A Ruthenium(II) Water Oxidation Catalyst Containing a pH-Responsive Ligand Framework

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1. 1

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   The art of splitting water

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   Plants produce oxygen from water, but the same chem. reaction is hard to achieve synthetically. A new family of catalysts could breathe fresh life into the quest for artificial photosynthesis.
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   Roger, I.; Shipman, M. A.; Symes, M. D. Earth-Abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 0003  DOI: 10.1038/s41570-016-0003

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   Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting

   Roger, Isolda; Shipman, Michael A.; Symes, Mark D.

   Nature Reviews Chemistry (2017), 1 (1), 0003CODEN: NRCAF7; ISSN:2397-3358. (Nature Research)

   Sunlight is by far the most plentiful renewable energy resource, providing Earth with enough power to meet all of humanity's needs several hundred times over. However, it is both diffuse and intermittent, which presents problems regarding how best to harvest this energy and store it for times when the sun is not shining. Devices that use sunlight to split water into hydrogen and oxygen could be one soln. to these problems, because hydrogen is an excellent fuel. However, if such devices are to become widely adopted, they must be cheap to produce and operate. Therefore, the development of electrocatalysts for water splitting that comprise only inexpensive, earth-abundant elements is crit. In this Review, we investigate progress towards such electrocatalysts, with special emphasis on how they might be incorporated into photoelectrocatalytic water-splitting systems and the challenges that remain in developing these devices.
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   Romain, S.; Vigara, L.; Llobet, A. Oxygen-Oxygen Bond Formation Pathways Promoted by Ruthenium Complexes. Acc. Chem. Res. 2009, 42, 1944– 1953,  DOI: 10.1021/ar900240w

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   Oxygen-Oxygen Bond Formation Pathways Promoted by Ruthenium Complexes

   Romain, Sophie; Vigara, Laura; Llobet, Antoni

   Accounts of Chemical Research (2009), 42 (12), 1944-1953CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

   A review. The photoprodn. of hydrogen from water and sunlight represents an attractive means of artificial energy conversion for a world still largely dependent on fossil fuels. A practical technol. for producing sun-derived hydrogen remains an unachieved goal, however, and is dependent on developing a better understanding of the key reaction, the oxidn. of water to dioxygen. The mol. complexity of this process is such that sophisticated transition metal complexes, which can access low-energy reaction pathways, are considered essential as catalysts. Complexes based on Mn, Co, Ir, and Ru have been described recently; a variety of ligands and nuclearities that comprise many complex topologies have been developed, but very few of them have been studied from a mechanistic perspective. One step in particular needs to be understood and better characterized for the transition-metal-catalyzed oxidn. of water to dioxygen, namely, the circumstances under which the formation of O-O bonds can occur. Although there is a large body of work related to the formation of C-C bonds promoted by metal complexes, the analogous literature for O-O bond formation is practically nonexistent and just beginning to emerge. In this Account, the authors describe the sparse literature existing on this topic, focusing on the Ru-aqua complexes. These complexes are capable of reaching high oxidn. states as a result of the sequential and simultaneous loss of protons and electrons. A solvent water mol. may or may not participate in the formation of the O-O bond; accordingly, the two main pathways are named (i) solvent water nucleophilic attack (WNA) and (ii) interaction of two M-O units (I2M). Most of the complexes described belong to the WNA class, including a variety of mononuclear and polynuclear complexes contg. one or several Ru-O units. A common feature of these complexes is the generation of formal oxidn. states as high as Ru(V) and Ru(VI), which render the oxygen atom of the Ru-O group highly electrophilic. On the other hand, only one sym. dinuclear complex that undergoes an intramol. O-O bond formation step has been described for the I2M class; it has a formal oxidn. state of Ru(IV). A special section is devoted to Ru-OH2 complexes that contain redox active ligands, such as the chelating quinone. These ligands are capable of undergoing reversible redox processes and thus generate a complex but fascinating electron-transfer process between the metal and the ligand. Despite the intrinsic exptl. difficulties in detg. reaction mechanisms, progress with these Ru complexes is now beginning to be reported. An understanding of recent successes, as well as pitfalls, is essential in the search for a practical water oxidn. catalyst.
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   Sala, X.; Romero, I.; Rodríguez, M.; Escriche, L.; Llobet, A. Molecular Catalysts That Oxidize Water to Dioxygen. Angew. Chem., Int. Ed. 2009, 48, 2842– 2852,  DOI: 10.1002/anie.200802659

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   Molecular catalysts that oxidize water to dioxygen

   Sala, Xavier; Romero, Isabel; Rodriguez, Montserrat; Escriche, Lluis; Llobet, Antoni

   Angewandte Chemie, International Edition (2009), 48 (16), 2842-2852CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

   A review of H2O-oxidn. catalysis - well-defined mols. are now enabling the design of more rugged and efficient catalysts. There is a need for clean and renewable fuel and the intrinsic human desire to mimic nature's reactions, in this case the O-evolving complex (OEC) of the photosystem II (PSII), stimulate developments. The basis for the oxidn. of H2O to dioxygen as well new developments are presented. The new avenues these developments are opening up with regard to catalyst design and performance, together with the questions they pose, esp. from a mechanistic perspective, are presented. The challenges the field are facing are also discussed.
6. 6

   Duan, L.; Wang, L.; Li, F.; Li, F.; Sun, L. Highly Efficient Bioinspired Molecular Ru Water Oxidation Catalysts with Negatively Charged Backbone Ligands. Acc. Chem. Res. 2015, 48, 2084– 2096,  DOI: 10.1021/acs.accounts.5b00149

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   Highly fficient bioinspired mol. Ru water oxidn. catalysts with neg. charged backbone ligands

   Duan, Lele; Wang, Lei; Li, Fusheng; Li, Fei; Sun, Licheng

   Accounts of Chemical Research (2015), 48 (7), 2084-2096CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

   A review. The oxygen evolving complex (OEC) of the natural photosynthesis system II (PSII) oxidizes water to produce oxygen and reducing equiv. (protons and electrons). The oxygen released from PSII provides the oxygen source of our atm.; the reducing equiv. are used to reduce carbon dioxide to org. products, which support almost all organisms on the Earth planet. The first photosynthetic organisms able to split water were proposed to be cyanobacteria-like ones appearing ca. 2.5 billion years ago. Since then, nature has chosen a sustainable way by using solar energy to develop itself. Inspired by nature, human beings started to mimic the functions of the natural photosynthesis system and proposed the concept of artificial photosynthesis (AP) with the view to creating energy-sustainable societies and reducing the impact on the Earth environments. Water oxidn. is a highly energy demanding reaction and essential to produce reducing equiv. for fuel prodn., and thereby effective water oxidn. catalysts (WOCs) are required to catalyze water oxidn. and reduce the energy loss. X-ray crystallog. studies on PSII have revealed that the OEC consists of a Mn4CaO5 cluster surrounded by oxygen rich ligands, such as oxyl, oxo, and carboxylate ligands. These neg. charged, oxygen rich ligands strongly stabilize the high valent states of the Mn cluster and play vital roles in effective water oxidn. catalysis with low overpotential. This Account describes our endeavors to design effective Ru WOCs with low overpotential, large turnover no., and high turnover frequency by introducing neg. charged ligands, such as carboxylate. Neg. charged ligands stabilized the high valent states of Ru catalysts, as evidenced by the low oxidn. potentials. Meanwhile, the oxygen prodn. rates of our Ru catalysts were improved dramatically as well. Thanks to the strong electron donation ability of carboxylate contg. ligands, a seven-coordinate RuIV species was isolated as a reaction intermediate, shedding light on the reaction mechanisms of Ru-catalyzed water oxidn. chem. Auxiliary ligands have dramatic effects on the water oxidn. catalysis in terms of the reactivity and the reaction mechanism. For instance, Ru-bda (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water oxidn. catalysts catalyze CeIV-driven water oxidn. extremely fast via the radical coupling of two RuV=O species, while Ru-pda (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid) water oxidn. catalysts catalyze the same reaction slowly via water nucleophilic attack on a RuV=O species. With a no. of active Ru catalysts in hands, light driven water oxidn. was accomplished using catalysts with low catalytic onset potentials. The structures of mol. catalysts could be readily tailored to introduce addnl. functional groups, which favors the fabrication of state-of-the-art Ru-based water oxidn. devices, such as electrochem. water oxidn. anodes and photo-electrochem. anodes. The development of efficient water oxidn. catalysts has led to a step forward in the sustainable energy system.
7. 7

