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1. Statistical analysis of C–H activation by oxo complexes supports diverse thermodynamic control over reactivity

   https://doi.org/10.1039/D0SC06058E  

   Schneider, Joseph E.; Goetz, McKenna K.; Anderson, John S. (March 2021, Chemical Science)

   null (Ed.)

   Transition metal oxo species are key intermediates for the activation of strong C–H bonds. As such, there has been interest in understanding which structural or electronic parameters of metal oxo complexes determine their reactivity. Factors such as ground state thermodynamics, spin state, steric environment, oxygen radical character, and asynchronicity have all been cited as key contributors, yet there is no consensus on when each of these parameters is significant or the relative magnitude of their effects. Herein, we present a thorough statistical analysis of parameters that have been proposed to influence transition metal oxo mediated C–H activation. We used density functional theory (DFT) to compute parameters for transition metal oxo complexes and analyzed their ability to explain and predict an extensive data set of experimentally determined reaction barriers. We found that, in general, only thermodynamic parameters play a statistically significant role. Notably, however, there are independent and significant contributions from the oxidation potential and basicity of the oxo complexes which suggest a more complicated thermodynamic picture than what has been shown previously. 

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2. Concerted proton–electron transfer reactions of manganese–hydroxo and manganese–oxo complexes

   https://doi.org/10.1039/d0cc01201g  

   Mayfield, Jaycee R.; Grotemeyer, Elizabeth N.; Jackson, Timothy A. (January 2020, Chemical Communications)

   The enzymes manganese superoxide dismutase and manganese lipoxygenase use Mn III –hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C–H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn–oxo motifs in attacking substrate C–H bonds, presumably by a concerted proton–electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn–hydroxo and Mn–oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of Mn III –hydroxo and Mn IV –oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around Mn III –hydroxo and Mn IV –oxo motifs permit elucidation of structure–activity relationships. For Mn III –hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the Mn III/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the p K a of the Mn II –aqua product, which represents the proton-transfer component for CPET. For Mn IV –oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the Mn IV –oxo unit results in large rate enhancements for C–H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of Mn III –hydroxo and Mn IV –oxo adducts. 

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3. Structure and Unprecedented Reactivity of a Mononuclear Nonheme Cobalt(III) Iodosylbenzene Complex

   https://doi.org/10.1002/anie.202005091  

   Yang, Jindou; Seo, Mi Sook; Kim, Kyung Ha; Lee, Yong‐Min; Fukuzumi, Shunichi; Shearer, Jason; Nam, Wonwoo (May 2020, Angewandte Chemie International Edition)

   Abstract A mononuclear nonheme cobalt(III) iodosylbenzene complex, [Co^III(TQA)(OIPh)(OH)]²⁺(1), is synthesized and characterized structurally and spectroscopically. While1is a sluggish oxidant in oxidation reactions, it becomes a competent oxidant in oxygen atom transfer reactions, such as olefin epoxidation, in the presence of a small amount of proton. More interestingly,1shows a nucleophilic reactivity in aldehyde deformylation reaction, demonstrating that1has an amphoteric reactivity. Another interesting observation is that1can be used as an oxygen atom donor in the generation of high‐valent metal‐oxo complexes. To our knowledge, we present the first crystal structure of a Co^IIIiodosylbenzene complex and the unprecedented reactivity of metal‐iodosylarene adduct. 

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4. Tuning the H‐Atom Transfer Reactivity of Iron(IV)‐Oxo Complexes as Probed by Infrared Photodissociation Spectroscopy

   https://doi.org/10.1002/anie.202016695  

   Tripodi, Guilherme L.; Dekker, Magda M. J.; Roithová, Jana; Que, Jr., Lawrence (February 2021, Angewandte Chemie International Edition)

   Abstract Reactivities of non‐heme iron(IV)‐oxo complexes are mostly controlled by the ligands. Complexes with tetradentate ligands such as [(TPA)FeO]²⁺(TPA=tris(2‐pyridylmethyl)amine) belong to the most reactive ones. Here, we show a fine‐tuning of the reactivity of [(TPA)FeO]²⁺by an additional ligand X (X=CH₃CN, CF₃SO₃⁻, ArI, and ArIO; ArI=2‐(^tBuSO₂)C₆H₄I) attached in solution and reveal a thus far unknown role of the ArIO oxidant. The HAT reactivity of [(TPA)FeO(X)]^+/2+decreases in the order of X: ArIO > MeCN > ArI ≈ TfO⁻. Hence, ArIO is not just a mere oxidant of the iron(II) complex, but it can also increase the reactivity of the iron(IV)‐oxo complex as a labile ligand. The detected HAT reactivities of the [(TPA)FeO(X)]^+/2+complexes correlate with the Fe=O and FeO−H stretching vibrations of the reactants and the respective products as determined by infrared photodissociation spectroscopy. Hence, the most reactive [(TPA)FeO(ArIO)]²⁺adduct in the series has the weakest Fe=O bond and forms the strongest FeO−H bond in the HAT reaction. 

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5. Modeling the roles of rigidity and dopants in single-atom methane-to-methanol catalysts

   https://doi.org/10.1039/D1TA08502F  

   Jia, Haojun; Nandy, Aditya; Liu, Mingjie; Kulik, Heather J. (January 2022, Journal of Materials Chemistry A)

   Doped graphitic single-atom catalysts (SACs) with isolated iron sites have similarities to natural enzymes and molecular biomimetics that can convert methane to methanol via a radical rebound mechanism with high-valent Fe( iv )O intermediates. To understand the relationship of SACs to these homogeneous analogues, we use range-separated hybrid density functional theory (DFT) to compare the energetics and structure of the direct metal-coordinating environment in the presence of 2p ( i.e. , N or O) and 3p ( i.e. , P or S) dopants and with increasing finite graphene model flake size to mimic differences in local rigidity. While metal–ligand bond lengths in SACs are significantly shorter than those in transition-metal complexes, they remain longer than SAC mimic macrocyclic complexes. In SACs or the macrocyclic complexes, this compressed metal–ligand environment induces metal distortion out of the plane, especially when reactive species are bound to iron. As a result of this modified metal-coordination environment, we observe SACs to simultaneously favor the formation of the metal–oxo while also allowing for methanol release. This reactivity is different from what has been observed for large sets of square planar model homogeneous catalysts. Overall, our calculations recommend broader consideration of dopants ( e.g. , P or S) and processing conditions that allow for local distortion around the metal site in graphitic SACs. 

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