Periodic TableTalks: An Oasis of Science within a Conference Desert

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This article references 51 other publications.

1. 1

   Baumann, A. E.; Han, X.; Butala, M. M.; Thoi, V. S. Lithium Thiophosphate Functionalized Zirconium MOFs for Li–S Batteries with Enhanced Rate Capabilities. J. Am. Chem. Soc. 2019, 141, 17891– 17899,  DOI: 10.1021/jacs.9b09538

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   1

   Lithium Thiophosphate Functionalized Zirconium MOFs for Li-S Batteries with Enhanced Rate Capabilities

   Baumann, Avery E.; Han, Xu; Butala, Megan M.; Thoi, V. Sara

   Journal of the American Chemical Society (2019), 141 (44), 17891-17899CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   Zirconium metal-org. frameworks (Zr-MOFs) are renowned for their extraordinary stability and versatile chem. tunability. Several Zr-MOFs demonstrate a tolerance for missing linker defects, which create "open sites" that can be used to bind guest mols. on the node cluster. Herein, these sites are strategically utilized to stabilize reactive lithium thiophosphate (Li3PS4) within the porous framework for targeted application in lithium-sulfur (Li-S) batteries. Successful functionalization of the Zr-MOF with PS43- is confirmed by an array of techniques including NMR, XPS, and Raman spectroscopy, X-ray pair distribution function anal., and various elemental analyses. During electrochem. cycling, it was found that even a low incorporation extent of lithium thiophosphate in Zr-MOFs improves sulfur utilization and polysulfide encapsulation to deliver a sustainably high capacity over prolonged cycling. The functionalized MOF additives also prevent cell damage under abusive cycling conditions and recover high capacities when the cell is returned to lower charge/discharge rates, imperative for future energy storage devices. The unique approach marries the promising chem. attributes of the purely inorg. Li3PS4 with the stability and high surface area of MOFs, creating a Li-S cathode architecture with a performance beyond the sum of its component parts. More broadly, this novel functionalization strategy opens new avenues for facile syntheses of "designer materials" where chem. components from discrete disciplines can be united and tailored for specific applications.

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2. 2

   Burns, D. A.; Baumann, A. E.; Bennett, K. J.; Díaz, J. C.; Thoi, V. S. Chemical Sulfide Tethering Improves Low-Temperature Li–S Battery Cycling. ACS Appl. Mater. Interfaces 2021, 13, 50862– 50868,  DOI: 10.1021/acsami.1c12129

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   2

   Chemical Sulfide Tethering Improves Low-Temperature Li-S Battery Cycling

   Burns, David A.; Baumann, Avery E.; Bennett, Kevin J.; Diaz, Jose C.; Thoi, V. Sara

   ACS Applied Materials & Interfaces (2021), 13 (43), 50862-50868CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)

   Demands for energy storage and delivery continue to rise worldwide, making it imperative that reliable performance is achievable in diverse climates. Lithium-sulfur (Li-S) batteries offer a promising alternative to lithium-ion batteries owing to their substantially higher specific capacity and energy d. However, improvements to Li-S systems are still needed in low-temp. environments where polysulfide clustering and soly. limitations prohibit complete charge/discharge cycles. We address these issues by introducing thiophosphate-functionalized metal-org. frameworks (MOFs), capable of tethering polysulfides, into the cathode architecture. Compared to cells with the parent MOFs, cells contg. the functionalized MOFs exhibit greater capacity delivery and decreased polarization for a range of temps. down to -10°. We conduct thorough electrochem. analyses to ascertain the origins of performance differences and report an altered Li-S redox mechanism enabled by the thiophosphate moiety. This investigation is the first low-temp. Li-S study using MOF additives and represents a promising direction in enabling energy storage in extreme environments.

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3. 3

   Liu, B.; Thoi, V. S. Improving Charge Transfer in Metal–Organic Frameworks through Open Site Functionalization and Porosity Selection for Li–S Batteries. Chem. Mater. 2020, 32, 8450– 8459,  DOI: 10.1021/acs.chemmater.0c02438

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   3

   Improving Charge Transfer in Metal-Organic Frameworks through Open Site Functionalization and Porosity Selection for Li-S Batteries

   Liu, Bingqian; Thoi, V. Sara

   Chemistry of Materials (2020), 32 (19), 8450-8459CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)

   The tunable nature of metal-org. frameworks (MOFs) enables versatility and precise control over structures and properties, making them feasible for potential applications including gas storage and sepn., catalysis, mol. sensing, and energy storage. However, porous MOFs are typically insulating, greatly limiting their utility in electrochem. devices. Introducing redox activity to MOFs can promote charge conduction and provide insights into redox mechanisms in a multidimensional coordination system. Toward this end, we prepd. a series of anthraquinone (AQ)-functionalized zirconium MOFs (MOF-AQ) to investigate the relationship between porosity and charge transfer reactions using the canonical MOF-808 and NU-1000 frameworks. We evaluated the ability of these frameworks as sulfur host materials to promote polysulfide redox, which are crit. conversions for Li-S batteries. Li-S batteries are promising contenders as high-capacity energy storage devices, with an energy d. surpassing that of Li ion batteries. We found that the incorporation of AQ on the nodal structure leads to improvement in specific capacity, particularly at high charge and discharge rates. More importantly, enhanced electrochem. behavior of NU-1000-AQ over MOF-808-AQ suggests that larger pore aperture favors overall charge transfer and diffusion. Our study demonstrates there is a delicate balance between AQ loading and available pore vol. for ion flux to achieve optimized charge transfer efficiency under fast charge-discharge conditions. Our work provides insight for future designs of novel redox-active MOFs to facilitate charge transport in porous coordination networks.

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4. 4

   Saund, S. S.; Siegler, M. A.; Thoi, V. S. Electrochemical Degradation of a Dicationic Rhenium Complex via Hoffman-Type Elimination. Inorg. Chem. 2021, 60, 13011– 13020,  DOI: 10.1021/acs.inorgchem.1c01427

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   4

   Electrochemical Degradation of a Dicationic Rhenium Complex via Hoffman-Type Elimination

   Saund, Simran S.; Siegler, Maxime A.; Thoi, V. Sara

   Inorganic Chemistry (2021), 60 (17), 13011-13020CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

   Electrocatalytic redn. of CO2 (CO2) by transition-metal catalysts is an attractive means for storing renewably sourced electricity in chem. bonds. Metal coordination compds. represent highly tunable platforms ideal for studying the fundamental stepwise transformations of CO2 into its reduced products. However, metal complexes can decomp. upon extended electrolysis and form chem. distinct mol. species or, in some cases, catalytically active electrode deposits. Deciphering the degradative pathways is important for understanding the nature of the active catalyst and designing robust metal complexes for small-mol. activation. Herein, the authors present a new dicationic Re bipyridyl complex capable of multielectron ligand-centered redns. electrochem. The authors' in-depth exptl. and computational study provides mechanistic insight into an unusual reductively induced Hoffman-type elimination. The authors identify benzylic tertiary ammonium groups as an electrolytically susceptible moiety and propose key intermediates in the degradative pathway. This study highlights the complex interplay between the ligand and metal ion and will guide the future design of metal-org. catalysts.

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5. 5

   Mosby, J. M.; Prieto, A. L. Direct Electrodeposition of Cu₂Sb for Lithium-Ion Battery Anodes. J. Am. Chem. Soc. 2008, 130, 10656– 10661,  DOI: 10.1021/ja801745n

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   5

   Direct Electrodeposition of Cu2Sb for Lithium-Ion Battery Anodes

   Mosby, James M.; Prieto, Amy L.

   Journal of the American Chemical Society (2008), 130 (32), 10656-10661CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   We describe the direct single potential electrodeposition of cryst. Cu2Sb, a promising anode material for lithium-ion batteries, from aq. solns. at room temp. The use of citric acid as a complexing agent increases the soly. of antimony salts and shifts the redn. potentials of copper and antimony toward each other, enabling the direct deposition of the intermetallic compd. at pH 6. Electrodeposition of Cu2Sb directly onto conducting substrates represents a facile synthetic method for the synthesis of high quality samples with excellent elec. contact to a substrate, which is crit. for further battery testing.

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6. 6

   Kraynak, L. A.; Schneider, J. D.; Prieto, A. L. Exploring the Role of Vinylene Carbonate in the Passivation and Capacity Retention of Cu₂Sb Thin Film Anodes. J. Phys. Chem. C 2020, 124, 26083– 26093,  DOI: 10.1021/acs.jpcc.0c04064

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   Exploring the Role of Vinylene Carbonate in the Passivation and Capacity Retention of Cu2Sb Thin Film Anodes

   Kraynak, Leslie A.; Schneider, Jacob D.; Prieto, Amy L.

   Journal of Physical Chemistry C (2020), 124 (48), 26083-26093CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

   Electrolyte additives such as vinylene carbonate (VC) have been demonstrated to improve the capacity retention for many types of Li-ion battery electrodes, including intermetallic alloying anodes, but it is still unclear why VC extends the cycle lifetime of copper antimonide (Cu2Sb) anodes so dramatically. Here, we have studied how VC affects the solid electrolyte interface formed on Cu2Sb thin film anodes in fluorine-free electrolyte solns. in order to better understand which nonfluorinated species may play an important role in effective Cu2Sb passivation. Using differential capacity anal. and XPS, it was found that VC effectively passivates Cu2Sb and prevents Cu/Cu2Sb oxidn. at high potentials. Carbonate species from the redn. of VC seem to play an important role in passivation, while inorg. species like LiClO4 from the F-free supporting electrolyte do not seem to be beneficial.

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7. 7

   Amy Prieto is building safer, more powerful batteries. https://cen.acs.org/energy/energy-storage-/Amy-Prieto-is-building-safer-more-powerful-batteries/98/i9 (accessed Feb 20, 2022).

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8. 8

   Arthur, T. S.; Bates, D. J.; Cirigliano, N.; Johnson, D. C.; Malati, P.; Mosby, J. M.; Perre, E.; Rawls, M. T.; Prieto, A. L.; Dunn, B. Three-dimensional electrodes and battery architectures. MRS Bull. 2011, 36, 523– 531,  DOI: 10.1557/mrs.2011.156

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   8

   Three-dimensional electrodes and battery architectures

   Arthur, Timothy S.; Bates, Daniel J.; Cirigliano, Nicolas; Johnson, Derek C.; Malati, Peter; Mosby, James M.; Perre, Emilie; Rawls, Matthew T.; Prieto, Amy L.; Dunn, Bruce

   MRS Bulletin (2011), 36 (7), 523-531CODEN: MRSBEA; ISSN:0883-7694. (Materials Research Society)

   A review. Three-dimensional (3D) battery architectures have emerged as a new direction for powering microelectromech. systems and other small autonomous devices. Although there are few examples to date of fully functioning 3D batteries, these power sources have the potential to achieve high power d. and high energy d. in a small footprint. This overview highlights the various architectures proposed for 3D batteries, the advances made in the fabrication of components designed for these devices, and the remaining tech. challenges. Efforts directed at establishing design rules for 3D architectures and modeling are providing insight concerning the energy d. and current uniformity achievable with these architectures. The significant progress made on the fabrication of electrodes and electrolytes designed for 3D batteries is an indication that a no. of these battery architectures will be successfully demonstrated within the next few years.

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9. 9

   Miller, R. C.; Neilson, J. R.; Prieto, A. L. Amide-Assisted Synthesis of Iron Germanium Sulfide (Fe₂GeS₄) Nanostars: The Effect of LiN(SiMe₃)₂ on Precursor Reactivity for Favoring Nanoparticle Nucleation or Growth. J. Am. Chem. Soc. 2020, 142, 7023– 7035,  DOI: 10.1021/jacs.0c00260

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   9

   Amide-Assisted Synthesis of Iron Germanium Sulfide (Fe2GeS4) Nanostars: The Effect of LiN(SiMe3)2 on Precursor Reactivity for Favoring Nanoparticle Nucleation or Growth

   Miller, Rebecca C.; Neilson, James R.; Prieto, Amy L.

