater. 2016, 28 (12), 2427– 2431,  DOI: 10.1002/adma.201505281
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      Single-Atom Pt as Co-Catalyst for Enhanced Photocatalytic H2 Evolution
      Li, Xiaogang; Bi, Wentuan; Zhang, Lei; Tao, Shi; Chu, Wangsheng; Zhang, Qun; Luo, Yi; Wu, Changzheng; Xie, Yi
      Advanced Materials (Weinheim, Germany) (2016), 28 (12), 2427-2431CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Isolated single Pt atoms as a new form of co-catalyst were developed, by embedding them in sub-nanoporosity in 2D g-C3N4, which maximizes the atom efficiency of the noble metal and represents a new, highly efficient photocatalytic system for H2 evolution. The cooperation of the single-atom co-catalyst in g-C3N4 provides a new strategy to modulate the electronic structure, resulting in a longer lifetime of photogenerated electrons due to the isolated single Pt atoms induced, intrinsic change of the surface trap states. Single-atom Pt co-catalyst eventually leads to tremendously enhanced photocatalytic H2 generation performance, 8.6 times higher than that of Pt nanoparticles on the per Pt atom basis, and nearly 50 times of that for bare g-C3N4 . It is believed that the single-atom co-catalyst strategy will provide a promising way to reduce the high cost of noble metals and pave a new avenue for the development of highly efficient co-catalysts.
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      Dong, P.; Wang, Y.; Zhang, A.; Cheng, T.; Xi, X.; Zhang, J. Platinum Single Atoms Anchored on a Covalent Organic Framework: Boosting Active Sites for Photocatalytic Hydrogen Evolution. ACS Catal. 2021, 11 (21), 13266– 13279,  DOI: 10.1021/acscatal.1c03441
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      Platinum Single Atoms Anchored on a Covalent Organic Framework: Boosting Active Sites for Photocatalytic Hydrogen Evolution
      Dong, Pengyu; Wang, Yan; Zhang, Aicaijun; Cheng, Ting; Xi, Xinguo; Zhang, Jinlong
      ACS Catalysis (2021), 11 (21), 13266-13279CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      It is of great importance to explore and achieve a more effective approach toward the controllable synthesis of single-atom-based photocatalysts with high metal content and long-term durability. Herein, single-atom platinum (Pt) with high loading content anchored on the pore walls of two-dimensional β-ketoenamine-linked covalent org. frameworks (TpPa-1-COF) is presented. Aided by advanced characterization techniques of aberration-cor. high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy, it has been demonstrated that atomically dispersed Pt is formed on the TpPa-1-COF support through a six-coordinated C3N-Pt-Cl2 species. The optimized Pt1@TpPa-1 catalyst exhibits a high photocatalytic H2 evolution rate of 719μmol g-1 h-1 under visible-light irradn., a high actual Pt loading content of 0.72 wt %, and a large turnover frequency (TOF) of 19.5 h-1, with activity equiv. to 3.9 and 48 times higher than those of Pt nanoparticles/TpPa-1 and bare TpPa-1, resp. The improved photocatalytic performance for H2 evolution is ascribed to the effective photogenerated charge sepn. and migration and well-dispersed active sites of single-atom Pt. Moreover, d. functional theory (DFT) calcns. further reveal the role of Pt single atoms in the enhanced photocatalytic activity for H2 evolution. Overall, this work provides some inspiration for designing single-atom-based photocatalysts with outstanding stability and efficiency using COFs as the support.