   Fan, T.; Duan, L.; Huang, P.; Chen, H.; Daniel, Q.; Ahlquist, M. S. G.; Sun, L. The Ru-tpc Water Oxidation Catalyst and Beyond: Water Nucleophilic Attack Pathway versus Radical Coupling Pathway. ACS Catal. 2017, 7, 2956– 2966,  DOI: 10.1021/acscatal.6b03393

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   The Ru-tpc Water Oxidation Catalyst and Beyond: Water Nucleophilic Attack Pathway versus Radical Coupling Pathway

   Fan, Ting; Duan, Lele; Huang, Ping; Chen, Hong; Daniel, Quentin; Ahlquist, Maarten S. G.; Sun, Licheng

   ACS Catalysis (2017), 7 (4), 2956-2966CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

   Many Ru water oxidn. catalysts have been documented in the literature. However, only a few can catalyze the O-O bond formation via the radical coupling pathway, while most go through the water nucleophilic attack pathway. Understanding the electronic effect on the reaction pathway is of importance in design of active water oxidn. catalysts. The Ru-bda (bda = 2,2'-bipyridine-6,6'-dicarboxylate) catalyst is one example that catalyzes the O-O bond formation via the radical coupling pathway. Herein, we manipulate the equatorial backbone ligand, change the doubly charged bda2- ligand to a singly charged tpc- (2,2':6',2''-terpyridine-6-carboxylate) ligand, and study the structure-activity relationship. Surprisingly, kinetics measurements revealed that the resulting Ru-tpc catalyst catalyzes water oxidn. via the water nucleophilic attack pathway, which is different from the Ru-bda catalyst. The O-O bond formation Gibbs free energy of activation (ΔG⧺) at T = 298.15 K was 20.2 ± 1.7 kcal mol-1. The electronic structures of a series of RuV=O species were studied by d. function theory calcns., revealing that the spin d. of ORu=O of RuV=O is largely dependent on the surrounding ligands. Seven coordination configuration significantly enhances the radical character of RuV=O.
8. 8

   Kärkäs, M. D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation. Chem. Rev. 2014, 114, 11863– 12001,  DOI: 10.1021/cr400572f

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   Artificial photosynthesis: molecular systems for catalytic water oxidation

   Karkas Markus D; Verho Oscar; Johnston Eric V; ¡ÑÜAkermark Bjorn

   Chemical reviews (2014), 114 (24), 11863-2001 ISSN:.

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9. 9

   Duan, L.; Bozoglian, F.; Mandal, S.; Stewart, B.; Privalov, T.; Llobet, A.; Sun, L. A Molecular Ruthenium Catalyst with Water-Oxidation Activity Comparable to That of Photosystem II. Nat. Chem. 2012, 4, 418– 423,  DOI: 10.1038/nchem.1301

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   A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II

   Duan, Lele; Bozoglian, Fernando; Mandal, Sukanta; Stewart, Beverly; Privalov, Timofei; Llobet, Antoni; Sun, Licheng

   Nature Chemistry (2012), 4 (5), 418-423CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)

   Across chem. disciplines, an interest in developing artificial H2O splitting to O2 and H2, driven by sunlight, was motivated by the need for practical and environmentally friendly power generation without the consumption of fossil fuels. The central issue in light-driven H2O splitting is the efficiency of the H2O oxidn., which in the best-known catalysts falls short of the desired level by approx. two orders of magnitude. Here, it is possible to close that two orders of magnitude' gap with a rationally designed mol. catalyst [Ru(bda)(isoq)2] (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline). This speeds up the H2O oxidn. to an unprecedentedly high reaction rate with a turnover frequency of >300 s-1. This value is, for the 1st time, moderately comparable with the reaction rate of 100-400 s-1 of the O-evolving complex of photosystem II in vivo.
10. 10

    Matheu, R.; Garrido-Barros, P.; Gil-Sepulcre, M.; Ertem, M. Z.; Sala, X.; Gimbert-Suriñach, C.; Llobet, A. The Development of Molecular Water Oxidation Catalysts. Nat. Rev. Chem. 2019, 3, 331– 341,  DOI: 10.1038/s41570-019-0096-0

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    The development of molecular water oxidation catalysts

    Matheu, Roc; Garrido-Barros, Pablo; Gil-Sepulcre, Marcos; Ertem, Mehmed Z.; Sala, Xavier; Gimbert-Surinach, Carolina; Llobet, Antoni

    Nature Reviews Chemistry (2019), 3 (5), 331-341CODEN: NRCAF7; ISSN:2397-3358. (Nature Research)

    A review. There is an urgent need to transition from fossil fuels to solar fuels - not only to lower CO2 emissions that cause global warming but also to ration fossil resources. Splitting H2O with sunlight emerges as a clean and sustainable energy conversion scheme that can afford practical technologies in the short-to-mid-term. A crucial component in such a device is a water oxidn. catalyst (WOC). These artificial catalysts have been developed mainly over the past two decades, which is in contrast to nature's WOCs, which have featured in its photosynthetic app. for more than a billion years. Recent times have seen the development of increasingly active mol. WOCs, the study of which affords an understanding of catalytic mechanisms and decompn. pathways. This Perspective offers a historical description of the landmark mol. WOCs, particularly ruthenium systems, that have guided research to our present degree of understanding.
11. 11

    Tong, L.; Zong, R.; Zhou, R.; Kaveevivitchai, N.; Zhang, G.; Thummel, R. P. Ruthenium Catalysts for Water Oxidation Involving Tetradentate Polypyridine-Type Ligands. Faraday Discuss. 2015, 185, 87– 104,  DOI: 10.1039/C5FD00051C

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    Ruthenium catalysts for water oxidation involving tetradentate polypyridine-type ligands

    Tong, Lianpeng; Zong, Ruifa; Zhou, Rongwei; Kaveevivitchai, Nattawut; Zhang, Gang; Thummel, Randolph P.