   Journal of the American Chemical Society (2020), 142 (15), 7023-7035CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   Olivine Fe2GeS4 has been identified as a promising photovoltaic absorber material introduced as an alternate candidate to iron pyrite, FeS2. The compds. share similar benefits in terms of elemental abundance and relative nontoxicity, but Fe2GeS4 was predicted to have higher stability with respect to decompn. to alternate phases and, therefore, more optimal device performance. Our initial report of the nanoparticle (NP) synthesis for Fe2GeS4 was not well understood and required an inefficient 24 h growth to dissolve an iron sulfide impurity. Here, we report an amide-assisted Fe2GeS4 NP synthesis that directly forms the phase-pure product in minutes. This significant advance was achieved by the replacement of the poorly understood hexamethyldisilazane (HMDS) additive and TMS2S by the conjugate base, lithium bis(trimethylsilyl)amide (LiN(SiMe3)2), and elemental S, resp. We hypothesized that fragments of both TMS2S and HMDS had carried out the roles that Bronsted bases play in amide-assisted NP syntheses and were necessary for Ge incorporation. Convolution of this role with the supply of S in TMS2S caused the iron sulfide impurities. Sepg. these effects in the use of LiN(SiMe3)2 and elemental S resulted in synthetic control over the ternary phase. Herein we explore the Fe-Ge-S reaction landscape and the role of the base. Its concn. was found to increase the reactivities of the Fe, Ge, and S precursors, and we discuss possible metal-amide intermediates. This affords tunability in two areas: favorability of NP nucleation vs. growth and phase formation. The phase-purity of Fe2GeS4 depends on the molar ratios of the cations, base, and amine as well as the Fe:Ge:S molar ratios. The resultant Fe2GeS4 NPs exhibit an interesting star anise-like morphol. with stacks of nanoplates that intersect along a 6-fold rotation axis. The optical properties of the Fe2GeS4 NPs are consistent with previously published measurements showing a measured band gap of 1.48 eV.

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10. 10

    Miller, R. C.; Geiss, R. H.; Prieto, A. L. Olivine Crystal Structure-Directed Twinning in Iron Germanium Sulfide (Fe₂GeS₄) Nanoparticles. ACS Nano 2021, 15, 11981– 11991,  DOI: 10.1021/acsnano.1c03237

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    10

    Olivine Crystal Structure-Directed Twinning in Iron Germanium Sulfide (Fe2GeS4) Nanoparticles

    Miller, Rebecca C.; Geiss, Roy H.; Prieto, Amy L.

    ACS Nano (2021), 15 (7), 11981-11991CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)

    Understanding the microstructure of complex crystal structures is crit. for controlling material properties in next-generation devices. Synthetic reports of twinning in bulk and nanostructured crystals with detailed crystallog. characterization are integral for advancing systematic studies of twinning phenomena. Herein, we report a synthetic route to controllably twinned olivine nanoparticles. Microstructural characterization of Fe2GeS4 nanoparticles via electron microscopy (imaging, diffraction, and crystallog. anal.) demonstrates the formation of triplets of twins, or trillings. We establish synthetic control over the particle crystallinity and crystal growth. We describe the geometrical basis for twin formation, hexagonal pseudosymmetry of the orthorhombic lattice, and rank all of the reported olivine compds. according to this favorability to form twins. The work in this study highlights an area ripe for future exploration with respect to the advancement of soln.-phase synthetic approaches that can control microstructure in compositionally complex, technol. relevant structures. Finally, we discuss the potential implications for olivine properties and performance in various applications.

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    Lin, Q.; Fu, Y.; Liu, P.; Diao, T. Monovalent Nickel-Mediated Radical Formation: A Concerted Halogen-Atom Dissociation Pathway Determined by Electroanalytical Studies. J. Am. Chem. Soc. 2021, 143, 14196– 14206,  DOI: 10.1021/jacs.1c05255

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    11

    Monovalent Nickel-Mediated Radical Formation: A Concerted Halogen-Atom Dissociation Pathway Determined by Electroanalytical Studies

    Lin, Qiao; Fu, Yue; Liu, Peng; Diao, Tianning

    Journal of the American Chemical Society (2021), 143 (35), 14196-14206CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    The recent success of nickel catalysts in stereoconvergent cross-coupling and cross-electrophile coupling reactions partly stems from the ability of monovalent nickel species to activate C(sp3) electrophiles and generate radical intermediates. This electroanal. study of the commonly applied (bpy)Ni catalyst elucidates the mechanism of this crit. step. Data rule out outer-sphere electron transfer and two-electron oxidative addn. pathways. The linear free energy relationship between rates and the bond-dissocn. free energies, the electronic and steric effects of the nickel complexes and the electrophiles, and DFT calcns. support a variant of the halogen-atom abstraction pathway, the inner-sphere electron transfer concerted with halogen-atom dissocn. This mechanism accounts for the obsd. reactivity of different electrophiles in cross-coupling reactions and provides a mechanistic rationale for the chemoselectivity obtained in cross-electrophile coupling over homocoupling.

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12. 12

    Lin, Q.; Diao, T. Mechanism of Ni-Catalyzed Reductive 1,2-Dicarbofunctionalization of Alkenes. J. Am. Chem. Soc. 2019, 141, 17937– 17948,  DOI: 10.1021/jacs.9b10026

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    12

    Mechanism of Ni-Catalyzed Reductive 1,2-Dicarbofunctionalization of Alkenes

    Lin, Qiao; Diao, Tianning

    Journal of the American Chemical Society (2019), 141 (44), 17937-17948CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Ni-catalyzed cross-electrophile coupling reactions have emerged as appealing methods to construct org. mols. without the use of stoichiometric organometallic reagents. The mechanisms are complex: plausible pathways, such as "radical chain" and "sequential redn." mechanisms, are dependent on the sequence of the activation of electrophiles. A combination of kinetic, spectroscopic, and organometallic studies reveals that a Ni-catalyzed, reductive 1,2-dicarbofunctionalization of alkenes proceeds through a "sequential redn." pathway. The redn. of Ni by Zn is the turnover-limiting step, consistent with Ni(II) intermediates as the catalyst resting-state. Zn is only sufficient to reduce (phen)Ni(II) to a Ni(I) species. As a result, commonly proposed Ni(0) intermediates are absent under these conditions. (Phen)Ni(I)-Br selectively activates aryl bromides via two-electron oxidn. addn., whereas alkyl bromides are activated by (phen)Ni(I)-Ar through single-electron activation to afford radicals. These findings could provide insight into achieving selectivity between different electrophiles.

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13. 13

    Diccianni, J.; Lin, Q.; Diao, T. Mechanisms of Nickel-Catalyzed Coupling Reactions and Applications in Alkene Functionalization. Acc. Chem. Res. 2020, 53, 906– 919,  DOI: 10.1021/acs.accounts.0c00032

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    13

    Mechanisms of Nickel-Catalyzed Coupling Reactions and Applications in Alkene Functionalization

    Diccianni, Justin; Lin, Qiao; Diao, Tianning

    Accounts of Chemical Research (2020), 53 (4), 906-919CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

    A review. Nickel complexes exhibit distinct properties from other group 10 metals, including a small nuclear radius, high paring energy, low electronegativity, and low redox potentials. These properties enable Ni catalysts to accommodate and stabilize paramagnetic intermediates, access radical pathways, and undergo slow β-H elimination. Our research program investigates how each of these fundamental attributes impact the catalytic properties of Ni, in particular in the context of alkene functionalization. Alkenes are versatile functional groups, but stereoselective carbofunctionalization reactions of alkenes have been underdeveloped. This challenge may derive from the difficulty of controlling selectivity via traditional two-electron migratory insertion pathways. Ni catalysts could lead to different stereodetermining steps via radical mechanisms, allowing access to mol. scaffolds that are otherwise difficult to prep. For example, an asym. alkene diarylation reaction developed by our group relies upon the radical properties of Ni(III) intermediates to control the enantioselectivity and give access to a library of chiral α,α,β-triarylethane mols. with biol. activity. Mechanistic studies on a two-component reductive 1,2-difunctionalization reaction have shed light on the origin of the cross-electrophile selectivity, as C sp2 and C sp3 electrophiles are independently activated at Ni(I) via two-electron and radical pathways, resp. Catalyst redn. has been identified to be the turnover-limiting step in this system. A closer investigation of the radical formation step using a (Xantphos)Ni(I)Ar model complex reveals that Ni(I) initiates radical formation via a concerted halogen-abstraction pathway. The low redox potentials of Ni have allowed us to develop a reductive, trans-selective diene cyclization, wherein a classic two-electron mechanism operates on a Ni(I)/Ni(III) platform, accounting for the chemo- and stereoselectivity. This reaction has found applications in the efficient synthesis of pharmaceutically relevant mols., such as 3,4-dimethylgababutin. The tendency of Ni to undergo one-electron redox processes prompted us to explore dinuclear Ni-mediated bond formations. These studies provide insight into Ni-Ni bonding and how two metal centers react cooperatively to promote C-C, C-X, and N-N bond forming reductive elimination. Finally, isolation of β-agostic Ni and Pd complexes has allowed for X-ray and neutron diffraction characterization of these highly reactive mols. The bonding parameters serve as unambiguous evidence for β-agostic interactions and help rationalize the slower β-H elimination at Ni relative to Pd. Overall, our research has elucidated the fundamental properties of Ni complexes in several contexts. Greater mechanistic understanding facilitates catalyst design and helps rationalize the reactivity and selectivity in Ni-catalyzed alkene functionalization reactions.

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14. 14

    Ju, L.; Lin, Q.; LiBretto, N. J.; Wagner, C. L.; Hu, C. T.; Miller, J. T.; Diao, T. Reactivity of (bi-Oxazoline)organonickel Complexes and Revision of a Catalytic Mechanism. J. Am. Chem. Soc. 2021, 143, 14458– 14463,  DOI: 10.1021/jacs.1c07139

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    14

    Reactivity of (bi-Oxazoline)organonickel Complexes and Revision of a Catalytic Mechanism

    Ju, Luchuan; Lin, Qiao; LiBretto, Nicole J.; Wagner, Clifton L.; Hu, Chunhua Tony; Miller, Jeffrey T.; Diao, Tianning

    Journal of the American Chemical Society (2021), 143 (36), 14458-14463CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Bi-Oxazoline (biOx) has emerged as an effective ligand framework for promoting nickel-catalyzed cross-coupling, cross-electrophile coupling, and photoredox-nickel dual catalytic reactions. This report fills the knowledge gap of the organometallic reactivity of (biOx)Ni complexes, including catalyst redn., oxidative electrophile activation, radical capture, and reductive elimination. The biOx ligand displays no redox activity in (biOx)Ni(I) complexes, in contrast to other chelating imine and oxazoline ligands. The lack of ligand redox activity results in more neg. redn. potentials of (biOx)Ni(II) complexes and accounts for the inability of zinc and manganese to reduce (biOx)Ni(II) species. On the basis of these results, we revise the formerly proposed "sequential redn." mechanism of a (biOx)Ni-catalyzed cross-electrophile coupling reaction by excluding catalyst redn. steps.

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15. 15

    Schultz, J. W.; Rath, N. P.; Mirica, L. M. Improved Oxidative C–C Bond Formation Reactivity of High-Valent Pd Complexes Supported by a Pseudo-Tridentate Ligand. Inorg. Chem. 2020, 59, 11782– 11792,  DOI: 10.1021/acs.inorgchem.0c01763

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    15

    Improved Oxidative C-C Bond Formation Reactivity of High-Valent Pd Complexes Supported by a Pseudo-Tridentate Ligand

    Schultz, Jason W.; Rath, Nigam P.; Mirica, Liviu M.

    Inorganic Chemistry (2020), 59 (16), 11782-11792CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    There is a large interest in developing oxidative transformations catalyzed by palladium complexes that employ environmentally friendly and economical oxidizing reagents such as dioxygen. Recently, we have reported the isolation and characterization of various mononuclear PdIII and PdIV complexes supported by the tetradentate ligands N,N'-dialkyl-2,11-diaza[3.3](2,6)pyridinophane (RN4, R = tBu, iPr, Me), and the aerobically induced C-C and C-heteroatom bond formation reactivity was investigated in detail. Given that the steric and electronic properties of the multidentate ligands were shown to tune the stability and reactivity of the corresponding high-valent Pd complexes, herein we report the use of an asym. N4 ligand, N-methyl-N'-tosyl-2,11-diaza[3.3](2,6)pyridinophane (TsMeN4), in which one amine N atom contains a tosyl group. The N-Ts donor atom exhibits a markedly reduced donating ability, which led to the formation of transiently stable PdIII and PdIV complexes, and consequently the corresponding O2 oxidn. reactivity and the subsequent C-C bond formation were improved significantly. These results suggest that altering the electron donating ability as well as the denticity of the multidentate ligand employed can finely tune the oxidatively induced reactivity of high-valent Pd complexes.