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      Ling, X.; Xu, Y.; Wu, S.; Liu, M.; Yang, P.; Qiu, C.; Zhang, G.; Zhou, H.; Su, C. A Visible-Light-Photocatalytic Water-Splitting Strategy for Sustainable Hydrogenation/Deuteration of Aryl Chlorides. Sci. China-Chem. 2020, 63 (3), 386– 392,  DOI: 10.1007/s11426-019-9672-8
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      A visible-light-photocatalytic water-splitting strategy for sustainable hydrogenation/deuteration of aryl chlorides
      Ling, Xiang; Xu, Yangsen; Wu, Shaoping; Liu, Mofan; Yang, Peng; Qiu, Chuntian; Zhang, Guoqiang; Zhou, Hongwei; Su, Chenliang
      Science China: Chemistry (2020), 63 (3), 386-392CODEN: SCCCCS; ISSN:1869-1870. (Science China Press)
      Abstr.: Hydrogenation/deuteration of carbon chloride (C-Cl) bonds is of high significance but remains a remarkable challenge in synthetic chem., esp. using safe and inexpensive hydrogen donors. In this article, a visible-light-photocatalytic water splitting hydrogenation technol. (WSHT) is proposed to in-situ generate active H-species (i.e., Had) for controllable hydrogenation of aryl chlorides instead of using flammable H2. When applying heavy water-splitting systems, we could selectively install deuterium at the C-Cl position of aryl chlorides under mild conditions for the sustainable synthesis of high-valued added deuterated chems. Sub-micrometer Pd nanosheets (Pd NSs) decorated crystallined polymeric carbon nitrides (CPCN) is developed as the bifunctional photocatalyst, whereas Pd NSs not only serve as a cocatalyst of CPCN to generate and stabilize H (D)-species but also play a significant role in the sequential activation and hydrogenation/deuteration of C-Cl bonds. This article highlights a photocatalytic-WSHT for controllable hydrogenation/deuteration of low-cost aryl chlorides, providing a promising way for the photosynthesis of high-valued added chems. instead of the hydrogen evolution.
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      Ji, S.; Chen, Y.; Wang, X.; Zhang, Z.; Wang, D.; Li, Y. Chemical Synthesis of Single Atomic Site Catalysts. Chem. Rev. 2020, 120 (21), 11900– 11955,  DOI: 10.1021/acs.chemrev.9b00818
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      Chemical Synthesis of Single Atomic Site Catalysts
      Ji, Shufang; Chen, Yuanjun; Wang, Xiaolu; Zhang, Zedong; Wang, Dingsheng; Li, Yadong
      Chemical Reviews (Washington, DC, United States) (2020), 120 (21), 11900-11955CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chem. synthesis. The recent emergence of single at. site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the max. efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.
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    51. 51
      Mo, Q.; Zhang, L.; Li, S.; Song, H.; Fan, Y.; Su, C. Y. Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2022, 144 (49), 22747– 22758,  DOI: 10.1021/jacs.2c10801
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      51
      Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction
      Mo, Qijie; Zhang, Li; Li, Sihong; Song, Haili; Fan, Yanan; Su, Cheng-Yong
      Journal of the American Chemical Society (2022), 144 (49), 22747-22758CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      As a type of heterogeneous catalyst expected for the max. atom efficiency, a series of single-atom catalysts (SACs) contg. spatially isolated metal single atoms (M-SAs) have been successfully prepd. by confining M-SAs in the pore-nanospaces of porphyrinic metal-org. frameworks (MOFs). The prepd. MOF composites of M-SAs@Pd-PCN-222-NH2 (M = Pt, Ir, Au, and Ru) display exceptionally high and persistent efficiency in the photocatalytic hydrogen evolution reaction with a turnover no. (TON) of up to 21713 in 32 h and a beginning/lasting turnover frequency (TOF) larger than 1200/600 h-1 based on M-SAs under visible light irradn. (λ ≥ 420 nm). The photo-/electrochem. property studies and d. functional theory calcns. disclose that the close proximity of the catalytically active Pt-SAs to the Pd-porphyrin photosensitizers with the confinement and stabilization effect by chem. binding could accelerate electron-hole sepn. and charge transfer in pore-nanospaces, thus promoting the catalytic H2 evolution reaction with lasting effectiveness.