    Faraday Discussions (2015), 185 (), 87-104CODEN: FDISE6; ISSN:1359-6640. (Royal Society of Chemistry)

    A series of RuII complexes that behave as water oxidn. catalysts were prepd. involving a tetradentate equatorial ligand and two 4-substituted pyridines as the axial ligands. Two of these complexes were derived from 2,9-di-(pyrid-2'-yl)-1,10-phenanthroline (dpp) and examine the effect of incorporating electron-donating amino and bulky t-Bu groups on catalytic activity. A third complex replaced the two distal pyridines with N-methylimidazoles that are more electron-donating than the pyridines of dpp and potentially stabilize higher oxidn. states of the metal. The tetradentate ligand 2-(pyrid-2'-yl)-6-(1",10"-phenanthrol-2"-yl)pyridine (bpy-phen), possessing a bonding cavity similar to dpp, was also prepd. The RuII complex of this ligand does not have two rotatable pyridines in the equatorial plane and thus shows different flexibility from the [Ru(dpp)] complexes. All the complexes showed activity towards water oxidn. Investigation of their catalytic behavior and electrochem. properties suggests that they may follow the same catalytic pathway as the prototype [Ru(dpp)pic2]2+ involving a seven-coordinated [RuIV(O)] intermediate. The influence of coordination geometry on catalytic performance is analyzed and discussed.
12. 12

    Zong, R.; Thummel, R. P. 2,9-Di-(2′-Pyridyl)-1,10-Phenanthroline: A Tetradentate Ligand for Ru(II). J. Am. Chem. Soc. 2004, 126, 10800– 10801,  DOI: 10.1021/ja047410y

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    2,9-Di-(2'-pyridyl)-1,10-phenanthroline: A Tetradentate Ligand for Ru(II)

    Zong, Ruifa; Thummel, Randolph P.

    Journal of the American Chemical Society (2004), 126 (35), 10800-10801CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    The tetradentate ligand 2,9-di-(2'-pyridyl)-1,10-phenanthroline (L) was synthesized in 62% yield by the Stille coupling of 2,9-dichloro-1,10-phenanthroline and 2-(tri-n-butylstannyl)pyridine. Treatment of this ligand with RuCl3·3H2O and a 4-substituted pyridine gave complexes [RuL(4-X-py)2](PF6)2 (X = NMe2, Me, CF3) in which the tetradentate ligand occupies the equatorial plane and two pyridines are bound axially. The crystal structure of [RuL(4-NMe2py)2](PF6)2·C3H6O was detd. The interior N-Ru-N angles vary from 76.1° to 125.6°, showing considerable distortion from the 90° ideal. The lowest energy electronic transition is sensitive to the electronegativity of the 4-substituent on the axial pyridines, varying from 516 nm for the CF3 group to 580 nm for the NMe2. The oxidn. potentials mirror this trend, spanning a range of 1.36-1.03 V, while the redn. potentials show less variation (-0.97 to -1.08 V). The complexes are nonemissive, presumably due to competitive nonradiative processes caused by distortion of the system.
13. 13

    Zhang, G.; Zong, R.; Tseng, H.-W.; Thummel, R. P. Ru(II) Complexes of Tetradentate Ligands Related to 2,9-Di(Pyrid-2′-yl)-1,10-Phenanthroline. Inorg. Chem. 2008, 47, 990– 998,  DOI: 10.1021/ic701798v

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    Ru(II) complexes of tetradentate ligands related to 2,9-di(pyrid-2'-yl)-1,10-phenanthroline

    Zhang, Gang; Zong, Ruifa; Tseng, Huan-Wei; Thummel, Randolph P.

    Inorganic Chemistry (2008), 47 (3), 990-998CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    1,10-Phenanthrolines were prepd. having addnl. ligating substituents at the 2,9-positions. These substituents were either a 4-substituted pyrid-2-yl, quinolin-2-yl, 1,8-naphthyrid-2-yl, N-Me imidazo-2-yl, or N-Me benzimidazo-2-yl group. Addnl., 3,6-di-(pyrid-2'-yl)-dipyrido[3,2-a:2',3'-c]phenazine was prepd. All but two of these ligands coordinated Ru(II) in a tetradentate equatorial fashion with two 4-methylpyridines bound in the axial sites. An x-ray structure anal. of the diimidazolyl system indicates considerable distortion from square planar geometry in the equatorial plane. Previously reported variations in the axial ligand for such complexes appear to have a stronger effect on the electronic absorption and redox properties of the system than similar changes in the equatorial ligand. In the presence of excess Ce(IV) as a sacrificial oxidant at pH 1, all the systems examd. catalyze the decompn. of H2O to generate O. Turnover nos. are modest, ranging from 146 to 416.
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    Wu, Q.; Zheng, K.; Liao, S.; Ding, Y.; Li, Y.; Mei, W. Arene Ruthenium(II) Complexes as Low-Toxicity Inhibitor against the Proliferation, Migration, and Invasion of MDA-MB-231 Cells through Binding and Stabilizing c-Myc G-Quadruplex DNA. Organometallics 2016, 35, 317– 326,  DOI: 10.1021/acs.organomet.5b00820

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    Arene Ruthenium(II) Complexes as Low-Toxicity Inhibitor against the Proliferation, Migration, and Invasion of MDA-MB-231 Cells through Binding and Stabilizing c-myc G-Quadruplex DNA

    Wu, Qiong; Zheng, Kangdi; Liao, Siyan; Ding, Yang; Li, Yangqiu; Mei, Wenjie

    Organometallics (2016), 35 (3), 317-326CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)

    Arene Ru(II) complexes have long been extensively studied as potential inhibitors against the proliferation of tumor cells, but their behavior against the migration and invasion of tumor cells needs further research. In this work, a series of arene Ru(II) complexes, [(η6-C6H6)Ru(p-XPIP)Cl]Cl (X = H, 1; F, 2; Cl, 3; Br, 4; and I, 5), have been synthesized, and their inhibitory activity against the migration and invasion of MDA-MB-231 breast cancer cells have been investigated. It is found that all of these complexes exhibit excellent inhibitory activity (IC50) against the growth of MDA-MB-231 breast cancer cells, and the value of IC50 for 1, 2, 3, 4, and 5 is about >300, 52.6, 11.4, 45.5, and 59.1 μM, resp. Further studies by wound-healing assay, FITC-geltain assay, and flow cytometry assay showed that 3 can apparently suppress the migration and invasion of MDA-MB-231 cells via the joint action of S-phase arrest and apoptosis. Moreover, the binding behavior of these arene Ru(II) complexes with c-myc G-quadruplex DNA has also been studied, and the results showed that these complexes can bind and stabilize c-myc G-quadruplex DNA in groove binding mode. Also, the low toxicity of 3 was confirmed by its low inhibitory activity against the growth of normal MCF-10A breast cells in vitro and the development of zebrafish embryos in vivo. In other words, these results indicated that synthetic arene Ru(II) complexes can be developed as low-toxicity agents against the proliferation, migration, and invasion of breast cancer cells.
15. 15

    Stumper, A.; Lämmle, M.; Mengele, A. K.; Sorsche, D.; Rau, S. One Scaffold, Many Possibilities - Copper(I)-Catalyzed Azide-Alkyne Cycloadditions, Strain-Promoted Azide-Alkyne Cycloadditions, and Maleimide-Thiol Coupling of Ruthenium(II) Polypyridyl Complexes. Eur. J. Inorg. Chem. 2018, 2018, 586– 596,  DOI: 10.1002/ejic.201701126