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16. 16

    Luo, J.; Tran, G. N.; Rath, N. P.; Mirica, L. M. Detection and Characterization of Mononuclear Pd(I) Complexes Supported by N2S2 and N4 Tetradentate Ligands. Inorg. Chem. 2020, 59, 15659– 15669,  DOI: 10.1021/acs.inorgchem.0c01938

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    16

    Detection and Characterization of Mononuclear Pd(I) Complexes Supported by N2S2 and N4 Tetradentate Ligands

    Luo, Jia; Tran, Giang N.; Rath, Nigam P.; Mirica, Liviu M.

    Inorganic Chemistry (2020), 59 (21), 15659-15669CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    Palladium is a versatile transition metal used to catalyze a large no. of chem. transformations, largely due to its ability to access various oxidn. states (0, I, II, III, and IV). Among these oxidn. states, Pd(I) is arguably the least studied, and while dinuclear Pd(I) complexes are more common, mononuclear Pd(I) species are very rare. Reported herein are spectroscopic studies of a series of Pd(I) intermediates generated by the chem. redn. at low temps. of Pd(II) precursors supported by the tetradentate ligands 2,11-dithia[3.3](2,6)pyridinophane (N2S2) and N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4): [(N2S2)PdII(MeCN)]2(OTf)4 (1), [(N2S2)PdIIMe]2(OTf)2 (2), [(N2S2)PdIICl](OTf) (3), [(N2S2)PdIIX](OTf)2 (X = tBuNC 4, PPh3 5), [(N2S2)PdIIMe(PPh3)](OTf) (6), and [(tBuN4)PdIIX2](OTf)2 (X = MeCN 8, tBuNC 9). In addn., a stable Pd(I) dinuclear species, [(N2S2)PdI(μ-tBuNC)]2(ClO4)2 (7), was isolated upon the electrochem. redn. of 4 and structurally characterized. Moreover, the (tBuN4)PdI intermediates, formed from the chem. redn. of [(tBuN4)PdIIX2](OTf)2 (X = MeCN 8, tBuNC 9) complexes, were investigated by EPR spectroscopy, x-ray absorption spectroscopy (XAS), and DFT calcns. and compared with the analogous (N2S2)PdI systems. Upon probing the stability of Pd(I) species under different ligand environments, it is apparent that the presence of soft ligands such as tBuNC and PPh3 significantly improves the stability of Pd(I) species, which should make the isolation of mononuclear Pd(I) species possible. Several mononuclear Pd(I) complexes supported by the tetradentate ligands 2,11-dithia[3.3](2,6)pyridinophane (N2S2) and N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4) ligands were generated and characterized by EPR, UV-vis, XAS, and DFT calcns. Upon probing the stability of Pd(I) species under different ligand environments, the presence of soft ligands such as tBuNC and PPh3 significantly improves the stability of Pd(I) species, which should make the isolation of mononuclear Pd(I) species possible.

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    Sinha, S.; Mirica, L. M. Electrocatalytic O₂ Reduction by an Organometallic Pd(III) Complex via a Binuclear Pd(III) Intermediate. ACS Catal. 2021, 11, 5202– 5211,  DOI: 10.1021/acscatal.0c05726

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    17

    Electrocatalytic O2 Reduction by an Organometallic Pd(III) Complex via a Binuclear Pd(III) Intermediate

    Sinha, Soumalya; Mirica, Liviu M.

    ACS Catalysis (2021), 11 (9), 5202-5211CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    The development of electrocatalysts for the selective O2-to-H2O conversion, the O2 redn. reaction (ORR), is of great interest for improving the performance of fuel cells. In this context, mol. catalysts that are known to mediate the 4H+/4e- redn. of O2 to H2O tend to be marred by limited stability and selectivity in controlling the multiproton and multielectron transfer steps. Thus, evaluation of transition metal complexes, including organometallic species, for ORR reactivity could uncover mol. catalysts with improved properties. We have previously reported the synthesis and characterization of various organometallic PdIII complexes stabilized by the tetradentate ligand N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4). These complexes were shown to react with O2 and undergo oxidatively induced C-C and C-heteroatom bond formation reactions in the presence of O2. These O2-induced oxidative transformations prompted us to evaluate the ORR reactivity of such organometallic Pd complexes, which to the best of our knowledge has never been studied before for any mol. Pd catalyst. Herein, we report the ORR reactivity of the [(tBuN4)PdIIIMeCl]+ complex under both homogeneous and heterogeneous conditions in a nonaq. and acidic aq. electrolyte, resp. Cyclic voltammetry and hydrodynamic electrochem. studies for [(tBuN4)PdIIIMeCl]+ revealed that the electrocatalytic redn. of O2 to H2O proceeds with Faradaic efficiencies (FEs) of 50-70% in the presence of acetic acid (AcOH) in MeCN. The selectivity toward H2O prodn. further improved the FE to 80-90% in an acidic aq. medium (pH 0), upon immobilization of the mol. catalyst onto edge plane graphite (EPG) electrodes. Anal. of electrochem. data suggests the formation of a binuclear PdIII intermediate in soln., likely a PdIII-peroxo-PdIII species, which dictates the thermochem. of the ORR process for [(tBuN4)PdIIIMeCl]+ in MeCN, thus being a rare example of a bimol. ORR process. The max. second-order turnover frequency TOFmax2 = 2.76 x 108 M-1 s-1 was detd. for 0.32 mM of [(tBuN4)PdIIIMeCl]+ in the presence of 1 M AcOH in O2-satd. MeCN with an overpotential of 0.32 V. By comparison, a comparatively lower TOFmax2 = 1.25 x 105 M-1 s-1 at a higher overpotential of 0.8 V was obsd. for [(tBuN4)PdIIIMeCl]PF6 adsorbed onto EPG electrodes in O2-satd. 1 M H2SO4 aq. soln. Overall, reported herein is a detailed ORR reactivity study using a PdIII organometallic complex to benchmark its selectivity and energetics toward O2 redn. in MeCN and acidic aq. solns.

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18. 18

    Gvozdetskyi, V.; Lee, S. J.; Owens-Baird, B.; Dolyniuk, J.-A.; Cox, T.; Wang, R.; Lin, Z.; Ho, K.-M.; Zaikina, J. V. Ternary Zinc Antimonides Unlocked Using Hydride Synthesis. Inorg. Chem. 2021, 60, 10686– 10697,  DOI: 10.1021/acs.inorgchem.1c01381

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    18

    Ternary Zinc Antimonides Unlocked Using Hydride Synthesis

    Gvozdetskyi, Volodymyr; Lee, Shannon J.; Owens-Baird, Bryan; Dolyniuk, Juli-Anna; Cox, Tori; Wang, Renhai; Lin, Zijing; Ho, Kai-Ming; Zaikina, Julia V.

    Inorganic Chemistry (2021), 60 (14), 10686-10697CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    Three new sodium zinc antimonides Na11Zn2Sb5, Na4Zn9Sb9, and NaZn3Sb3 were synthesized utilizing sodium hydride NaH as a reactive sodium source. In comparison to the synthesis using sodium metal, salt-like NaH can be ball-milled, leading to the easy and uniform mixing of precursors in the desired stoichiometric ratios. Such comprehensive compositional control enables a fast screening of the Na-Zn-Sb system and identification of new compds., followed by their prepn. in bulk with high purity. Na11Zn2Sb5 crystallizes in the triclinic P1 space group (No. 2, Z = 2, a = 8.8739(6) Å, b = 10.6407(7) Å, c = 11.4282(8) Å, α = 103.453(2)°, β = 96.997(2)°, γ = 107.517(2)°) and features polyanionic [Zn2Sb5]11- clusters with unusual 3-coordinated Zn atoms. Both Na4Zn9Sb9 (Z = 4, a = 28.4794(4) Å, b = 4.47189(5) Å, c = 17.2704(2) Å, β = 98.3363(6)°) and NaZn3Sb3 (Z = 8, a = 32.1790(1) Å, b = 4.51549(1) Å, c = 9.64569(2) Å, β = 98.4618(1)°) crystallize in the monoclinic C2/m space group (No. 12) and have complex new structure types. For both compds., their frameworks are built from ZnSb4 distorted tetrahedra, which are linked via edge-, vertex-sharing, or both, while Na cations fill in the framework channels. Due to the complex structures, Na4Zn9Sb9 and NaZn3Sb3 compds. exhibit low thermal conductivities (0.97-1.26 W·m-1 K-1) at room temp., pos. Seebeck coeffs. (19-32μV/K) suggestive of holes as charge carriers, and semimetallic elec. resistivities (~ 1.0-2.3 x 10-4 Ω·m). Na4Zn9Sb9 and NaZn3Sb3 decomp. into the equiat. NaZnSb above ~ 800 K, as detd. by in situ synchrotron powder x-ray diffraction. The discovery of multiple ternary compds. highlights the importance of judicious choice of the synthetic method.

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19. 19

    Gvozdetskyi, V.; Owens-Baird, B.; Hong, S.; Cox, T.; Bhaskar, G.; Harmer, C.; Sun, Y.; Zhang, F.; Wang, C.-Z.; Ho, K.-M. From NaZn₄Sb₃ to HT-Na_1–xZn_4–ySb₃: Panoramic Hydride Synthesis, Structural Diversity, and Thermoelectric Properties. Chem. Mater. 2019, 31, 8695– 8707,  DOI: 10.1021/acs.chemmater.9b02239

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    19

    From NaZn4Sb3 to HT-Na1-xZn4-ySb3: Panoramic hydride synthesis, structural diversity, and thermoelectric properties

    Gvozdetskyi, Volodymyr; Owens-Baird, Bryan; Hong, Sangki; Cox, Tori; Bhaskar, Gourab; Harmer, Colin; Sun, Yang; Zhang, Feng; Wang, Cai-Zhuang; Ho, Kai-Ming; Zaikina, Julia V.

    Chemistry of Materials (2019), 31 (21), 8695-8707CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)

    Two new sodium zinc antimonides NaZn4Sb3 and HT-Na1-xZn4-ySb3 were synthesized by using reactive sodium hydride, NaH, as a precursor. The hydride route provides uniform mixing and comprehensive control over the compn., facilitating fast reactions and high-purity samples, whereas traditional synthesis using sodium metal results in inhomogeneous samples with a significant fraction of the more stable NaZnSb compd. NaZn4Sb3 crystallizes in the hexagonal P63/mmc space group (No. 194, Z = 2, a = 4.43579(4) Å, c = 23.41553(9) Å) and is stable upon heating in vacuum up to 736 K. The layered crystal structure of NaZn4Sb3 is related to the structure of the well-studied thermoelec. antimonides AeZn2Sb2 (Ae = Ca, Sr, Eu). Upon heating in vacuum, NaZn4Sb3 transforms to HT-Na1-xZn4-ySb3 (x = 0.047(3), y = 0.135(1)) due to partial Na/Zn evapn./elimination, as was detd. from high-temp. in situ synchrotron powder X-ray diffraction. HT-Na1-xZn4-ySb3 has a complex monoclinic structure with considerable degrees of structural disorder (P21/c (No. 14), Z = 32, a = 19.5366(7) Å, b = 14.7410(5) Å, c = 20.7808(7) Å, β = 90.317(2)°) and is stable exclusively in a narrow temp. range of 736-885 K. Further heating of HT-Na1-xZn4-ySb3 leads to a reversible transformation to NaZnSb above 883 K. Both compds. exhibit similarly low thermal cond. at room temp. (0.9 W m-1 K-1) and pos. Seebeck coeffs. (38-52μV/K) indicative of holes as the main charge carriers. However, resistivities of the two phases differ by 2 orders of magnitude.

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20. 20

    Bhaskar, G.; Gvozdetskyi, V.; Batuk, M.; Wiaderek, K. M.; Sun, Y.; Wang, R.; Zhang, C.; Carnahan, S. L.; Wu, X.; Ribeiro, R. A. Topochemical Deintercalation of Li from Layered LiNiB: toward 2D MBene. J. Am. Chem. Soc. 2021, 143, 4213– 4223,  DOI: 10.1021/jacs.0c11397

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    20

    Topochemical deintercalation of Li from layered LiNiB: toward 2D MBene

    Bhaskar, Gourab; Gvozdetskyi, Volodymyr; Batuk, Maria; Wiaderek, Kamila M.; Sun, Yang; Wang, Renhai; Zhang, Chao; Carnahan, Scott L.; Wu, Xun; Ribeiro, Raquel A.; Bud'ko, Sergey L.; Canfield, Paul C.; Huang, Wenyu; Rossini, Aaron J.; Wang, Cai-Zhuang; Ho, Kai-Ming; Hadermann, Joke; Zaikina, Julia V.