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    52. 52
      Jin, X.; Wang, R.; Zhang, L.; Si, R.; Shen, M.; Wang, M.; Tian, J.; Shi, J. Electron Configuration Modulation of Nickel Single Atoms for Elevated Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2020, 59 (17), 6827– 6831,  DOI: 10.1002/anie.201914565
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      52
      Electron Configuration Modulation of Nickel Single Atoms for Elevated Photocatalytic Hydrogen Evolution
      Jin, Xixiong; Wang, Rongyan; Zhang, Lingxia; Si, Rui; Shen, Meng; Wang, Min; Tian, Jianjian; Shi, Jianlin
      Angewandte Chemie, International Edition (2020), 59 (17), 6827-6831CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The emerging metal single-atom catalyst has aroused extensive attention in multiple fields, such as clean energy, environmental protection, and biomedicine. Unfortunately, though it has been shown to be highly active, the origins of the activity of the single-atom sites remain unrevealed to date owing to the lack of deep insight on electronic level. Now, partially oxidized Ni single-atom sites were constructed in polymeric carbon nitride (CN), which elevates the photocatalytic performance by over 30-fold. The 3d orbital of the partially oxidized Ni single-atom sites is filled with unpaired d-electrons, which are ready to be excited under irradn. Such an electron configuration results in elevated light response, cond., charge sepn., and mobility of the photocatalyst concurrently, thus largely augmenting the photocatalytic performance.
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    53. 53
      Sholl, D. S.; Lively, R. P. Seven Chemical Separations to Change the World. Nature 2016, 532 (7600), 435– 437,  DOI: 10.1038/532435a
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      53
      Seven chemical separations to change the world
      Sholl David S; Lively Ryan P
      Nature (2016), 532 (7600), 435-7 ISSN:.
      There is no expanded citation for this reference.
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    54. 54
      Arcudi, F.; Đorđevic, L.; Schweitzer, N.; Stupp, S. I.; Weiss, E. A. Selective Visible-Light Photocatalysis of Acetylene to Ethylene Using a Cobalt Molecular Catalyst and Water as a Proton Source. Nat. Chem. 2022, 14 (9), 1007– 1012,  DOI: 10.1038/s41557-022-00966-5
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      54
      Selective visible-light photocatalysis of acetylene to ethylene using a cobalt molecular catalyst and water as a proton source
      Arcudi, Francesca; Dordjevic, Luka; Schweitzer, Neil; Stupp, Samuel I.; Weiss, Emily A.
      Nature Chemistry (2022), 14 (9), 1007-1012CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
      The prodn. of polymers from ethylene requires the ethylene feed to be sufficiently purified of acetylene contaminant. Accomplishing this task by thermally hydrogenating acetylene requires a high temp., an external feed of H2 gas and noble-metal catalysts. It is not only expensive and energy-intensive, but also prone to overhydrogenating to ethane. Here we report a photocatalytic system that reduces acetylene to ethylene with ≥99% selectivity under both non-competitive (no ethylene co-feed) and competitive (ethylene co-feed) conditions, and near 100% conversion under the latter industrially relevant conditions. Our system uses a mol. catalyst based on earth-abundant cobalt operating under ambient conditions and sensitized by either [Ru(bpy)3]2+ or an inexpensive org. semiconductor (metal-free mesoporous graphitic carbon nitride) under visible light. These features and the use of water as a proton source offer advantages over current hydrogenation technologies with respect to selectivity and sustainability.