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    One Scaffold, Many Possibilities - Copper(I)-Catalyzed Azide-Alkyne Cycloadditions, Strain-Promoted Azide-Alkyne Cycloadditions, and Maleimide-Thiol Coupling of Ruthenium(II) Polypyridyl Complexes

    Stumper, Anne; Laemmle, Martin; Mengele, Alexander K.; Sorsche, Dieter; Rau, Sven

    European Journal of Inorganic Chemistry (2018), 2018 (5), 586-596CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)

    The applicability of RuII polypyridyl complexes with appropriate functionalities as substrates for bioorthogonal coupling reactions was studied. In detail, Cu(I)-catalyzed azide-alkyne cycloaddns. (CuAAC), strain-promoted azide-alkyne cycloaddns. (SPAAC), and maleimide-thiol coupling reactions of Ru complexes were examd. The first examples of SPAAC in which the org. azide is provided by the metal complex are presented. All of the chromophores belong to one easy-to-synthesize scaffold, which proved to be convenient for the application of metal chromophores. The fundamental photophys. properties of the examd. compds. do not change with substitution, which is important for the design of chromophore conjugates. Also, the limitations of CuAAC reactions are discussed with regard to Cu impurities in the products formed.
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    Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry; Oxford University Press, 2001.

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    Duan, L.; Fischer, A.; Xu, Y.; Sun, L. Isolated Seven-Coordinate Ru(IV) Dimer Complex with [HOHOH]^- Bridging Ligand as an Intermediate for Catalytic Water Oxidation. J. Am. Chem. Soc. 2009, 131, 10397– 10399,  DOI: 10.1021/ja9034686

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    Isolated Seven-Coordinate Ru(IV) Dimer Complex with [HOHOH]- Bridging Ligand as an Intermediate for Catalytic Water Oxidation

    Duan, Lele; Fischer, Andreas; Xu, Yunhua; Sun, Licheng

    Journal of the American Chemical Society (2009), 131 (30), 10397-10399CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    With the inspiration from an oxygen evolving complex (OEC) in Photosystem II (PSII), a mononuclear Ru(II) complex with a tetradentate ligand contg. two carboxylate groups has been synthesized and structurally characterized. This Ru(II) complex showed efficient catalytic properties toward water oxidn. by the chem. oxidant cerium(IV) ammonium nitrate. During the process of catalytic water oxidn., Ru(III) and Ru(IV) species have been successfully isolated as intermediates. To our surprise, X-ray crystallog. together with HR-MS revealed that the Ru(IV) species is a seven-coordinate Ru(IV) dimer complex contg. a [HOHOH]- bridging ligand. This bridging ligand has a short O···O distance and is hydrogen bonded to two water mols. The discovery of this very uncommon seven-coordinate Ru(IV) dimer together with a hydrogen bonding network may contribute to a deeper understanding of the mechanism for catalytic water oxidn. It will also provide new possibilities for the design of more efficient catalysts for water oxidn., which is the key step for solar energy conversion into hydrogen by light-driven water splitting, the ultimate challenge in artificial photosynthesis.
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    TheoDORE: A toolbox for a detailed and automated analysis of electronic excited state computations

    Plasser, F.

    Journal of Chemical Physics (2020), 152 (8), 084108CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)

    The advent of ever more powerful excited-state electronic structure methods has led to a tremendous increase in the predictive power of computation, but it has also rendered the anal. of these computations much more challenging and time-consuming. TheoDORE tackles this problem through providing tools for post-processing excited-state computations, which automate repetitive tasks and provide rigorous and reproducible descriptors. Interfaces are available for ten different quantum chem. codes and a range of excited-state methods implemented therein. This article provides an overview of three popular functionalities within TheoDORE, a fragment-based anal. for assigning state character, the computation of exciton sizes for measuring charge transfer, and the natural transition orbitals used not only for visualization but also for quantifying multiconfigurational character. Using the examples of an org. push-pull chromophore and a transition metal complex, it is shown how these tools can be used for a rigorous and automated assignment of excited-state character. In the case of a conjugated polymer, we venture beyond the limits of the traditional MO picture to uncover spatial correlation effects using electron-hole correlation plots and conditional densities. (c) 2020 American Institute of Physics.
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    Plasser, F.; Wormit, M.; Dreuw, A. New Tools for the Systematic Analysis and Visualization of Electronic Excitations. I. Formalism. J. Chem. Phys. 2014, 141, 024106  DOI: 10.1063/1.4885819

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    New tools for the systematic analysis and visualization of electronic excitations. I. Formalism

    Plasser, Felix; Wormit, Michael; Dreuw, Andreas

    Journal of Chemical Physics (2014), 141 (2), 024106/1-024106/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)

    A variety of d. matrix based methods for the anal. and visualization of electronic excitations are discussed and their implementation within the framework of the algebraic diagrammatic construction of the polarization propagator is reported. Their math. expressions are given and an extensive phenomenol. discussion is provided to aid the interpretation of the results. Starting from several std. procedures, e.g., population anal., natural orbital decompn., and d. plotting, we proceed to more advanced concepts of natural transition orbitals and attachment/detachment densities. In addn., special focus is laid on information coded in the transition d. matrix and its phenomenol. anal. in terms of an electron-hole picture. Taking advantage of both the orbital and real space representations of the d. matrixes, the phys. information in these anal. methods is outlined, and similarities and differences between the approaches are highlighted. Moreover, new anal. tools for excited states are introduced including state averaged natural transition orbitals, which give a compact description of a no. of states simultaneously, and natural difference orbitals (defined as the eigenvectors of the difference d. matrix), which reveal details about orbital relaxation effects. (c) 2014 American Institute of Physics.
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    Mai, S.; Plasser, F.; Dorn, J.; Fumanal, M.; Daniel, C.; González, L. Quantitative Wave Function Analysis for Excited States of Transition Metal Complexes. Coord. Chem. Rev. 2018, 361, 74– 97,  DOI: 10.1016/j.ccr.2018.01.019

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    Quantitative wave function analysis for excited states of transition metal complexes

    Mai, Sebastian; Plasser, Felix; Dorn, Johann; Fumanal, Maria; Daniel, Chantal; Gonzalez, Leticia

    Coordination Chemistry Reviews (2018), 361 (), 74-97CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)

    A review. The character of an electronically excited state is one of the most important descriptors employed to discuss the photophysics and photochem. of transition metal complexes. In transition metal complexes, the interaction between the metal and the different ligands gives rise to a rich variety of excited states, including metal-centered, intra-ligand, metal-to-ligand charge transfer, ligand-to-metal charge transfer, and ligand-to-ligand charge transfer states. Most often, these excited states are identified by considering the most important wave function excitation coeffs. and inspecting visually the involved orbitals. This procedure is tedious, subjective, and imprecise. Instead, automatic and quant. techniques for excited-state characterization are desirable. In this contribution we review the concept of charge transfer nos.-as implemented in the TheoDORE package-and show its wide applicability to characterize the excited states of transition metal complexes. Charge transfer nos. are a formal way to analyze an excited state in terms of electron transitions between groups of atoms based only on the well-defined transition d. matrix. Its advantages are many: it can be fully automatized for many excited states, is objective and reproducible, and provides quant. data useful for the discussion of trends or patterns. We also introduce a formalism for spin-orbit-mixed states and a method for statistical anal. of charge transfer nos. The potential of this technique is demonstrated for a no. of prototypical transition metal complexes contg. Ir, Ru, and Re. Topics discussed include orbital delocalization between metal and carbonyl ligands, nonradiative decay through metal-centered states, effect of spin-orbit couplings on state character, and comparison among results obtained from different electronic structure methods.
22. 22