    Journal of the American Chemical Society (2021), 143 (11), 4213-4223CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    The pursuit of two-dimensional (2D) borides, MBenes, has proven to be challenging, not the least because of the lack of a suitable precursor prone to the deintercalation. Here, we studied room-temp. topochem. deintercalation of lithium from the layered polymorphs of the LiNiB compd. with a considerable amt. of Li stored in between [NiB] layers (33 at. % Li). Deintercalation of Li leads to novel metastable borides (Li~ 0.5NiB) with unique crystal structures. Partial removal of Li is accomplished by exposing the parent phases to air, water, or dil. HCl under ambient conditions. Scanning transmission electron microscopy and solid-state 7Li and 11B NMR spectroscopy, combined with X-ray pair distribution function (PDF) anal. and DFT calcns., were utilized to elucidate the novel structures of Li~ 0.5NiB and the mechanism of Li-deintercalation. We have shown that the deintercalation of Li proceeds via a "zip-lock" mechanism, leading to the condensation of single [NiB] layers into double or triple layers bound via covalent bonds, resulting in structural fragments with Li[NiB]2 and Li[NiB]3 compns. The crystal structure of Li~ 0.5NiB is best described as an intergrowth of the ordered single [NiB], double [NiB]2, or triple [NiB]3 layers alternating with single Li layers; this explains its structural complexity. The formation of double or triple [NiB] layers induces a change in the magnetic behavior from temp.-independent paramagnets in the parent LiNiB compds. to the spin-glassiness in the deintercalated Li~ 0.5NiB counterparts. LiNiB compds. showcase the potential to access a plethora of unique materials, including 2D MBenes (NiB).

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21. 21

    Weiland, A.; Felder, J. B.; McCandless, G. T.; Chan, J. Y. One Ce, Two Ce, Three Ce, Four? An Intermetallic Homologous Series to Explore: A_n+1B_nX_3n+1. Chem. Mater. 2020, 32, 1575– 1580,  DOI: 10.1021/acs.chemmater.9b04743

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    21

    One Ce, Two Ce, Three Ce, Four? An Intermetallic Homologous Series to Explore: An+1BnX3n+1

    Weiland, Ashley; Felder, Justin B.; McCandless, Gregory T.; Chan, Julia Y.

    Chemistry of Materials (2020), 32 (4), 1575-1580CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)

    Low-dimensional solids are highly anisotropic by nature and show promise as new quantum materials, leading to exotic phys. properties not realized in three-dimensional materials. To discover correlations in low-dimensional systems, studying robust crystal structures that allow for chem. tuning is crit. for optimizing materials properties. In our search for novel quantum intermetallic materials, we discovered a new homologous series, An+1BnX3n+1 (A = rare earth; B = transition metal; X = tetrels; n = 1-5) which crystallizes in orthorhombic space groups Cmmm (for odd "n") and Cmcm (for even "n"). This series, best characterized by the stacking of structural subunits of AlB2, AuCu3, and BaNiSn3, represents a bulk architecture of highly correlated quantum materials. Though not a conventional "low dimensional" material with a van der Waals gap, the lattice parameters of the members of this series have a high aspect ratio (b/a) and can systematically be "tuned" as a function of dimensionality. This new homologous series can serve as a robust intermetallic system to study collective phenomena in quantum materials.

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22. 22

    Weiland, A.; Frith, M. G.; Lapidus, S. H.; Chan, J. Y. In Situ Methods for Metal-Flux Synthesis in Inert Environments. Chem. Mater. 2021, 33, 7657– 7664,  DOI: 10.1021/acs.chemmater.1c02413

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    22

    In Situ Methods for Metal-Flux Synthesis in Inert Environments

    Weiland, Ashley; Frith, Matthew G.; Lapidus, Saul H.; Chan, Julia Y.

    Chemistry of Materials (2021), 33 (19), 7657-7664CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)

    Flux growth synthesis is an advantageous synthetic method as it allows for the growth of single crystals of both congruently melting and metastable phases. The detn. of synthetic parameters for the flux growth of new cryst. phases is complex as many factors and parameters need to be considered, such as the purity and morphol. of the starting material and heating profile variables including max. temp., dwell time, cooling rate, and flux removal temp. In situ monitoring of crystallite growth can lead to elucidation of reaction intermediates and growth mechanisms. The detn. of pivotal reaction parameters can revolutionize the way growth parameters are selected. Herein, we report a new sample environment and furnace app. for synchrotron in situ synthesis of cryst. materials, including flux grown intermetallics.

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23. 23

    Kyrk, T. M.; Scheifers, J. P.; Thanabalasingam, K.; McCandless, G. T.; Young, D. P.; Chan, J. Y. It Runs in the BaAl₄ Family: Relating the Structure and Properties of Middle Child Ln₂Co₃Ge₅ (Ln = Pr, Nd, and Sm) to its Siblings LnCo₂Ge₂ and LnCoGe₃. Inorg. Chem. 2021, 60, 15343– 15350,  DOI: 10.1021/acs.inorgchem.1c01978

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    23

    It Runs in the BaAl4 Family: Relating the Structure and Properties of Middle Child Ln2Co3Ge5 (Ln = Pr, Nd, and Sm) to its Siblings LnCo2Ge2 and LnCoGe3

    Kyrk, Trent M.; Scheifers, Jan P.; Thanabalasingam, Kulatheepan; McCandless, Gregory T.; Young, David P.; Chan, Julia Y.

    Inorganic Chemistry (2021), 60 (20), 15343-15350CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    The BaAl4 prototype structure and its derivs. have been identified to host several topol. quantum materials and noncentrosym. superconductors. Single crystals up to ~ 3 mm x 3 mm x 5 mm of Ln2Co3Ge5 (Ln = Pr, Nd, and Sm) are obtained via flux growth utilizing Sn as metallic flux. The crystal structure is isostructural to the Lu2Co3Si5 structure type in the crystallog. space group C2/c. The temp.-dependent magnetization indicates magnetic ordering at 30 K for all three compds. Pr2Co3Ge5 and Nd2Co3Ge5 exhibit complex magnetic behavior with spin reorientations before ordering antiferromagnetically around 6 K, whereas Sm2Co3Ge5 shows a clear antiferromagnetic behavior at 26 K. The structures and properties of Ln2Co3Ge5 (Ln = Pr, Nd, and Sm) are compared to those of the ThCr2Si2 and BaNiSn3 structure types. Herein, we present the optimized crystal growth, structure, and phys. properties of Ln2Co3Ge5 (Ln = Pr, Nd, Sm).

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24. 24

    Wang, Q.; Brooks, S. H.; Liu, T.; Tomson, N. C. Tuning metal–metal interactions for cooperative small molecule activation. Chem. Commun. 2021, 57, 2839– 2853,  DOI: 10.1039/D0CC07721F

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    24

    Tuning metal-metal interactions for cooperative small molecule activation

    Wang, Qiuran; Brooks, Sam H.; Liu, Tianchang; Tomson, Neil C.

    Chemical Communications (Cambridge, United Kingdom) (2021), 57 (23), 2839-2853CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

    A review. Cluster complexes have attracted interest for decades due to their promise of drawing analogies to metallic surfaces and metalloenzyme active sites, but only recently have chemists started to develop ligand scaffolds that are specifically designed to support multinuclear transition metal cores. Such ligands not only hold multiple metal centers in close proximity but also allow for fine-tuning of their electronic structures and surrounding steric environments. This Feature Article highlights ligand designs that allow for cooperative small mol. activation at cluster complexes, with a particular focus on complexes that contain metal-metal bonds. Two useful ligand-design elements have emerged from this work: a degree of geometric flexibility, which allows for novel small mol. activation modes, and the use of redox-active ligands to provide electronic flexibility to the cluster core. The authors have incorporated these factors into a unique class of dinucleating macrocycles (nPDI2). Redox-active fragments in nPDI2 mimic the weak-overlap covalent bonding that is characteristic of M-M interactions, and aliph. linkers in the ligand backbone provide geometric flexibility, allowing for interconversion between a range of geometries as the dinuclear core responds to the requirements of various small mol. substrates. The union of these design elements appears to be a powerful combination for analogizing crit. aspects of heterogeneous and metalloenzyme catalysts.

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25. 25

    Liu, T.; Gau, M. R.; Tomson, N. C. Mimicking the Constrained Geometry of a Nitrogen-Fixation Intermediate. J. Am. Chem. Soc. 2020, 142, 8142,  DOI: 10.1021/jacs.0c01861

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    25

    Mimicking the Constrained Geometry of a Nitrogen-Fixation Intermediate

    Liu, Tianchang; Gau, Michael R.; Tomson, Neil C.

    Journal of the American Chemical Society (2020), 142 (18), 8142-8146CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Both biol. and industrial nitrogen redn. catalysts activate N2 at multinuclear binding sites with constrained Fe-Fe distances. This contrasts with mol. diiron systems, which routinely form linear N2 bridges to minimize steric interactions. Model compds. that capture the salient geometric features of N2 binding by the nitrogenase enzymes and Mittasch catalysts would contribute to understanding their high N2-redn. activity. It is shown in the present study that use of a geometrically flexible, dinucleating macrocycle allows for the formation of a bridging N2 ligand with an unusual Fe-CtN2-Fe angle of 150° (CtN2 = centroid of N2), a geometry that approximates the α-N2 binding mode on Fe(111) surfaces that precedes N2 bond cleavage. The cavity size of the macrocycle prevents the formation of a linear Fe-N2-Fe unit and leads to orbital interactions that are distinct from those available to the linear configuration.

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26. 26

    Cui, P.; Wang, Q.; McCollom, S. P.; Manor, B. C.; Carroll, P. J.; Tomson, N. C. Ring-Size-Modulated Reactivity of Putative Dicobalt-Bridging Nitrides: C–H Activation versus Phosphinimide Formation. Angew. Chem., Int. Ed. 2017, 56, 15979– 15983,  DOI: 10.1002/anie.201708966

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    26

    Ring-Size-Modulated Reactivity of Putative Dicobalt-Bridging Nitrides: C-H Activation versus Phosphinimide Formation

    Cui, Peng; Wang, Qiuran; McCollom, Samuel P.; Manor, Brian C.; Carroll, Patrick J.; Tomson, Neil C.

    Angewandte Chemie, International Edition (2017), 56 (50), 15979-15983CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Dicobalt complexes supported by flexible macrocyclic ligands were used to target the generation of the bridging nitrido species [(nPDI2)Co2(μ-N)(PMe3)2]3+ (PDI = 2,6-pyridyldiimine; n = 2, 3, corresponding to the no. of catenated methylene units between imino nitrogen atoms). Depending on the size of the macrocycle and the reaction conditions (soln. vs. solid-state), the thermolysis of azide precursors yielded bridging phosphinimido [(2PDI2)Co2(μ-NPMe3)(PMe3)2]3+, amido [(nPDI2)Co2(μ-NH2)(PMe3)2]3+ (n = 2, 3), and C-H amination [(3PDI2*-μ-NH)Co2(PMe3)2]3+ products. All results are consistent with the initial formation of [(nPDI2)Co2(μ-N)(PMe3)2]3+, followed by (1) PMe3 attack on the nitride, (2) net hydrogen-atom transfer to form N-H bonds, or (3) C-H amination of the alkyl linker of the nPDI2 ligand.

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27. 27

    Spentzos, A. Z.; Tomson, N. C. Mapping the Reactivity of Dicobalt Bridging Nitrides in Constrained Geometries. Inorg. Chem. 2021, 60, 6889– 6899,  DOI: 10.1021/acs.inorgchem.0c03774

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    27

    Mapping the Reactivity of Dicobalt Bridging Nitrides in Constrained Geometries

    Spentzos, Ariana Z.; Tomson, Neil C.

