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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. 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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4. Hydrogen peroxide adducts of triarylphosphine oxides

   https://doi.org/10.1039/c9dt03070k  

   Arp, Fabian F.; Bhuvanesh, Nattamai; Blümel, Janet (October 2019, Dalton Transactions)

   Five new hydrogen peroxide adducts of phosphine oxides ( p -Tol 3 PO·H 2 O 2 ) 2 ( 1 ), ( o -Tol 3 PO·H 2 O 2 ) 2 ( 2 ), ( o -Tol 2 PhPO·H 2 O 2 ) 2 ( 3 ), ( p -Tol 3 PO) 2 ·H 2 O 2 ( 4 ), and ( o -TolPh 2 PO) 2 ·H 2 O 2 ( 5 ), and the water adduct ( o -Tol 2 PhPO·H 2 O) 2 ( 6 ) have been synthesized and fully characterized. Their single crystal X-ray structures have been determined and analyzed. The IR and 31 P NMR data are in accordance with strong hydrogen bonding of the hydrogen peroxide. The mono- versus dimeric nature of the adduct assemblies has been investigated by DOSY NMR experiments. Raman spectroscopy of the symmetric adducts and the ν (O–O) stretching bands confirm the presence of hydrogen-bonded hydrogen peroxide in the solid materials. The solubilities in organic solvents have been quantified. Due to the high solubilities of 1–6 in organic solvents their 17 O NMR spectra could be recorded in natural abundance, providing well-resolved signals for the PO and O–O groups. The adducts 1–5 have been probed regarding their stability in solution at 105 °C. The decomposition of the adduct 1 takes place by loss of the active oxygen atoms in two steps. 

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5. Di(hydroperoxy)adamantane adducts: synthesis, characterization and application as oxidizers for the direct esterification of aldehydes

   https://doi.org/10.1039/d1dt03243g  

   Arp, Fabian F.; Ashirov, Rahym; Bhuvanesh, Nattamai; Blümel, Janet (November 2021, Dalton Transactions)

   The di(hydroperoxy)adamantane adducts of water (1) and phosphine oxides p -Tol 3 PO·(HOO) 2 C(C 9 H 14 ) (2), o -Tol 3 PO·(HOO) 2 C(C 9 H 14 ) (3), and Cy 3 PO·(HOO) 2 C(C 9 H 14 ) (4), as well as a CH 2 Cl 2 adduct of a phosphole oxide dimer (8), have been created and investigated by multinuclear NMR spectroscopy, and by Raman and IR spectroscopy. The single crystal X-ray structures for 1–4 and 8 are reported. The IR and 31 P NMR data are in accordance with strong hydrogen bonding of the di(hydroperoxy)adamantane adducts. The Raman ν (O–O) stretching bands of 1–4 prove that the peroxo groups are present in the solids. Selected di(hydroperoxy)alkane adducts, in combination with AlCl 3 as catalyst, have been applied for the direct oxidative esterification of n -nonyl aldehyde, benzaldehyde, p -methylbenzaldehyde, p -bromobenzaldehyde, and o -hydroxybenzaldehyde to the corresponding methyl esters. The esterification takes place in an inert atmosphere, under anhydrous and oxygen-free conditions, within a time frame of 45 minutes to 5 hours at room temperature. Hereby, two oxygen atoms per adduct assembly are active with respect to the quantitative transformation of the aldehyde into the ester. 

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