    Okamura, M.; Yoshida, M.; Kuga, R.; Sakai, K.; Kondo, M.; Masaoka, S. A Mononuclear Ruthenium Complex Showing Multiple Proton-Coupled Electron Transfer toward Multi-Electron Transfer Reactions. Dalton Trans. 2012, 41, 13081– 13089,  DOI: 10.1039/c2dt30773a

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    22

    A mononuclear ruthenium complex showing multiple proton-coupled electron transfer toward multi-electron transfer reactions

    Okamura, Masaya; Yoshida, Masaki; Kuga, Reiko; Sakai, Ken; Kondo, Mio; Masaoka, Shigeyuki

    Dalton Transactions (2012), 41 (42), 13081-13089CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)

    Two new ruthenium(II) complexes bearing dissociable protons, [Ru(trpy)(H2bim)Cl]PF6 (1) and [Ru(trpy)(H2bim)(OH2)](PF6)2 (2) (H2bim = 2,2'-biimidazole and trpy = 2,2':6',2''-terpyridine), were synthesized and characterized, where the H2bim and M-OH2 moieties are expected to serve as proton-dissocn. sites. Single crystal x-ray diffraction analyses revealed that the H2bim and M-OH2 moieties act as proton donors in intermol. hydrogen bonds. Two pKa values of 2 (pKa1 = 9.0 and pKa2 = 11.3) were spectrophotometrically detd., where the 1st proton dissocn. is assigned to that from H2bim and the 2nd is from M-OH2. This assignment was supported by the d. functional theory (DFT) and time-dependent d. functional theory (TD-DFT) calcns. for two sets of conjugated bases, [Ru(trpy)(Hbim)(OH2)]+ and [Ru(trpy)(H2bim)(OH)]+ for the 1st proton dissocn., and [Ru(trpy)(Hbim)(OH)]+ and [Ru(trpy)(bim)(OH2)]+ for the 2nd dissocn. Electrochem. studies in aq. solns. under various pH conditions afforded the Pourbaix diagram (potential vs. pH diagram) of 2, where the pKa values found from the diagram agree well with those detd. spectrophotometrically. Also 2 demonstrates four-step proton-coupled electron transfer (PCET) reactions to give the four-electron oxidized species, [RuIV(trpy)(bim)(O)]2+, without electrostatic charge buildup during the reactions. The multiple PCET ability of 2 would be applicable to various multi-electron oxidn. reactions. Catalysis of electrochem. water oxidn. was indeed evaluated in the initial attempt to demonstrate multi-electron oxidn. reactions, revealing that the water oxidn. potential for 2 is lower than that for other ruthenium catalysts, [Ru(trpy)(bpy)(OH2)]2+, [Ru(trpy)(bpm)(OH2)]2+ and [Ru(tmtacn)(bpy)(OH2)]2+ (bpy = 2,2'-bipyridine, bpm = 2,2'-bipyrimidine, and tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), which are known to be active catalysts for water oxidn.
23. 23

    Kirchhoff, J. R.; McMillin, D. R.; Marnot, P. A.; Sauvage, J. P. Photochemistry and Photophysics of Bis(Terpyridyl) Complexes of Ruthenium(II) in Fluid Solution. Evidence for the Formation of an η²-Diphenylterpyridine Complex. J. Am. Chem. Soc. 1985, 107, 1138– 1141,  DOI: 10.1021/ja00291a009

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    23

    Photochemistry and photophysics of bis(terpyridyl) complexes of ruthenium(II) in fluid solution. Evidence for the formation of an η2-diphenylterpyridine complex

    Kirchhoff, Jon R.; McMillin, David R.; Marnot, Pascal A.; Sauvage, Jean Pierre

    Journal of the American Chemical Society (1985), 107 (5), 1138-41CODEN: JACSAT; ISSN:0002-7863.

    The photochem. and photophys. properties of Ru(dpt)22+ (dpt = 6,6''-diphenyl-2,2':6',2''-terpyridine) are described. The absorption max. occurs at 477 nm in CH2Cl2 and is ascribed to a metal-to-ligand charge-transfer band. Excitation at 477 nm yields little or no detectable emission in CH2Cl2 at room temp. or in a 4:1 EtOH/MeOH glass at 77 K. Upon irradn. in CH2Cl2 at 25° with added SCN-, Ru(dpt)22+ undergoes photoanation. The quantum yield for the reaction is (4.0(± 0.5) × 10-3. Several possible structures for the photoproduct are considered, and on the basis of the available data, it is formulated as Ru(dpt)(η2-dpt)(NCS)+. The photophys. properties of Ru(dpt)22+ are compared with other Ru(II) bis(terpyridyl) complexes, and is concluded that the extremely short lifetimes of these complexes cannot be explained by solvent interactions with the metal center. A kinetics scheme is proposed in which nonradiative decay occurs via a d-d excited state which is prone to undergo ligand substitution processes. The model is used to explain the disparate behavior of 3 different complexes of Ru(II) involving terpyridine-based ligands.
24. 24

    Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. Photochemistry and Photophysics of Coordination Compounds: Ruthenium. Photochem. Photophys. Coord. Compd. I 2007, 280, 117– 214,  DOI: 10.1007/128_2007_133

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    There is no corresponding record for this reference.
25. 25

    Pannwitz, A.; Prescimone, A.; Wenger, O. S. Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents. Eur. J. Inorg. Chem. 2017, 2017, 609– 615,  DOI: 10.1002/ejic.201601403

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    25

    Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents

    Pannwitz, Andrea; Prescimone, Alessandro; Wenger, Oliver S.

    European Journal of Inorganic Chemistry (2017), 2017 (3), 609-615CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Complexes of the type [Ru(bpy)2pyimH]2+ [bpy = 2,2'-bipyridine; pyimH = 2-(2-pyridyl)imidazole] with various substituents on the bpy ligands can act as photoreductants. Their reducing power in the ground state and in the long-lived 3MLCT excited state is increased significantly upon deprotonation, and they can undergo proton-coupled electron transfer (PCET) in the ground and excited state. PCET with both the proton and electron originating from a single donor resembles hydrogen atom transfer (HAT) and can be described thermodynamically by formal bond dissocn. free energies (BDFEs). Whereas the class of complexes studied herein has long been known, their N-H BDFEs were not detd. even though this is important in view of assessing their reactivity. The authors' study demonstrates that the N-H BDFEs in the 3MLCT excited states are 34-52 kcal mol-1 depending on the chem. substituents at the bpy spectator ligands. Specifically, the authors report on the electrochem. and PCET thermochem. of three heteroleptic complexes in 1:1 (vol./vol.) CH3CN/H2O with CF3, tBu, and NMe2 substituents on the bpy ligands.
26. 26

    Pannwitz, A.; Wenger, O. S. Proton Coupled Electron Transfer from the Excited State of a Ruthenium(II) Pyridylimidazole Complex. Phys. Chem. Chem. Phys. 2016, 18, 11374– 11382,  DOI: 10.1039/c6cp00437g

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    26

    Proton coupled electron transfer from the excited state of a ruthenium(II) pyridylimidazole complex

    Pannwitz Andrea; Wenger Oliver S

    Physical chemistry chemical physics : PCCP (2016), 18 (16), 11374-82 ISSN:.