    Inorganic Chemistry (2021), 60 (10), 6889-6899CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    Low-nuclearity nitrides of the late transition metals are rare and reactive mol. species, with little exptl. precedent. The first putative examples of dicobalt bridging nitrides, [(nPDI2)Co2(μ-N)(PMe3)2][OTf]3 (n[Co2N]3+; PDI = pyridyldiimine; n = 2 or 3, representing the length of the aliph. chain linking PDI imino groups), were reported recently and shown to undergo a range of intramol. reaction pathways, including N-H bond formation, C-H bond insertion, and P:N bond formation at the bridging nitride. The specific mode of reactivity changed with the phase of the reaction and the size of the macrocycle used to support the transient species. The present contribution offers a computational investigation into both the geometric and electronic structures of these nitrides as well as the factors governing their reaction selectivity. The compds. n[Co2N]3+ exhibit μ-N-based lowest unoccupied MOs (LUMOs) that are consistent with subvalent, electrophilic nitrides. The specific orientations of the LUMOs induce ring-size-dependent stereoelectronic effects, thereby causing the product selectivity obsd. exptl. Notably, the nitrides also exhibit a degree of nucleophilicity at μ-N by way of a high-energy, μ-N-based lone pair. This ambiphilic character appears to be a direct result of the constrained environment imposed by the folded-ligand geometries of n[Co2N]3+. When combined with the exptl. findings, these data led to the conclusion that the folded-ligand isomers are the reactive species and that the constrained geometry imposed by the macrocyclic ligand plays an important role in controlling the reaction outcome.

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28. 28

    Zhang, S.; Wang, Q.; Thierer, L. M.; Weberg, A. B.; Gau, M. R.; Carroll, P. J.; Tomson, N. C. Tuning Metal–Metal Interactions through Reversible Ligand Folding in a Series of Dinuclear Iron Complexes. Inorg. Chem. 2019, 58, 12234– 12244,  DOI: 10.1021/acs.inorgchem.9b01673

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    Tuning Metal-Metal Interactions through Reversible Ligand Folding in a Series of Dinuclear Iron Complexes

    Zhang, Shaoguang; Wang, Qiuran; Thierer, Laura M.; Weberg, Alexander B.; Gau, Michael R.; Carroll, Patrick J.; Tomson, Neil C.

    Inorganic Chemistry (2019), 58 (18), 12234-12244CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    A dinucleating macrocyclic ligand with two redox-active, pyridyldiimine components was shown to undergo reversible ligand folding to accommodate various substitution patterns, metal ion spin states, and degrees of Fe-Fe bonding within the cluster. An unfolded-ligand geometry with a rectangular Fe2(μ-Cl)2 core and an Fe-Fe distance of 3.3262(5) Å served as a direct precursor to two different folded-ligand complexes. Chem. redn. in the presence of PPh3 resulted in a diamagnetic, folded ligand complex with an Fe-Fe bonding interaction (dFe-Fe = 2.7096(17) Å) between two intermediate spin (SFe = 1) Fe(II) centers. Ligand folding was also induced through anion exchange on the unfolded-ligand species, producing a complex with three PhS- ligands and a temp.-dependent Fe-Fe distance. In this latter example, the weak ligand field of the thiolate ligands led to a product with weakly coupled, high-spin Fe(II) ions (SFe = 2; J = -50.1 cm-1) that form a bonding interaction in the ground state and a nonbonding interaction in the excited state(s), as detd. by SQUID magnetometry and variable temp. crystallog. Finally, both folded-ligand complexes were shown to reform an unfolded-ligand geometry through convergent syntheses of a complex with an Fe-Fe bonded Fe2(μ-SPh)2 core (dFe-Fe = 2.7320(11) Å). Exptl. validated DFT calcns. were used to investigate the electronic structures of all species as a way to understand the origin of Fe-Fe bonding interactions, the extent of ligand redn., and the nature of the spin systems that result from multiple, weakly interacting spin centers.

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29. 29

    Wang, Q.; Zhang, S.; Cui, P.; Weberg, A. B.; Thierer, L. M.; Manor, B. C.; Gau, M. R.; Carroll, P. J.; Tomson, N. C. Interdependent Metal–Metal Bonding and Ligand Redox-Activity in a Series of Dinuclear Macrocyclic Complexes of Iron, Cobalt, and Nickel. Inorg. Chem. 2020, 59, 4200– 4214,  DOI: 10.1021/acs.inorgchem.9b02339

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    29

    Interdependent Metal-Metal Bonding and Ligand Redox-Activity in a Series of Dinuclear Macrocyclic Complexes of Iron, Cobalt, and Nickel

    Wang, Qiuran; Zhang, Shaoguang; Cui, Peng; Weberg, Alexander B.; Thierer, Laura M.; Manor, Brian C.; Gau, Michael R.; Carroll, Patrick J.; Tomson, Neil C.

    Inorganic Chemistry (2020), 59 (7), 4200-4214CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    This report describes an isostructural series of dinuclear iron, cobalt, and nickel complexes bound by a redox-active macrocyclic ligand. The series spans five redox levels (34-38 e-/cluster core), allowing for a detailed investigation into both the degree of metal-metal interaction and the extent of ligand-based redox-activity. Magnetometry, electrochem., UV-vis-NIR absorption spectroscopy, and crystallog. were used in conjunction with DFT computational analyses to ext. the electronic structures of the six homodinuclear complexes. The isoelectronic, 34 e- species [(3PDI2)Fe2(PMe3)2(μ-Cl)](OTf) and [(3PDI2)Co2(PMe3)2(μ-Cl)](OTf)3 exhibit metal-metal single bonds, with varying amts. of electron d. delocalization into the ligand as a function of the effective nuclear charge of the metal ions. One- and two-electron redns. of [(3PDI2)Co2(PMe3)2(μ-Cl)](OTf)3 lead to isolable products, which show successive increases in both the Co-Co distances and the extent of redn. of the ligand manifold. This trend results from redn. of a Co-Co σ* orbital, which was heavily mixed with the redox-active manifold of the 3PDI2 ligand. A similar trend was obsd. in the 37 and 38 e- dinickel complexes [(3PDI2)Ni2(PMe3)2(μ-Cl)](OTf)2 and [(3PDI2)Ni2(PMe3)2(μ-Cl)](OTf); however, their higher electron counts lead to high-spin ground states that result from occupation of a high-lying δ/δ* manifold with significant Ni-NPDI σ* character. This change in ground state configuration reforms a M-M bonding interaction in the 37 e- complex, but formation of the 38 e- species again disrupts the M-M bond alongside the transfer of electron d. to the ligand. This study investigates the metal-metal bonding and ligand-based redn. in a six-membered electron transfer series of homobimetallic complexes supported by a redox-active macrocyclic ligand. The formal metal-metal bond order varies from 0 to 0.5 and 1, while the ligand's pyridinediimine moieties work in tandem to hold between 0 and 3 electrons.

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30. 30

    Galley, S. S.; Pattenaude, S. A.; Ray, D.; Gaggioli, C. A.; Whitefoot, M. A.; Qiao, Y.; Higgins, R. F.; Nelson, W. L.; Baumbach, R.; Sperling, J. M. Using Redox-Active Ligands to Generate Actinide Ligand Radical Species. Inorg. Chem. 2021, 60, 15242– 15252,  DOI: 10.1021/acs.inorgchem.1c01766

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    30

    Using Redox-Active Ligands to Generate Actinide Ligand Radical Species

    Galley, Shane S.; Pattenaude, Scott A.; Ray, Debmalya; Gaggioli, Carlo Alberto; Whitefoot, Megan A.; Qiao, Yusen; Higgins, Robert F.; Nelson, W. L.; Baumbach, Ryan; Sperling, Joseph M.; Zeller, Matthias; Collins, Tyler S.; Schelter, Eric J.; Gagliardi, Laura; Albrecht-Schonzart, Thomas E.; Bart, Suzanne C.

    Inorganic Chemistry (2021), 60 (20), 15242-15252CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    Using a redox-active dioxophenoxazine ligand, DOPO (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazine-9-olate), a family of actinide (U, Th, Np, Pu) and Hf tris(ligand) coordination compds. were synthesized. Full characterization of these species using 1H NMR spectroscopy, electronic absorption spectroscopy, SQUID magnetometry, and x-ray crystallog. showed these compds. are analogous, existing in the form M(DOPOq)2(DOPOsq), where two ligands are of the oxidized, quinone form (DOPOq), and the third is of the reduced, semiquinone (DOPOsq) form. The electronic structures of these complexes were further investigated using CASSCF calcns., which revealed electronic structures consistent with metals in the +4 formal oxidn. state and one unpaired electron localized on one ligand in each complex. Furthermore, f-orbitals of the early actinides show a sizable bonding overlap with the ligand 2p orbitals. Notably, this is the first example of a plutonium-ligand radical species and a rare example of magnetic data recorded for a homogeneous plutonium coordination complex.

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31. 31

    Galley, S. S.; Pattenaude, S. A.; Gaggioli, C. A.; Qiao, Y.; Sperling, J. M.; Zeller, M.; Pakhira, S.; Mendoza-Cortes, J. L.; Schelter, E. J.; Albrecht-Schmitt, T. E. Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides. J. Am. Chem. Soc. 2019, 141, 2356– 2366,  DOI: 10.1021/jacs.8b10251

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    31

    Synthesis and Characterization of Tris-chelate Complexes for Understanding f-Orbital Bonding in Later Actinides

    Galley, Shane S.; Pattenaude, Scott A.; Gaggioli, Carlo Alberto; Qiao, Yusen; Sperling, Joseph M.; Zeller, Matthias; Pakhira, Srimanta; Mendoza-Cortes, Jose L.; Schelter, Eric J.; Albrecht-Schmitt, Thomas E.; Gagliardi, Laura; Bart, Suzanne C.

    Journal of the American Chemical Society (2019), 141 (6), 2356-2366CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    An isostructural family of f-element compds. (Ce, Nd, Sm, Gd; Am, Bk, Cf) of the redox-active dioxophenoxazine ligand (DOPOq; DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate) was prepd. This family, of the form M(DOPOq)3, represents the first nonaq. isostructural series, including the later actinides berkelium and californium. The lanthanide derivs. were fully characterized using 1H NMR spectroscopy and SQUID magnetometry, while all species were structurally characterized by x-ray crystallog. and electronic absorption spectroscopy. In order to probe the electronic structure of this new family, CASSCF calcns. were performed and revealed these systems to be largely ionic in contrast to previous studies, where berkelium and californium typically have a small degree of covalent character. To validate the zeroth order regular approxn. (ZORA) method, the same CASSCF anal. using exptl. structures vs. UDFT-ZORA optimized structures does not exhibit sizable changes in bonding patterns. UDFT-ZORA combined with CASSCF could be a useful first approxn. to predict and investigate the structure and electronic properties of actinides and lanthanides that are difficult to synthesize or characterize.

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32. 32

    Perales, D.; Ford, S. A.; Salpage, S. R.; Collins, T. S.; Zeller, M.; Hanson, K.; Bart, S. C. Conversion of Trivalent Uranium Anilido to Tetravalent Uranium Imido Species via Oxidative Deprotonation. Inorg. Chem. 2020, 59, 11910– 11914,  DOI: 10.1021/acs.inorgchem.0c01704

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    32

    Conversion of Trivalent Uranium Anilido to Tetravalent Uranium Imido Species via Oxidative Deprotonation

    Perales, Diana; Ford, Shannon A.; Salpage, Sahan R.; Collins, Tyler S.; Zeller, Matthias; Hanson, Kenneth; Bart, Suzanne C.

    Inorganic Chemistry (2020), 59 (17), 11910-11914CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    Two U(III) anilido complexes were synthesized, Tp*2U(NH-C6H4-p-terpyridine) (2-terpy) and Tp*2U(NH-C6H4-p-CH3) (2-ptol), where Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, by protonation of Tp*2UBn (1-Bn; Bn = benzyl) with 4-[2,6-di(pyridin-2-yl)pyridin-4-yl]benzenamine or p-toluidine, resp. Conversion to the resp. U(IV) imido species was possible by oxidn. and deprotonation, forming Tp*2U(N-C6H4-p-terpyridine) (3-terpy) and Tp*2U(N-C6H4-p-CH3) (3-ptol). These compds. were characterized by multinuclear NMR spectroscopy, IR spectroscopy, electronic absorption spectroscopy, and x-ray crystallog. Two U(III) anilido complexes were synthesized with small and large ligands, Tp*2U(terpy-anilido) and Tp*2U(ptol-anilido) [Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, terpy = 2,6-bis(2-pyridyl)pyridine, and ptol = p-tolyl]. Both were converted to the resp. U(IV) imido species by 1-electron oxidn. and deprotonation forming Tp*2U(terpy-imido) and Tp*2U(ptol-imido), showing that the robust bis(Tp*) ligand framework can support imido species under reducing conditions. These compds. were characterized by multinuclear NMR spectroscopy, IR spectroscopy, electronic absorption spectroscopy, and x-ray crystallog.