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    55. 55
      Shi, X.; Huang, Y.; Bo, Y.; Duan, D.; Wang, Z.; Cao, J.; Zhu, G.; Ho, W.; Wang, L.; Huang, T.; Xiong, Y. Highly Selective Photocatalytic CO2 Methanation with Water Vapor on Single-Atom Platinum-Decorated Defective Carbon Nitride. Angew. Chem., Int. Ed. 2022, 61 (27), e202203063  DOI: 10.1002/anie.202203063
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      55
      Highly Selective Photocatalytic CO2 Methanation with Water Vapor on Single-Atom Platinum-Decorated Defective Carbon Nitride
      Shi, Xianjin; Huang, Yu; Bo, Yanan; Duan, Delong; Wang, Zhenyu; Cao, Junji; Zhu, Gangqiang; Ho, Wingkei; Wang, Liqin; Huang, Tingting; Xiong, Yujie
      Angewandte Chemie, International Edition (2022), 61 (27), e202203063CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Solar-driven CO2 methanation with water is an important route to simultaneously address carbon neutrality and produce fuels. It is challenging to achieve high selectivity in CO2 methanation due to competing reactions. Nonetheless, aspects of the catalyst design can be controlled with meaningful effects on the catalytic outcomes. We report highly selective CO2 methanation with water vapor using a photocatalyst that integrates polymeric carbon nitride (CN) with single Pt atoms. As revealed by exptl. characterization and theor. simulations, the widely explored Pt-CN catalyst is adapted for selective CO2 methanation with our rationally designed synthetic method. The synthesis creates defects in CN along with formation of hydroxyl groups proximal to the coordinated Pt atoms. The photocatalyst exhibits high activity and carbon selectivity (99%) for CH4 prodn. in photocatalytic CO2 redn. with pure water. This work provides at. scale insight into the design of photocatalysts for selective CO2 methanation.
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    56. 56
      Liu, C.; Chen, Z.; Su, C.; Zhao, X.; Gao, Q.; Ning, G. H.; Zhu, H.; Tang, W.; Leng, K.; Fu, W.; Tian, B.; Peng, X.; Li, J.; Xu, Q. H.; Zhou, W.; Loh, K. P. Controllable Deuteration of Halogenated Compounds by Photocatalytic D2O Splitting. Nat. Commun. 2018, 9 (1), 80,  DOI: 10.1038/s41467-017-02551-8
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      56
      Controllable deuteration of halogenated compounds by photocatalytic D2O splitting
      Liu Cuibo; Su Chenliang; Gao Qiang; Tian Bingbing; Li Jing; Xu Qing-Hua; Loh Kian Ping; Liu Cuibo; Chen Zhongxin; Su Chenliang; Zhao Xiaoxu; Gao Qiang; Ning Guo-Hong; Zhu Hai; Leng Kai; Fu Wei; Tian Bingbing; Peng Xinwen; Li Jing; Xu Qing-Hua; Loh Kian Ping; Chen Zhongxin; Zhao Xiaoxu; Tang Wei; Zhou Wu
      Nature communications (2018), 9 (1), 80 ISSN:.
      Deuterium labeling is of great value in organic synthesis and the pharmaceutical industry. However, the state-of-the-art C-H/C-D exchange using noble metal catalysts or strong bases/acids suffers from poor functional group tolerances, poor selectivity and lack of scope for generating molecular complexity. Herein, we demonstrate the deuteration of halides using heavy water as the deuteration reagent and porous CdSe nanosheets as the catalyst. The deuteration mechanism involves the generation of highly active carbon and deuterium radicals via photoinduced electron transfer from CdSe to the substrates, followed by tandem radicals coupling process, which is mechanistically distinct from the traditional methods involving deuterium cations or anions. Our deuteration strategy shows better selectivity and functional group tolerances than current C-H/C-D exchange methods. Extending the synthetic scope, deuterated boronic acids, halides, alkynes, and aldehydes can be used as synthons in Suzuki coupling, Click reaction, C-H bond insertion reaction etc. for the synthesis of complex deuterated molecules.
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    57. 57
      Nan, X. L.; Wang, Y.; Li, X. B.; Tung, C. H.; Wu, L. Z. Site-selective D2O-Mediated Deuteration of Diaryl Alcohols via Quantum Dots Photocatalysis. Chem. Commun. 2021, 57 (55), 6768– 6771,  DOI: 10.1039/D1CC02551A
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      57
      Site-selective D2O-mediated deuteration of diaryl alcohols via quantum dots photocatalysis
      Nan, Xiao-Lei; Wang, Yao; Li, Xu-Bing; Tung, Chen-Ho; Wu, Li-Zhu
      Chemical Communications (Cambridge, United Kingdom) (2021), 57 (55), 6768-6771CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Owing to the high synthetic value of deuteration in the pharmaceutical industry, the conversion of a range of arom. ketones to deuterium-labeled products such as RR1CD(OH) [R = Ph, 4-ClC6H4, 4-BrC6H4, 4-CNC6H4, 4-PhC6H4; R1 = Ph, 2-thienyl, 2-naphthyl, etc.] in good to excellent yields was described. Efficient and site-selective deuteration of benzyl alcs. by D2O with visible light irradn. of quantum dots (QDs), together with gram-scale synthesis and photocatalyst recycling expts. indicated the potential of the developed method in practical org. synthesis.