    Proton coupled electron transfer (PCET) from the excited state of [Ru(bpy)2pyimH](2+) (bpy = 2,2'-bipyridine; pyimH = 2-(2'-pyridyl)imidazole) to N-methyl-4,4'-bipyridinium (monoquat, MQ(+)) was studied. While this complex has been investigated previously, our study is the first to show that the formal bond dissociation free energy (BDFE) of the imidazole-N-H bond decreases from (91 ± 1) kcal mol(-1) in the electronic ground state to (43 ± 5) kcal mol(-1) in the lowest-energetic (3)MLCT excited state. This makes the [Ru(bpy)2pyimH](2+) complex a very strong (formal) hydrogen atom donor even when compared to metal hydride complexes, and this is interesting for light-driven (formal) hydrogen atom transfer (HAT) reactions with a variety of different substrates. Mechanistically, formal HAT between (3)MLCT excited [Ru(bpy)2pyimH](2+) and monoquat in buffered 1 : 1 (v : v) CH3CN/H2O was found to occur via a sequence of reaction steps involving electron transfer from Ru(ii) to MQ(+) coupled to release of the N-H proton to buffer base, followed by protonation of reduced MQ(+) by buffer acid. Our study is relevant in the larger contexts of photoredox catalysis and light-to-chemical energy conversion.
27. 27

    Walba, H.; Isensee, R. W. Acidity Constants of Some Arylimidazoles and Their Cations. J. Org. Chem. 1961, 26, 2789– 2791,  DOI: 10.1021/jo01066a039

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    27

    Acidity constants of some arylimidazoles and their cations

    Walba, Harold; Isensee, Robert W.

    Journal of Organic Chemistry (1961), 26 (), 2789-91CODEN: JOCEAH; ISSN:0022-3263.

    A comparative study of the thermodn. acidity consts., both pK°a and pK°a+, of imidazole (I), the mono- and diphenylimidazoles, benzimidazole (II), and 2-phenylbenzimidazole (III) was presented. The procedure for the calcn. of the thermodn. consts. was described. The following thermodn. acidity consts. at 25° were obtained (compd., pK°a, and pK°a+ given): I, 14.52, 7.05; 2-phenylimidazole, 13.32, 6.48; 4(or 5)-phenylimidazole, 13.42, 6.10; 2,4(or 2,5)-diphenylimidazole, 12.53, 5.64; 4,5-diphenylimidazole, 12.80, 5.90; II, 12.78, 5.55; III, 11.91, 5.23. The effect of phenylation on the acidity consts. of I and some arylimidazoles was given in a figure. The results were discussed.
28. 28

    Huber, F. L.; Amthor, S.; Schwarz, B.; Mizaikoff, B.; Streb, C.; Rau, S. Multi-Phase Real-Time Monitoring of Oxygen Evolution Enables in Operando Water Oxidation Catalysis Studies. Sustainable Energy Fuels 2018, 2, 1974– 1978,  DOI: 10.1039/C8SE00328A

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    28

    Multi-phase real-time monitoring of oxygen evolution enables in operando water oxidation catalysis studies

    Huber, Fabian L.; Amthor, Sebastian; Schwarz, Benjamin; Mizaikoff, Boris; Streb, Carsten; Rau, Sven

    Sustainable Energy & Fuels (2018), 2 (9), 1974-1978CODEN: SEFUA7; ISSN:2398-4902. (Royal Society of Chemistry)

    Understanding the reactivity of water oxidn. catalysts (WOCs) is crit. for designing advanced materials for sustainable energy technologies. Here, we show how optical oxygen sensors based on fluorescence quenching enable the simultaneous real-time in operando monitoring of O2 evolution in soln. and in the gas phase, thereby opening new avenues for advanced mechanistic studies under true operating conditions. The advantages of the system are demonstrated by rationalizing and overcoming reactivity limitations of two prototype mol. WOCs.
29. 29

    Muckerman, J. T.; Kowalczyk, M.; Badiei, Y. M.; Polyansky, D. E.; Concepcion, J. J.; Zong, R.; Thummel, R. P.; Fujita, E. New Water Oxidation Chemistry of a Seven-Coordinate Ruthenium Complex with a Tetradentate Polypyridyl Ligand. Inorg. Chem. 2014, 53, 6904– 6913,  DOI: 10.1021/ic500709h

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    29

    New Water Oxidation Chemistry of a Seven-Coordinate Ruthenium Complex with a Tetradentate Polypyridyl Ligand

    Muckerman, James T.; Kowalczyk, Marta; Badiei, Yosra M.; Polyansky, Dmitry E.; Concepcion, Javier J.; Zong, Ruifa; Thummel, Randolph P.; Fujita, Etsuko

    Inorganic Chemistry (2014), 53 (13), 6904-6913CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    The mononuclear ruthenium(II) complex [Ru]2+ (Ru = Ru(dpp)(pic)2, where dpp is the tetradentate 2,9-dipyrid-2'-yl-1,10-phenanthroline ligand and pic is 4-picoline) reported by Thummel's group (Inorg. Chem. 2008, 47, 1835-1848) that contains no water mol. in its primary coordination shell is evaluated as a catalyst for water oxidn. in artificial photosynthesis. A detailed theor. characterization of the energetics, thermochem., and spectroscopic properties of intermediates allowed us to interpret new electrochem. and spectroscopic exptl. data, and propose a mechanism for the water oxidn. process that involves an unprecedented sequence of seven-coordinate ruthenium complexes as intermediates. This anal. provides insights into a mechanism that generates four electrons and four protons in the soln. and a gas-phase oxygen mol. at different pH values. On the basis of the calcns. and corroborated substantially by expts., the catalytic cycle goes through [2RuIII]3+ and [2RuV(O)]3+ to [1RuIV(OOH)]3+ then [2RuIII(···3O2)]3+ at pH 0, and through [3RuIV(O)]2+, [2RuV(O)]3+, and [1RuIV(OO)]2+ at pH 9 before reaching the same [2RuIII(···3O2)]3+ species, from which the liberation of the weakly bound O2 might require an addnl. oxidn. to form [3RuIV(O)]2+ to initiate further cycles involving all seven-coordinate species.
30. 30

    Pineda-Galvan, Y.; Ravari, A. K.; Shmakov, S.; Lifshits, L.; Kaveevivitchai, N.; Thummel, R.; Pushkar, Y. Detection of the Site Protected 7-Coordinate Ru^V = O Species and Its Chemical Reactivity to Enable Catalytic Water Oxidation. J. Catal. 2019, 375, 1– 7,  DOI: 10.1016/j.jcat.2019.05.014

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    30

    Detection of the site protected 7-coordinate RuV = O species and its chemical reactivity to enable catalytic water oxidation

    Pineda-Galvan, Yuliana; Ravari, Alireza K.; Shmakov, Sergei; Lifshits, Liubov; Kaveevivitchai, Nattawut; Thummel, Randolph; Pushkar, Yulia

    Journal of Catalysis (2019), 375 (), 1-7CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)