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33. 33

    Lee, H. B.; Ciolkowski, N.; Winslow, C.; Rittle, J. High Spin Cobalt Complexes Supported by a Trigonal Tris(Phosphinimide) Ligand. Inorg. Chem. 2021, 60, 11830– 11837,  DOI: 10.1021/acs.inorgchem.1c01400

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    33

    High Spin Cobalt Complexes Supported by a Trigonal Tris(Phosphinimide) Ligand

    Lee, Heui Beom; Ciolkowski, Nicholas; Winslow, Charles; Rittle, Jonathan

    Inorganic Chemistry (2021), 60 (16), 11830-11837CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    Terminal, π-basic moieties occupy a prominent position in the stabilization of unusual or reactive inorg. species. The electron-releasing, π-basic properties of phosphinimides (PN) have been employed to stabilize electron-deficient early transition metals and lanthanides. In principle, a ligand field comprised of terminal PN groups should enable access to high-valent states of late first row transition metals. Herein, we report a new class of multidentate phosphinimide ligands to logically explore this hypothesis. Access to such ligands is made possible by a new procedure for the electrophilic amination of rigid, sterically encumbering, multidentate phosphines. Such frameworks facilitate terminal PN coordination to cobalt as demonstrated by the synthesis of a trinuclear CoII3 complex and a homoleptic, three-coordinate CoIII complex. Interestingly, the CoIII complex exhibits an exceedingly rare S = 2 ground state. Combined XRD, magnetic susceptibility, and DFT studies highlight that terminally bound PNs engage in strong dπ-pπ interactions that present a weak ligand field appropriate to stabilize high-spin states of late transition metals.

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34. 34

    Winslow, C.; Lee, H. B.; Field, M. J.; Teat, S. J.; Rittle, J. Structure and Reactivity of a High-Spin, Nonheme Iron(III)- Superoxo Complex Supported by Phosphinimide Ligands. J. Am. Chem. Soc. 2021, 143, 13686,  DOI: 10.1021/jacs.1c05276

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    34

    Structure and Reactivity of a High-Spin, Nonheme Iron(III)-Superoxo Complex Supported by Phosphinimide Ligands

    Winslow, Charles; Lee, Heui Beom; Field, Mackenzie J.; Teat, Simon J.; Rittle, Jonathan

    Journal of the American Chemical Society (2021), 143 (34), 13686-13693CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Nonheme Fe oxygenases use dioxygen to accomplish challenging chem. oxidns. A further understanding of the Fe-O2 intermediates implicated in these processes is challenged by their highly transient nature. To that end, the authors have developed a ligand platform featuring phosphinimide donors intended to stabilize oxidized, high-spin Fe complexes. O2 exposure of single crystals of a three-coordinate Fe(II) complex of this framework allowed for in crystallo trapping of a terminally bound Fe-O2 complex suitable for XRD characterization. Spectroscopic and computational studies of this species support a high-spin Fe(III) center antiferromagnetically coupled to a superoxide ligand, similar to that proposed for numerous nonheme Fe oxygenases. In addn. to the apparent stability of this synthetic Fe-O2 complex, its ability to engage in a range of stoichiometric and catalytic oxidn. processes demonstrates that this Fe-phosphinimide system is primed for development in modeling oxidizing bioinorg. intermediates and green oxidn. chem.

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35. 35

    Mbughuni, M. M.; Chakrabarti, M.; Hayden, J. A.; Bominaar, E. L.; Hendrich, M. P.; Munck, E.; Lipscomb, J. D. Trapping and spectroscopic characterization of an Fe^III-superoxo intermediate from a nonheme mononuclear iron-containing enzyme. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 16788– 16793,  DOI: 10.1073/pnas.1010015107

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    35

    Trapping and spectroscopic characterization of an FeIII-superoxo intermediate from a nonheme mononuclear iron-containing enzyme

    Mbughuni, Michael M.; Chakrabarti, Mirinmoy; Hayden, Joshua A.; Bominaar, Emile L.; Hendrich, Michael P.; Munck, Eckard; Lipscomb, John D.

    Proceedings of the National Academy of Sciences of the United States of America (2010), 107 (39), 16788-16793, S16788/1-S16788/7CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)

    FeIII-O2·- intermediates are well known in heme enzymes, but none have been characterized in the nonheme mononuclear FeII enzyme family. Many steps in the O2 activation and reaction cycle of FeII-contg. homoprotocatechuate 2,3-dioxygenase are made detectable by using the alternative substrate 4-nitrocatechol (4NC) and mutation of the active site His200 to Asn (H200N). Here, the first intermediate (Int-1) obsd. after adding O2 to the H200N-4NC complex is trapped and characterized using EPR and Mossbauer (MB) spectroscopies. Int-1 is a high-spin (S1 = 5/2) FeIII antiferromagnetically (AF) coupled to an S2 = 1/2 radical (J ≈ 6 cm-1 in = H = JS1/S2). It exhibits parallel-mode EPR signals at g = 8.17 from the S = 2 multiplet, and g = 8.8 and 11.6 from the S = 3 multiplet. These signals are broadened significantly by 17O2 hyperfine interactions (A17O ≈ 180 MHz). Thus, Int-1 is an AF-coupled FeIII-O2·- species. The exptl. observations are supported by d. functional theory calcns. that show nearly complete transfer of spin d. to the bound O2- Int-1 decays to form a second intermediate (Int-2). MB spectra show that it is also an AF-coupled FeIII-radical complex. Int-2 exhibits an EPR signal at g = 8.05 arising from an S = 2 state. The signal is only slightly broadened by 17O2 (<3% spin delocalization), suggesting that Int-2 is a peroxo-FeIII-4NC semiquinone radical species. Our results demonstrate facile electron transfer between FeII, O2, and the org. ligand, thereby supporting the proposed wild-type enzyme mechanism.

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36. 36

    Rittle, J.; Green, M. T. Cytochrome P450 Compound I: Capture, Characterization, and C-H Bond Activation Kinetics. Science 2010, 330, 933– 937,  DOI: 10.1126/science.1193478

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    36

    Cytochrome P450 Compound I: Capture, Characterization, and C-H Bond Activation Kinetics

    Rittle, Jonathan; Green, Michael T.

    Science (Washington, DC, United States) (2010), 330 (6006), 933-937CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)

    Cytochrome P 450 enzymes are responsible for the phase I metab. of approx. 75% of known pharmaceuticals. P 450s perform this and other important biol. functions through the controlled activation of C-H bonds. Here, we report the spectroscopic and kinetic characterization of the long-sought principal intermediate involved in this process, P 450 compd. I (P 450-I), which we prepd. in approx. 75% yield by reacting ferric CYP119 with m-chloroperbenzoic acid. The Mossbauer spectrum of CYP119-I is similar to that of chloroperoxidase compd. I, although its ESR spectrum reflects an increase in |J|/D, the ratio of the exchange coupling to the zero-field splitting. CYP119-I hydroxylates the unactivated C-H bonds of lauric acid [D(C-H) ~ 100 kcal per mol], with an apparent second-order rate const. of kapp = 1.1 × 107 per M per s at 4°. Direct measurements put a lower limit of k ≥ 210 per s on the rate const. for bound substrate oxidn., whereas analyses involving kinetic isotope effects predict a value in excess of 1400 per s.

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37. 37

    Rutledge, H. L.; Rittle, J.; Williamson, L. M.; Xu, W. A.; Gagnon, D. M.; Tezcan, F. A. Redox-Dependent Metastability of the Nitrogenase P-Cluster. J. Am. Chem. Soc. 2019, 141, 10091– 10098,  DOI: 10.1021/jacs.9b04555

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    37

    Redox-Dependent Metastability of the Nitrogenase P-Cluster

    Rutledge, Hannah L.; Rittle, Jonathan; Williamson, Laura M.; Xu, Wanqing A.; Gagnon, Derek M.; Tezcan, F. Akif

    Journal of the American Chemical Society (2019), 141 (25), 10091-10098CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Molybdenum nitrogenase catalyzes the redn. of dinitrogen into ammonia, which requires the coordinated transfer of eight electrons to the active site cofactor (FeMoco) through the intermediacy of an [8Fe-7S] cluster (P-cluster), both housed in the molybdenum-iron protein (MoFeP). Previous studies on MoFeP from two different organisms, Azotobacter vinelandii (Av) and Gluconacetobacter diazotrophicus (Gd), have established that the P-cluster is conformationally flexible and can undergo substantial structural changes upon two-electron oxidn. to the POX state, whereby a backbone amidate and an oxygenic residue (Ser or Tyr) ligate to two of the cluster's Fe centers. This redox-dependent change in coordination has been implicated in the conformationally gated electron transfer in nitrogenase. Here, we have investigated the role of the oxygenic ligand in Av MoFeP, which natively contains a Ser ligand (βSer188) to the P-cluster. Three variants were generated in which (1) the oxygenic ligand was eliminated (βSer188Ala), (2) the P-cluster environment was converted to the one in Gd MoFeP (βPhe99Tyr/βSer188Ala), and (3) two oxygenic ligands were simultaneously included (βPhe99Tyr). Our studies have revealed that the P-cluster can become compositionally labile upon oxidn. and reversibly lose one or two Fe centers in the absence of the oxygenic ligand, while still retaining wild-type-like dinitrogen redn. activity. Our findings also suggest that Av and Gd MoFePs evolved with specific preferences for Ser and Tyr ligands, resp., and that the structural control of these ligands must extend beyond the primary and secondary coordination spheres of the P-cluster. The P-cluster adds to the increasing no. of examples of inherently labile Fe-S clusters whose compositional instability may be an obligatory feature to enable redox-linked conformational changes to facilitate multielectron redox reactions.

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38. 38

    Rittle, J.; Field, M. J.; Green, M. T.; Tezcan, F. A. An efficient, step-economical strategy for the design of functional metalloproteins. Nat. Chem. 2019, 11, 434– 441,  DOI: 10.1038/s41557-019-0218-9

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    38

    An efficient, step-economical strategy for the design of functional metalloproteins

    Rittle, Jonathan; Field, Mackenzie J.; Green, Michael T.; Tezcan, F. Akif

    Nature Chemistry (2019), 11 (5), 434-441CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)

    The bottom-up design and construction of functional metalloproteins remains a formidable task in biomol. design. Although numerous strategies have been used to create new metalloproteins, pre-existing knowledge of the tertiary and quaternary protein structure is often required to generate suitable platforms for robust metal coordination and activity. Here we report an alternative and easily implemented approach (metal active sites by covalent tethering or MASCoT) in which folded protein building blocks are linked by a single disulfide bond to create diverse metal coordination environments within evolutionarily naive protein-protein interfaces. Metalloproteins generated using this strategy uniformly bind a wide array of first-row transition metal ions (MnII, FeII, CoII, NiII, CuII, ZnII and vanadyl) with physiol. relevant thermodn. affinities (dissocn. consts. ranging from 700 nM for MnII to 50 fM for CuII). MASCoT readily affords coordinatively unsatd. metal centers-including a penta-His-coordinated non-heme Fe site-and well-defined binding pockets that can accommodate modifications and enable coordination of exogenous ligands such as nitric oxide to the interfacial metal center.

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    Austin, R. N.; Chang, H.-K.; Zylstra, G. J.; Groves, J. T. The Non-Heme Diiron Alkane Monooxygenase of Pseudomonas oleovorans (AlkB) Hydroxylates via a Substrate Radical Intermediate. J. Am. Chem. Soc. 2000, 122, 11747– 11748,  DOI: 10.1021/ja001500v

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    The Non-Heme Diiron Alkane Monooxygenase of Pseudomonas oleovorans (AlkB) Hydroxylates via a Substrate Radical Intermediate

    Austin, Rachel N.; Chang, Hung-Kuang; Zylstra, Gerben J.; Groves, John T.

