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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Mechanistic insight into rapid oxygen-atom transfer from a calix-functionalized polyoxovanadate
We report accelerated rates of oxygen-atom transfer from a polyoxovanadate–alkoxide cluster following functionalization with a 4- tert butylcalix[4]arene ligand. Incorporation of this electron withdrawing ligand modifies the electronics of the metal oxide core, favoring a mechanism in which the rate of oxygen-atom transfer is limited by outer-sphere electron transfer.
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- Award ID(s):
- 1653195
- NSF-PAR ID:
- 10337038
- Date Published:
- Journal Name:
- Chemical Communications
- Volume:
- 58
- Issue:
- 40
- ISSN:
- 1359-7345
- Page Range / eLocation ID:
- 6004 to 6007
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Hydrogen-atom (H-atom) transfer at the surface of heterogeneous metal oxides has received significant attention owing to its relevance in energy conversion and storage processes. Here, we present the synthesis and characterization of an organofunctionalized polyoxovanadate cluster, (calix)V6O5(OH2)(OMe) 8 (calix = 4- tert -butylcalix[4]arene). Through a series of equilibrium studies, we establish the BDFE(O–H) avg of the aquo ligand as 62.4 ± 0.2 kcal mol −1 , indicating substantial bond weaking of water upon coordination to the cluster surface. Subsequent kinetic isotope effect studies and Eyring analysis indicate the mechanism by which the hydrogenation of organic substrates occurs proceeds through a concerted proton–electron transfer from the aquo ligand. Atomistic resolution of surface reactivity presents a novel route of hydrogenation reactivity from metal oxide surfaces through H-atom transfer from surface-bound water molecules.more » « less
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Photoinduced atom transfer radical polymerization (photo-ATRP) has risen to the forefront of modern polymer chemistry as a powerful tool giving access to well-defined materials with complex architecture. However, most photo-ATRP systems can only generate radicals under biocidal UV light and are oxygen-sensitive, hindering their practical use in the synthesis of polymer biohybrids. Herein, inspired by the photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT) polymerization, we demonstrate a dual photoredox/copper catalysis that allows open-air ATRP under green light irradiation. Eosin Y was used as an organic photoredox catalyst (PC) in combination with a copper complex (X–Cu II /L). The role of PC was to trigger and drive the polymerization, while X–Cu II /L acted as a deactivator, providing a well-controlled polymerization. The excited PC was oxidatively quenched by X–Cu II /L, generating Cu I /L activator and PC˙ + . The ATRP ligand (L) used in excess then reduced the PC˙ + , closing the photocatalytic cycle. The continuous reduction of X–Cu II /L back to Cu I /L by excited PC provided high oxygen tolerance. As a result, a well-controlled and rapid ATRP could proceed even in an open vessel despite continuous oxygen diffusion. This method allowed the synthesis of polymers with narrow molecular weight distributions and controlled molecular weights using Cu catalyst and PC at ppm levels in both aqueous and organic media. A detailed comparison of photo-ATRP with PET-RAFT polymerization revealed the superiority of dual photoredox/copper catalysis under biologically relevant conditions. The kinetic studies and fluorescence measurements indicated that in the absence of the X–Cu II /L complex, green light irradiation caused faster photobleaching of eosin Y, leading to inhibition of PET-RAFT polymerization. Importantly, PET-RAFT polymerizations showed significantly higher dispersity values (1.14 ≤ Đ ≤ 4.01) in contrast to photo-ATRP (1.15 ≤ Đ ≤ 1.22) under identical conditions.more » « less
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Abstract Decarbonylation along with P‐atom transfer from the phosphaethynolate anion, PCO−, to the NbIVcomplex [(PNP)NbCl2(N
t BuAr)] (1 ) (PNP=N[2‐Pi Pr2‐4‐methylphenyl]2−; Ar=3,5‐Me2C6H3) results in its coupling with one of the phosphine arms of the pincer ligand to produce a phosphanylidene phosphorane complex [(PNPP)NbCl(Nt BuAr)] (2 ). Reduction of2 with CoCp*2cleaves the P−P bond to form the first neutral and terminal phosphido complex of a group 5 transition metal, namely, [(PNP)Nb≡P(Nt BuAr)] (3 ). Theoretical studies have been used to understand both the coupling of the P‐atom and the reductive cleavage of the P−P bond. Reaction of3 with a two‐electron oxidant such as ethylene sulfide results in a diamagnetic sulfido complex having a P−P coupled ligand, namely [(PNPP)Nb=S(Nt BuAr)] (4 ).
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2CC01228F