    Artificial photosynthesis could promise cheap and abundant energy but requires the discovery of new water oxidn. catalysts. A RuV = O 7-coordinate intermediate is implicated in the reactivity of the fastest water oxidn. catalysts. Previously we reported in situ characterization of the 7-coordinate [RuV = O(L)2(bda)]+ stabilized on the electrode. Here we use ligand protection to stabilize [RuV = O(pic)2(dpp)]3+. We report the transformation of [RuII(pic)2(dpp)]2+ (dpp = 2,9-di-(pyrid-2'-yl)-1,10-phenanthroline, pic = 4-picoline) water oxidn. catalyst using a combination of ESR (EPR), X-ray diffraction, resonance Raman (RR) and d. functional theory (DFT). We observe the generation of [RuV = O(pic)2(dpp)]3+ and its chem. reactivity in soln. Lag phase in the oxygen evolution by [RuII(pic)2(dpp)]2+ is due to catalyst activation via an oxygen atom transfer from [RuV = O(pic)2(dpp)]3+ to the polypyridine dpp ligand and formation of [RuIII(pic)2(dpp-NO,NO)]3+. Detailed information regarding catalyst activation during the reaction will enable the design of more active and stable catalysts.
31. 31

    Liu, Y.; Ng, S.-M.; Yiu, S.-M.; Lam, W. W. Y.; Wei, X.-G.; Lau, K.-C.; Lau, T.-C. Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (Qpy) Complexes: Evidence for Ruthenium(III) Qpy-N, N‴-Dioxide as the Real Catalysts. Angew. Chem. 2014, 126, 14696– 14699,  DOI: 10.1002/ange.201408795

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    There is no corresponding record for this reference.
32. 32

    Rau, S.; Büttner, T.; Temme, C.; Ruben, M.; Görls, H.; Walther, D.; Duati, M.; Fanni, S.; Vos, J. G. A Bibenzimidazole-Containing Ruthenium(II) Complex Acting as a Cation-Driven Molecular Switch. Inorg. Chem. 2000, 39, 1621– 1624,  DOI: 10.1021/ic991225h

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    32

    A Bibenzimidazole-Containing Ruthenium(II) Complex Acting as a Cation-Driven Molecular Switch

    Rau, Sven; Buettner, Torsten; Temme, Christian; Ruben, Mario; Goerls, Helmar; Walther, Dirk; Duati, Marco; Fanni, Stefano; Vos, Johannes G.

    Inorganic Chemistry (2000), 39 (7), 1621-1624CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    The nonluminescent complex [Ru(tbbpy)(bibzim)] (1; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; bibzimH2 = 2,2'-bibenzimidazole) was prepd. by the deprotonation of [Ru(tbbpy)(bibzimH2)] (2). The x-ray crystal structures of both 1 and 2 were obtained. The emission of 1 is switched on in the presence of metal ions such as Zn(II), Mg(II) and Cu(I). The emission intensity and wavelength are dependent on the concn. and nature of the metal ion. The emission is due to the formation of complexes such as [Ru(tbbpy)(μ-bibzim)ZnCl2].
33. 33

    Duan, L.; Xu, Y.; Zhang, P.; Wang, M.; Sun, L. Visible Light-Driven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System. Inorg. Chem. 2010, 49, 209– 215,  DOI: 10.1021/ic9017486

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    33

    Visible Light-Driven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System

    Duan, Le-Le; Xu, Yun-Hua; Zhang, Pan; Wang, Mei; Sun, Li-Cheng

    Inorganic Chemistry (2010), 49 (1), 209-215CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    Discovery of an efficient catalyst bearing low overpotential toward H2O oxidn. is a key step for light-driven H2O splitting into dioxygen and dihydrogen. A mononuclear Ru complex, Ru(II)L(pic)2 (1) (H2L = 2,2'-bipyridine-6,6'-dicarboxylic acid; pic = 4-picoline), was found capable of oxidizing H2O electrochem. at a relatively low potential and promoting light-driven H2O oxidn. using a 3-component system composed of a photosensitizer, sacrificial electron acceptor, and complex 1. The detailed electrochem. properties of 1 were studied, and the onset potentials of the electrochem. catalytic curves in pH 7.0 and pH 1.0 solns. are 1.0 and 1.5 V, resp. The low catalytic potential of 1 under neutral conditions allows the use of [Ru(bpy)3]2+ and even [Ru(dmbpy)3]2+ as a photosensitizer for photochem. H2O oxidn. Two different sacrificial electron acceptors, [Co(NH3)5Cl]Cl2 and Na2S2O8, were used to generate the oxidized state of Ru tris(2,2'-bipyridyl) photosensitizers. A 2-h photolysis of 1 in a pH 7.0 phosphate buffer did not lead to obvious degrdn., indicating the good photostability of the catalyst. However, under conditions of light-driven H2O oxidn., the catalyst deactivates quickly. In both soln. and the solid state under aerobic conditions, complex 1 gradually decompd. via oxidative degrdn. of its ligands, and 2 of the decompd. products, sp3 C-H bond oxidized Ru complexes, were identified. The capability of oxidizing the sp3 C-H bond implies the presence of a highly oxidizing Ru species, which might also cause the final degrdn. of the catalyst.
34. 34

    Matheu, R.; Ertem, M. Z.; Benet-Buchholz, J.; Coronado, E.; Batista, V. S.; Sala, X.; Llobet, A. Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex. J. Am. Chem. Soc. 2015, 137, 10786– 10795,  DOI: 10.1021/jacs.5b06541

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    Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

    Matheu, Roc; Ertem, Mehmed Z.; Benet-Buchholz, Jordi; Coronado, Eugenio; Batista, Victor S.; Sala, Xavier; Llobet, Antoni

    Journal of the American Chemical Society (2015), 137 (33), 10786-10795CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    We introduce a new family of complexes with the general formula [Run(tda)(py)2]m+ (n = 2, m = 0, 1; n = 3, m = 1, 2+; n = 4, m = 2, 32+), with tda2- being [2,2':6',2''-terpyridine]-6,6''-dicarboxylate, including complex [RuIV(OH)(tda-κ-N3O)(py)2]+, 4H+, which we find to be an impressive water oxidn. catalyst, formed by hydroxo coordination to 32+ under basic conditions. The complexes are synthesized, isolated, and thoroughly characterized by anal., spectroscopic (UV-vis, NMR, ESR), computational, and electrochem. techniques (cyclic voltammetry, differential pulse voltammetry, coulometry), including solid-state monocrystal X-ray diffraction anal. In oxidn. state IV, the Ru center is seven-coordinated and diamagnetic, whereas in oxidn. state II, the complex has an unbonded dangling carboxylate and is six-coordinated while still diamagnetic. With oxidn. state III, the coordination no. is halfway between the coordination of oxidn. states II and IV. Species generated in situ have also been characterized by spectroscopic, computational, and electrochem. techniques, together with the related species derived from a different degree of protonation and oxidn. states. 4H+ can be generated potentiometrically, or voltammetrically, from 32+, and both coexist in soln. While complex 32+ is not catalytically active, the catalytic performance of complex 4H+ is characterized by the foot of the wave anal., giving an impressive turnover frequency record of 8000 s-1 at pH 7.0 and 50 000 s-1 at pH 10.0. D. functional theory calcns. provide a complete description of the water oxidn. catalytic cycle of 4H+, manifesting the key functional role of the dangling carboxylate in lowering the activation free energies that lead to O-O bond formation.
35. 35

    Imidazole moieties from histidine-like H332 directly bound to Manganese 1 of the OEC and H337 located in the second coordination sphere both affect catalytic activity through changes in the degree of protonation

    (a) Retegan, M.; Neese, F.; Pantazis, D. A. Convergence of QM/MM and Cluster Models for the Spectroscopic Properties of the Oxygen-Evolving Complex in Photosystem II. J. Chem. Theory Comput. 2013, 9, 3832– 3842,  DOI: 10.1021/ct400477j

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    35a

    Convergence of QM/MM and cluster models for the spectroscopic properties of the oxygen-evolving complex in photosystem II

    Retegan, Marius; Neese, Frank; Pantazis, Dimitrios A.