    Journal of the American Chemical Society (2000), 122 (47), 11747-11748CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    One of the most remarkable oxidns. found in nature is the insertion of oxygen into a carbon-hydrogen bond. Although the chem. inertness of paraffins has been well-known to chemists for centuries, the hydroxylation of these materials is a common biol. strategy. The soil organism Pseudomonas oleovorans TF4-1L (ATCC 29347)' can grow on octane as its sole source of carbon, a facility that has been successfully directed toward the large-scale prodn. of octanol from octane. The monooxygenase AlkB from TF4-1L is a dinuclear non-heme iron enzyme that catalyzes the terminal hydroxylation of simple alkanes. This is the initial step in the metabolic process by which TF4-1L obtains energy from alkanes. Recent genetic expts. have suggested that enzymes with high homol. to AlkB, esp. in the histidine-rich region thought to be essential for iron binding, are widely distributed in nature. Hence, mechanistic insights into AlkB hydroxylation may shed light on a significant portion of the alkane transformations that take place in the environment and also on the differences and similarities between this enzyme and two, more extensively studied alkane hydroxylating enzymes, cytochrome P 450 and sol. methane monooxygenase (sMMO). We have investigated the mechanism of alkane hydroxylation catalyzed by AlkB using the diagnostic substrates norcarane (bicyclo[4.1.0]heptane) and 2-methyl-1-phenylcyclopropane, both in wild-type TF4-1L and Escherichia coli expressing the cloned alkane hydroxylase genes. The results give conclusive evidence for a carbon-centered radical intermediate with a lifetime of approx. 1 ns in the hydroxylation process.

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40. 40

    Brazeau, B. J.; Austin, R. N.; Tarr, C.; Groves, J. T.; Lipscomb, J. D. Intermediate Q from Soluble Methane Monooxygenase Hydroxylates the Mechanistic Substrate Probe Norcarane: Evidence for a Stepwise Reaction. J. Am. Chem. Soc. 2001, 123, 11831– 11837,  DOI: 10.1021/ja016376+

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    Intermediate Q from Soluble Methane Monooxygenase Hydroxylates the Mechanistic Substrate Probe Norcarane: Evidence for a Stepwise Reaction

    Brazeau, Brian J.; Austin, Rachel N.; Tarr, Carly; Groves, John T.; Lipscomb, John D.

    Journal of the American Chemical Society (2001), 123 (48), 11831-11837CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Norcarane is a valuable mechanistic probe for enzyme-catalyzed hydrocarbon oxidn. reactions because different products or product distributions result from concerted, radical, and cation based reactions. Sol. methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b catalyzes the oxidn. of norcarane to afford 3-hydroxymethylcyclohexene and 3-cycloheptenol, compds. characteristic of radical and cationic intermediates, resp., in addn. to 2- and 3-norcaranols. Past single turnover transient kinetic studies have identified several optically distinct intermediates from the catalytic cycle of the hydroxylase component of sMMO. Thus, the reaction between norcarane and key reaction intermediates can be directly monitored. The presence of norcarane increases the rate of decay of only one intermediate, the high-valent bis-μ-oxo Fe(IV)2 cluster-contg. species compd. Q, showing that it is responsible for the majority of the oxidn. chem. The observation of products from both radical and cationic intermediates from norcarane oxidn. catalyzed by sMMO is consistent with a mechanism in which an initial substrate radical intermediate is formed by hydrogen atom abstraction. This intermediate then undergoes either oxygen rebound, intramol. rearrangement followed by oxygen rebound, or loss of a second electron to yield a cationic intermediate to which OH- is transferred. The estd. lower limit of 20 ps for the lifetime of the putative radical intermediate is in accord with values detd. from previous studies of sterically hindered sMMO probes.

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41. 41

    Cooper, H. L. R.; Mishra, G.; Huang, X.; Pender-Cudlip, M.; Austin, R. N.; Shanklin, J.; Groves, J. T. Parallel and Competitive Pathways for Substrate Desaturation, Hydroxylation, and Radical Rearrangement by the Non-heme Diiron Hydroxylase AlkB. J. Am. Chem. Soc. 2012, 134, 20365– 20375,  DOI: 10.1021/ja3059149

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    41

    Parallel and Competitive Pathways for Substrate Desaturation, Hydroxylation, and Radical Rearrangement by the Non-heme Diiron Hydroxylase AlkB

    Cooper, Harriet L. R.; Mishra, Girish; Huang, Xiongyi; Pender-Cudlip, Marilla; Austin, Rachel N.; Shanklin, John; Groves, John T.

    Journal of the American Chemical Society (2012), 134 (50), 20365-20375CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    A purified and highly active form of the non-heme diiron hydroxylase AlkB was investigated using the diagnostic probe substrate norcarane. The reaction afforded C2 (26%) and C3 (43%) hydroxylation and desatn. products (31%). Initial C-H cleavage at C2 led to 7% C2 hydroxylation and 19% 3-hydroxymethylcyclohexene, a rearrangement product characteristic of a radical rearrangement pathway. A deuterated substrate analog, 3,3,4,4-norcarane-d4, afforded drastically reduced amts. of C3 alc. (8%) and desatn. products (5%), while the radical rearranged alc. was now the major product (65%). This change in product ratios indicates a large kinetic hydrogen isotope effect of ∼20 for both the C-H hydroxylation at C3 and the desatn. pathway, with all of the desatn. originating via hydrogen abstraction at C3 and not C2. The data indicate that AlkB reacts with norcarane via initial C-H hydrogen abstraction from C2 or C3 and that the three pathways, C3 hydroxylation, C3 desatn., and C2 hydroxylation/radical rearrangement, are parallel and competitive. Thus, the incipient radical at C3 either reacts with the iron-oxo center to form an alc. or proceeds along the desatn. pathway via a second H-abstraction to afford both 2-norcarene and 3-norcarene. Subsequent reactions of these norcarenes lead to detectable amts. of hydroxylation products and toluene. By contrast, the 2-norcaranyl radical intermediate leads to C2 hydroxylation and the diagnostic radical rearrangement, but this radical apparently does not afford desatn. products. The results indicate that C-H hydroxylation and desatn. follow analogous stepwise reaction channels via carbon radicals that diverge at the product-forming step.

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42. 42

    Mahdavi-Shakib, A.; Sempel, J.; Babb, L.; Oza, A.; Hoffman, M.; Whittaker, T. N.; Chandler, B. D.; Austin, R. N. Combining Benzyl Alcohol Oxidation Saturation Kinetics and Hammett Studies as Mechanistic Tools for Examining Supported Metal Catalysts. ACS Catal. 2020, 10, 10207– 10215,  DOI: 10.1021/acscatal.0c02212

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    42

    Combining Benzyl Alcohol Oxidation Saturation Kinetics and Hammett Studies as Mechanistic Tools for Examining Supported Metal Catalysts

    Mahdavi-Shakib, Akbar; Sempel, Janine; Babb, Lauren; Oza, Aisha; Hoffman, Maya; Whittaker, Todd N.; Chandler, Bert D.; Austin, Rachel Narehood

    ACS Catalysis (2020), 10 (17), 10207-10215CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)

    Understanding and quantifying how the active sites in supported metal catalysts can be modified are crit. for rationally designing catalysts. This problem is particularly complex for reactions that occur at the metal-support interface (MSI) because of the multiple chemistries assocd. with the metal and the support. In this study, we used the oxidn. of substituted benzyl alc. over Au/TiO2 and Au/Al2O3 to probe MSI chem. Substituents impacted substrate binding, deprotonation, and the rate-limiting transfer of a hydride from benzyl alc. to Au, as shown by a combination of Michaelis-Menten (M-M) satn. kinetics and kinetic isotope effects. Hammett studies performed with a single substrate vs. those done with two substrates together in competition expts. showed significant differences, which were attributable to stronger competitive adsorption on the support by more electron-rich alcs. The M-M anal. showed that alc. substitution impacts substrate binding and deprotonation equil., which in turn affect the no. of active alkoxides adsorbed at the MSI. Hammett slopes should therefore be measured under satg. conditions using one substrate at a time. The Hammett slopes measured for heterogeneous systems in this manner agree well with the KIE-Hammett slope relationship detd. in homogeneous systems, which provide information on the early or late nature of the transition state. Our results show that the combination of Michaelis-Menten and Hammett techniques for benzyl alc. oxidn. provides mechanistic information assocd. with the MSI chem. of supported Au catalysts as well as information on active site electronics.

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43. 43

    Hsieh, C. H.; Huang, X.; Amaya, J. A.; Rutland, C. D.; Keys, C. L.; Groves, J. T.; Austin, R. N.; Makris, T. M. The Enigmatic P450 Decarboxylase OleT Is Capable of, but Evolved To Frustrate, Oxygen Rebound Chemistry. Biochemistry 2017, 56, 3347– 3357,  DOI: 10.1021/acs.biochem.7b00338

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    43

    The enigmatic P450 decarboxylase OleT is capable of, but evolved To frustrate, oxygen rebound chemistry

    Hsieh, Chun H.; Huang, Xiongyi; Amaya, Jose A.; Rutland, Cooper D.; Keys, Carson L.; Groves, John T.; Austin, Rachel N.; Makris, Thomas M.

    Biochemistry (2017), 56 (26), 3347-3357CODEN: BICHAW; ISSN:0006-2960. (American Chemical Society)

    OleT is a cytochrome P 450 enzyme that catalyzes the removal of carbon dioxide from variable chain length fatty acids to form 1-alkenes. In this work, we examine the binding and metabolic profile of OleT with shorter chain length (n ≤ 12) fatty acids that can form liq. transportation fuels. Transient kinetics and product analyses confirm that OleT capably activates hydrogen peroxide with shorter substrates to form the high-valent intermediate Compd. I and largely performs C-C bond scission. However, the enzyme also produces fatty alc. side products using the high-valent iron oxo chem. commonly assocd. with insertion of oxygen into hydrocarbons. When presented with a short chain fatty acid that can initiate the formation of Compd. I, OleT oxidizes the diagnostic probe mols. norcarane and methylcyclopropane in a manner that is reminiscent of reactions of many CYP hydroxylases with radical clock substrates. These data are consistent with a decarboxylation mechanism in which Compd. I abstrs. a substrate hydrogen atom in the initial step. Positioning of the incipient substrate radical is a crucial element in controlling the efficiency of activated OH rebound.

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44. 44

    Williams, S. C.; Forsberg, A. P.; Lee, J.; Vizcarra, C. L.; Lopatkin, A. J.; Austin, R. N. Investigation of the prevalence and catalytic activity of rubredoxin-fused alkane monooxygenases (AlkBs). J. Inorg. Biochem. 2021, 219, 111409,  DOI: 10.1016/j.jinorgbio.2021.111409

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    44

    Investigation of the prevalence and catalytic activity of rubredoxin-fused alkane monooxygenases (AlkBs)

    Williams, Shoshana C.; Forsberg, Allison P.; Lee, Juliet; Vizcarra, Christina L.; Lopatkin, Allison J.; Austin, Rachel N.

    Journal of Inorganic Biochemistry (2021), 219 (), 111409CODEN: JIBIDJ; ISSN:0162-0134. (Elsevier Inc.)

    Interest in understanding the environmental distribution of the alkane monooxygenase (AlkB) enzyme led to the identification of over 100 distinct alkane monooxygenase (AlkB) enzymes contg. a covalently bound, or fused, rubredoxin. The rubredoxin-fused AlkB from Dietzia cinnamea was cloned as a full-length protein and as a truncated protein with the rubredoxin domain deleted. A point mutation (V91W) was introduced into the full-length protein, with the goal of assessing how steric bulk in the putative substrate channel might affect selectivity. Based on activity studies with alkane and alkene substrates, the rubredoxin-fused AlkB oxidizes a similar range of alkane substrates relative to its rubredoxin domain-deletion counterpart. Oxidn. of terminal alkenes generated both an epoxide and a terminal aldehyde. The products of V91W-mutant-catalyzed oxidn. of alkenes had a higher aldehyde-to-epoxide ratio than the products formed in the presence of the wild type protein. These results are consistent with this mutation causing a structural change impacting substrate positioning.