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    58. 58
      Jia, T.; Meng, D.; Ji, H.; Sheng, H.; Chen, C.; Song, W.; Zhao, J. Visible-Light-Driven Semihydrogenation of Alkynes via Proton Reduction over Carbon Nitride Supported Nickel. Appl. Catal., B 2022, 304, 121004,  DOI: 10.1016/j.apcatb.2021.121004
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      58
      Visible-light-driven semihydrogenation of alkynes via proton reduction over carbon nitride supported nickel
      Jia, Tongtong; Meng, Di; Ji, Hongwei; Sheng, Hua; Chen, Chuncheng; Song, Wenjing; Zhao, Jincai
      Applied Catalysis, B: Environmental (2022), 304 (), 121004CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Semihydrogenation of alkynes represents one of the most viable route to produce functional alkene products. Herein we describe the visible-light-driven alc. or water donating semihydrogenation catalyzed by nickel supported on carbon nitride scaffold (Ni/C3N4) under ambient condition, exhibiting excellent alkene selectivity and broad substrate scope. The catalyst design takes advantage of C3N4 to harvest visible irradn. and to tune the interaction of Ni with hydrogenation intermediates, which is essential for the excellent selectivity toward alkene products. The hydrogen atom incorporated in alkene products originates from hydroxyl group of methanol or water, via a Ni catalyzed proton redn. by photogenerated electrons to give the active surface hydrogen species (H*). Such hydrogenation pathway not only avoids harsh reaction condition but also enables facile synthesis of valuable deuterated alkenes using deuterated alcs. or D2O, promising enormous application potential for well-designed catalyst architectures in the light-driven selective transfer hydrogenation (deuteration) of alkynes and other org. substrates.
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    59. 59
      Han, C.; Han, G.; Yao, S.; Yuan, L.; Liu, X.; Cao, Z.; Mannodi-Kanakkithodi, A.; Sun, Y. Defective Ultrathin ZnIn2S4 for Photoreductive Deuteration of Carbonyls Using D2O as the Deuterium Source. Adv. Sci. 2022, 9 (3), 2103408  DOI: 10.1002/advs.202103408
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      59
      Defective Ultrathin ZnIn2S4 for Photoreductive Deuteration of Carbonyls Using D2O as the Deuterium Source
      Han, Chuang; Han, Guanqun; Yao, Shukai; Yuan, Lan; Liu, Xingwu; Cao, Zhi; Mannodi-Kanakkithodi, Arun; Sun, Yujie
      Advanced Science (Weinheim, Germany) (2022), 9 (3), 2103408CODEN: ASDCCF; ISSN:2198-3844. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Deuterium (D) labeling is of great value in org. synthesis, pharmaceutical industry, and materials science. However, the state-of-the-art deuteration methods generally require noble metal catalysts, expensive deuterium sources, or harsh reaction conditions. Herein, noble metal-free and ultrathin ZnIn2S4 (ZIS) is reported as an effective photocatalyst for visible light-driven reductive deuteration of carbonyls to produce deuterated alcs. using heavy water (D2O) as the sole deuterium source. Defective two-dimensional ZIS nanosheets (D-ZIS) are prepd. in a surfactant assisted bottom-up route exhibited much enhanced performance than the pristine ZIS counterpart. A systematic study is carried out to elucidate the contributing factors and it is found that the in situ surfactant modification enabled D-ZIS to expose more defect sites for charge carrier sepn. and active D-species generation, as well as high sp. surface area, all of which are beneficial for the desirable deuteration reaction. This work highlights the great potential in developing low-cost semiconductor-based photocatalysts for org. deuteration in D2O, circumventing expensive deuterium reagents and harsh conditions.