    Journal of Chemical Theory and Computation (2013), 9 (8), 3832-3842CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)

    The latest crystal structure of photosystem II at 1.9 Å resoln., which resolves the topol. of the Mn4CaO5 oxygen evolving complex (OEC) at atomistic detail, enables a better correlation between structural features and spectroscopic properties than ever before. Here, building on the refined crystallog. model of the OEC and the protein, the authors present combined quantum mech./mol. mech. (QM/MM) studies of the spectroscopic properties of the natural catalyst embedded in the protein matrix. Focusing on the S2 state of the catalytic cycle, the authors examd. the convergence of not only structural parameters but also of the intracluster magnetic interactions in terms of exchange coupling consts. and of exptl. relevant 55Mn, 17O, and 14N hyperfine coupling consts. with respect to QM/MM partitioning using 5 QM regions of increasing size. This enabled the authors to assess the performance of the method and to probe 2nd sphere effects by identifying amino acid residues that principally affected the spectroscopic properties of the OEC. Comparison between QM-only and QM/MM treatments revealed that whereas QM/MM models converge quickly to stable values, the QM cluster models need to incorporate significantly larger parts of the second coordination sphere and surrounding water mols. to achieve convergence for certain properties. This was mainly due to the sensitivity of the QM-only models to fluctuations in the H-bonding network and ligand acidity. Addnl., a H-bond that is typically omitted in QM-only treatments is shown to det. the hyperfine coupling tensor of the unique Mn(III) ion by regulating the rotation plane of the ligated D1-His-332 imidazole ring, the only N-donor ligand of the OEC.

    (b) Askerka, M.; Brudvig, G. W.; Batista, V. S. The O₂-Evolving Complex of Photosystem II: Recent Insights from Quantum Mechanics/Molecular Mechanics (QM/MM), Extended X-Ray Absorption Fine Structure (EXAFS), and Femtosecond X-Ray Crystallography Data. Acc. Chem. Res. 2017, 50, 41– 48,  DOI: 10.1021/acs.accounts.6b00405

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    35b

    The O2-Evolving Complex of Photosystem II: Recent Insights from Quantum Mechanics/Molecular Mechanics (QM/MM), extended X-ray Absorption Fine Structure (EXAFS), and Femtosecond X-ray Crystallography Data

    Askerka, Mikhail; Brudvig, Gary W.; Batista, Victor S.

    Accounts of Chemical Research (2017), 50 (1), 41-48CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

    A review. Efficient photoelectrochem. water oxidn. may open a way to produce energy from renewable solar power. In biol., generation of fuel due to water oxidn. happens efficiently on an immense scale during the light reactions of photosynthesis. To oxidize water, photosynthetic organisms have evolved a highly conserved protein complex, photosystem II. Within that complex, water oxidn. occurs at the CaMn4O5 inorg. catalytic cluster, the so-called O2-evolving complex (OEC), which cycles through storage 'S' states as it accumulates oxidizing equiv and produces O2. In recent years, there has been significant progress in understanding the OEC as it evolves through the catalytic cycle. Studies have combined conventional and femtosecond x-ray crystallog. with EXAFS and QM/MM methods and have addressed changes in protonation states of μ-oxo bridges and the coordination of substrate water through the anal. of ammonia binding as a chem. analog of water. These advances are thought to be crit. to understanding the catalytic cycle since protonation states regulate the relative stability of different redox states and the geometry of the OEC. Therefore, establishing the mechanism for substrate water binding and the nature of protonation/redox state transitions in the OEC is essential for understanding the catalytic cycle of O2 evolution. Here, the authors summarize the most recent models of the OEC that have emerged from QM/MM, EXAFS, and femtosecond x-ray crystallog. methods.
36. 36

    Theerasilp, M.; Crespy, D. Halochromic Polymer Nanosensors for Simple Visual Detection of Local pH in Coatings. Nano Lett. 2021, 21, 3604– 3610,  DOI: 10.1021/acs.nanolett.1c00620

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    36

    Halochromic Polymer Nanosensors for Simple Visual Detection of Local pH in Coatings

    Theerasilp, Man; Crespy, Daniel

    Nano Letters (2021), 21 (8), 3604-3610CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)

    Replacing metallic structures before crit. damage is beneficial for safety and for saving energy and resources. One simple approach consists in visually monitoring the early stage of corrosion, and related change of pH, of coated metals. We prep. smart nanoparticle additives for coatings which act as a pH sensor. The nanoparticles are formed with a terpolymer contg. two dyes as side chains, acting as donor and acceptor for a FRET process. Real time monitoring of the extent of localized corrosion on metallic structures is then carried out with a smartphone camera. Colored pH mapping can be then manually retrieved by an operator or automatically recorded by a surveillance camera.
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    Chakraborty, S.; Nandi, S.; Bhattacharyya, K.; Mukherjee, S. Time Evolution of Local pH Around a Photo-Acid in Water and a Polymer Hydrogel: Time Resolved Fluorescence Spectroscopy of Pyranine. ChemPhysChem 2019, 20, 3221– 3227,  DOI: 10.1002/cphc.201900845

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    Time Evolution of Local pH Around a Photo-Acid in Water and a Polymer Hydrogel: Time Resolved Fluorescence Spectroscopy of Pyranine

    Chakraborty, Subhajit; Nandi, Somen; Bhattacharyya, Kankan; Mukherjee, Saptarshi

    ChemPhysChem (2019), 20 (23), 3221-3227CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)

    In this work, we propose a new anal. of the time resolved emission spectra of a photo-acid, HA, pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS) based on time resolved area normalized emission spectra (TRANES). Presence of an isoemissive point in TRANES confirms the presence of two emissive species (HA and A-) inside the system in bulk water and inside a co-polymer hydrogel [F127, (PEO)100-(PPO)70-(PEO)100]. We show that following electronic excitation, the local pH around HPTS, is much lower than the bulk pH presumably because of ejection of proton from the photo-acid in the excited state. With increase in time, the local pH increases and reaches the bulk value. We further, demonstrate that the excited state pKa of HPTS may be estd. from the emission intensities of HA and A- at long time. The time const. for time evolution of pH is ~ 630 ps in water, ~ 1300 ps in F127 gel and ~ 4700 ps in CTAB micelle. The location and local viscosity sensed by the probe is ascertained using fluorescence correlation spectroscopy (FCS) and fluorescence anisotropy decay. The different values of the local viscosity reported by these two methods are reconciled.