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45. 45

    Kephart, J. A.; Mitchell, B. S.; Chirila, A.; Anderton, K. J.; Rogers, D.; Kaminsky, W.; Velian, A. Atomically Defined Nanopropeller Fe₃Co₆Se₈(Ph₂PNTol)₆: Functional Model for the Electronic Metal–Support Interaction Effect and High Catalytic Activity for Carbodiimide Formation. J. Am. Chem. Soc. 2019, 141, 19605– 19610,  DOI: 10.1021/jacs.9b12473

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    45

    Atomically Defined Nanopropeller Fe3Co6Se8(Ph2PNTol)6: Functional Model for the Electronic Metal-Support Interaction Effect and High Catalytic Activity for Carbodiimide Formation

    Kephart, Jonathan A.; Mitchell, Benjamin S.; Chirila, Andrei; Anderton, Kevin J.; Rogers, Dylan; Kaminsky, Werner; Velian, Alexandra

    Journal of the American Chemical Society (2019), 141 (50), 19605-19610CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Atomically defined interfaces that maximize the d. of active sites and harness the electronic metal-support interaction are desirable to facilitate challenging multielectron transformations, but their synthesis remains a considerable challenge. The authors report the rational synthesis of the atomically defined metal chalcogenide nanopropellers Fe3Co6Se8L6 (L = Ph2PNTol) featuring three Fe edge sites, and its ensuing catalytic activity for carbodiimide formation. The complex interaction between the Fe edges and Co6Se8 support, including the interplay between oxidn. state, substrate coordination, and metal-support interaction, is probed in detail using chem. and electrochem. methods, extensive single crystal x-ray diffraction, and electronic absorption and Mossbauer spectroscopy.

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46. 46

    Mitchell, B. S.; Kaminsky, W.; Velian, A. Tuning the Electronic Structure of Atomically Precise Sn/Co/Se Nanoclusters via Redox Matching of Tin(IV) Surface Sites. Inorg. Chem. 2021, 60, 6135– 6139,  DOI: 10.1021/acs.inorgchem.1c00313

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    46

    Tuning the Electronic Structure of Atomically Precise Sn/Co/Se Nanoclusters via Redox Matching of Tin(IV) Surface Sites

    Mitchell, Benjamin S.; Kaminsky, Werner; Velian, Alexandra

    Inorganic Chemistry (2021), 60 (9), 6135-6139CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)

    A new strategy is reported to tailor the electronic properties of a superat. metal chalcogenide cluster by redox-matching the cluster core with surface Sn(IV) sites. Two ternary clusters (SnR2)3Co6Se8L6 (R = Me, nBu) are synthesized by salt metathesis from the hexalithiated salt [Li2(py)2]3Co6Se8L6 and R2SnCl2. Cyclic and differential pulse voltammetry studies reveal that the tristannylated clusters feature two new, near-degenerate, electronic states within the HOMO-LUMO gap of Co6Se8 core, that are attributed to the redn. of a surface tin site. Single crystal x-ray diffraction anal. reveals no Sn···Se coordination is present in the solid-state. The single crystal x-ray structure of the hexalithiated salt starting material is reported for the THF adduct variant [Li2(THF)2]6Co6Se8L6.

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47. 47

    Mitchell, B. S.; Krajewski, S. M.; Kephart, J. A.; Rogers, D.; Kaminsky, W.; Velian, A. Redox-Switchable Allosteric Effects in Molecular Clusters. JACS Au 2022, 2, 92– 96,  DOI: 10.1021/jacsau.1c00491

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    47

    Redox-Switchable Allosteric Effects in Molecular Clusters

    Mitchell, Benjamin S.; Krajewski, Sebastian M.; Kephart, Jonathan A.; Rogers, Dylan; Kaminsky, Werner; Velian, Alexandra

    JACS Au (2022), 2 (1), 92-96CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)

    We demonstrate that allosteric effects and redox state changes can be harnessed to create a switch that selectively and reversibly regulates the coordination chem. of a single site on the surface of a mol. cluster. This redox-switchable allostery is employed as a guiding force to assemble the mol. clusters Zn3Co6Se8L'6 (L' = Ph2PN(H)Tol, Ph = Ph, Tol = 4-tolyl) into materials of predetd. dimensionality (1- or 2-D) and to encode them with emissive properties. This work paves the path to program the assembly and function of inorg. clusters into stimuli-responsive, atomically precise materials.

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48. 48

    Lu, H.-C.; Katyal, N.; Henkelman, G.; Milliron, D. J. Controlling the Shape Anisotropy of Monoclinic Nb₁₂O₂₉ Nanocrystals Enables Tunable Electrochromic Spectral Range. J. Am. Chem. Soc. 2021, 143, 15745– 15755,  DOI: 10.1021/jacs.1c06901

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    48

    Controlling the Shape Anisotropy of Monoclinic Nb12O29 Nanocrystals Enables Tunable Electrochromic Spectral Range

    Lu, Hsin-Che; Katyal, Naman; Henkelman, Graeme; Milliron, Delia J.

    Journal of the American Chemical Society (2021), 143 (38), 15745-15755CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Electrochromic smart windows that modulate the solar transmittance in a wide and selective spectral range can optimize building energy efficiency. However, for conventional materials such as bulk transition metal oxides, the electrochromic spectral range is constrained by their crystal structure with limited tunability. Herein, we report a method to control the shape anisotropy of monoclinic Nb12O29 nanocrystals and obtain a tunable electrochromic spectral range. We demonstrate the synthesis of monoclinic Nb12O29 nanorods (NRs), extending one-dimensionally along the b direction, and monoclinic Nb12O29 nanoplatelets (NPLs), extending two-dimensionally along the b and c directions. Upon electrochem. redn. accompanied by Li insertion, the NR films show increasing absorbance mostly in the near IR region. In contrast, the NPL films show increasing absorbance in the near IR region first followed by increasing absorbance in both visible and near IR regions. To elucidate the influence of shape anisotropy, we used d. functional theory to construct the lithiated structures of monoclinic Nb12O29 and in these structures we identified the presence of square planar sites and crystallog. shear sites for Li insertion. By calcg. the theor. spectra of the lithiated structures, we demonstrate that the Li insertion into the square planar sites results in absorption in the near IR region in both NRs and NPLs due to their extension in the b direction, while the subsequent insertion of Li into the crystallog. shear sites leads to absorption in both visible and near IR regions, which only occurs in NPLs due to their extension in the c direction.

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49. 49

    Tandon, B.; Gibbs, S. L.; Zydlewski, B. Z.; Milliron, D. J. Quantitative Analysis of Plasmonic Metal Oxide Nanocrystal Ensembles Reveals the Influence of Dopant Selection on Intrinsic Optoelectronic Properties. Chem. Mater. 2021, 33, 6955– 6964,  DOI: 10.1021/acs.chemmater.1c01951

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    49

    Quantitative Analysis of Plasmonic Metal Oxide Nanocrystal Ensembles Reveals the Influence of Dopant Selection on Intrinsic Optoelectronic Properties

    Tandon, Bharat; Gibbs, Stephen L.; Zydlewski, Benjamin Z.; Milliron, Delia J.

    Chemistry of Materials (2021), 33 (17), 6955-6964CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)

    Localized surface plasmon resonance (LSPR) arising from free charge carriers in doped metal oxide nanocrystals (NCs) has attracted abundant attention in the past decade for its potential in applications such as electrochromics, sensing, and photothermal therapy. While a lot is already known about the LSPR of doped metal oxide NCs, there is still much to learn about the effect of dopant identity on the electronic structure of the host and, in particular, the effect on surface depletion layers. Here, using indium oxide as the host lattice, we discuss the contribution of a dopant to the electronic structure and rationalize an empirical understanding on how a particular dopant can impact surface depletion, carrier concn., and carrier damping in doped metal oxide NCs. To do this, we leverage a slow-injection synthesis to incorporate four different dopants (Sn, Zr, Ti, and Ce) in indium oxide NCs. For each dopant, we synthesized NCs with different radius but the similar nominal doping level (∼1 atom %) and measured the optical response of dil. dispersions. This allowed us to deconvolute the effects of size and doping identity on LSPR. By fitting their plasmonic response to the heterogeneous ensemble Drude approxn., we extd. intrinsic electronic properties of the NCs such as surface depletion layer thickness, carrier concn., and carrier damping and rationalized the influence of dopant selection on each parameter. We find that the identity of the dopant does not have a significant impact on the extent of the depletion layer but it does impact carrier concn. and damping. In general, dopants with a greater electropositivity, similar radius to the host atom, and a stable aliovalent oxidn. state will have higher dopant activation, lower damping, and higher optical extinction. This study employs a broad sample set to empirically illustrate the effect of dopant identity on LSPR of doped metal oxide NCs and this new understanding will facilitate their implementation in different applications.

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50. 50

    Dahlman, C. J.; Heo, S.; Zhang, Y.; Reimnitz, L. C.; He, D.; Tang, M.; Milliron, D. J. Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles. J. Am. Chem. Soc. 2021, 143, 8278– 8294,  DOI: 10.1021/jacs.0c10628

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    50

    Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

    Dahlman, Clayton J.; Heo, Sungyeon; Zhang, Youtian; Reimnitz, Lauren C.; He, Daniel; Tang, Ming; Milliron, Delia J.

    Journal of the American Chemical Society (2021), 143 (22), 8278-8294CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Nanocryst. anatase TiO2 is a robust model anode for Li insertion in batteries. The influence of nanocrystal size on the equil. potential and kinetics of Li insertion is investigated with in operando spectroelectrochem. of thin film electrodes. Distinct visible and IR responses correlate with Li insertion and electron accumulation, resp., and these optical signals are used to deconvolute bulk Li insertion from other electrochem. responses, such as double-layer capacitance, pseudocapacitance, and electrolyte leakage. Electrochem. titrn. and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO2 systematically tunes the Li-insertion potentials with the particle size. However, the particle size does not affect the kinetics of Li insertion in ensemble electrodes. Rather, the Li-insertion rates depend on the applied overpotential, electrolyte concn., and initial state of charge. We conclude that Li diffusivity and phase propagation are not rate limiting during Li insertion in TiO2 nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO2 particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute the nonequil. charging behavior in nanocryst. electrodes through a combination of colloidal synthesis, phase field simulations, and spectroelectrochem.

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51. 51

    Skjærvø, S. L.; Ong, G. K.; Grendal, O. G.; Wells, K. H.; van Beek, W.; Ohara, K.; Milliron, D. J.; Tominaka, S.; Grande, T.; Einarsrud, M.-A. Understanding the Hydrothermal Formation of NaNbO₃: Its Full Reaction Scheme and Kinetics. Inorg. Chem. 2021, 60, 7632– 7640,  DOI: 10.1021/acs.inorgchem.0c02763

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    51

    Understanding the Hydrothermal Formation of NaNbO3: Its Full Reaction Scheme and Kinetics

    Skjaervo Susanne Linn; Grendal Ola Gjonnes; Wells Kristin Hoydalsvik; Grande Tor; Einarsrud Mari-Ann; Ong Gary K; Milliron Delia J; van Beek Wouter; Ohara Koji; Tominaka Satoshi

    Inorganic chemistry (2021), 60 (11), 7632-7640 ISSN:.

    Sodium niobate (NaNbO3) attracts attention for its great potential in a variety of applications, for instance, due to its unique optical properties. Still, optimization of its synthetic procedures is hard due to the lack of understanding of the formation mechanism under hydrothermal conditions. Through in situ X-ray diffraction, hydrothermal synthesis of NaNbO3 was observed in real time, enabling the investigation of the reaction kinetics and mechanisms with respect to temperature and NaOH concentration and the resulting effect on the product crystallite size and structure. Several intermediate phases were observed, and the relationship between them, depending on temperature, time, and NaOH concentration, was established. The reaction mechanism involved a gradual change of the local structure of the solid Nb2O5 precursor upon suspending it in NaOH solutions. Heating gave a full transformation of the precursor to HNa7Nb6O19·15H2O, which destabilized before new polyoxoniobates appeared, whose structure depended on the NaOH concentration. Following these polyoxoniobates, Na2Nb2O6·H2O formed, which dehydrated at temperatures ≥285 °C, before converting to the final phase, NaNbO3. The total reaction rate increased with decreasing NaOH concentration and increasing temperature. Two distinctly different growth regimes for NaNbO3 were observed, depending on the observed phase evolution, for temperatures below and above ≈285 °C. Below this temperature, the growth of NaNbO3 was independent of the reaction temperature and the NaOH concentration, while for temperatures ≥285 °C, the temperature-dependent crystallite size showed the characteristics of a typical dissolution-precipitation mechanism.

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