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    60. 60
      Yuan, J.; Li, S.; Yu, L.; Liu, Y.; Cao, Y. Efficient Catalytic Hydrogenolysis of Glycerol Using Formic Acid as Hydrogen Source. Chin. J. Catal. 2013, 34 (11), 2066– 2074,  DOI: 10.1016/S1872-2067(12)60656-1
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    61. 61
      Yan, D.-M.; Xu, S.-H.; Qian, H.; Gao, P.-P.; Bi, M.-H.; Xiao, W.-J.; Chen, J.-R. Photoredox-Catalyzed and Copper(II) Salt-Assisted Radical Addition/Hydroxylation Reaction of Alkenes, Sulfur Ylides, and Water. ACS Catal. 2022, 12 (6), 3279– 3285,  DOI: 10.1021/acscatal.2c00638
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      61
      Photoredox-Catalyzed and Copper(II) Salt-Assisted Radical Addition/Hydroxylation Reaction of Alkenes, Sulfur Ylides, and Water
      Yan, Dong-Mei; Xu, Shuang-Hua; Qian, Hao; Gao, Pan-Pan; Bi, Ming-Hang; Xiao, Wen-Jing; Chen, Jia-Rong
      ACS Catalysis (2022), 12 (6), 3279-3285CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A visible light-driven photoredox-catalyzed and copper(II)-assisted three-component radical addn./hydroxylation reaction of alkenes, sulfur ylides, and water is reported. This process shows broad substrate scope and high functional group tolerance, with respect to both readily available sulfur ylides and alkenes, providing high-yielding and practical access to valuable γ-hydroxy carbonyl compds. Key to the success of the reaction is the controlled generation of α-carbonyl carbon radicals from sulfur ylides via sulfonium salts by a visible-light-driven proton-coupled electron transfer (PCET) strategy in a mixt. of 2,2,2-trifluoroethanol/CH2Cl2. Addn. of Cu(TFA)2·H2O helps to accelerate the radical-cation crossover to improve the reaction efficiency. Mechanistic studies suggest that the hydroxy moiety in the products stems from water. This study also builds up a platform for further investigation into the radical synthetic chem. of sulfur ylides.
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    62. 62
      Tian, X.; Guo, Y.; An, W.; Ren, Y. L.; Qin, Y.; Niu, C.; Zheng, X. Coupling Photocatalytic Water Oxidation with Reductive Transformations of Organic Molecules. Nat. Commun. 2022, 13 (1), 6186,  DOI: 10.1038/s41467-022-33778-9
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      62
      Coupling photocatalytic water oxidation with reductive transformations of organic molecules
      Tian, Xinzhe; Guo, Yinggang; An, Wankai; Ren, Yun-Lai; Qin, Yuchen; Niu, Caoyuan; Zheng, Xin
      Nature Communications (2022), 13 (1), 6186CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
      The utilization of readily available and non-toxic water by photocatalytic water splitting is highly attractive in green chem. Herein, the light-induced oxidative half-reaction of water splitting is effectively coupled with redn. of org. compds., which provides a light-induced avenue to use water as an electron donor to enable reductive transformations of org. substances is reported. The present strategy allows various aryl bromides to undergo smoothly the reductive coupling with Pd/g-C3N4* as the photocatalyst, giving a pollutive reductant-free method for synthesizing biaryl skeletons. Moreover, the use of green visible-light energy endows this process with more advantages including mild conditions and good functional group tolerance. Although this method has some disadvantages such as a use of environmentally unfriendly 1,2-dioxane, an addn. of Na2CO3 and so on, it can guide chemists to use water as a reducing agent to develop clean procedures for various org. reactions.
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