Synthetically useful hydrocarbon oxidations are catalysed by bio-inspired non-heme iron complexes using hydrogen peroxide as oxidant, and carboxylic acid addition enhances their selectivity and catalytic efficiency. Talsi has identified a low-intensity g=2.7 electron paramagnetic resonance signal in such catalytic systems and attributed it to an oxoiron(V)-carboxylate oxidant. Herein we report the use of FeII (TPA*) (TPA*=tris(3,5- dimethyl-4-methoxypyridyl-2-methyl)amine) to generate this intermediate in 50% yield, and have characterized it by ultraviolet-visible, resonance Raman, Mössbauer and electrospray ionization mass spectrometric methods as a low-spin acylperoxoiron(III) species. Kinetic studies show that this intermediate is not itself the oxidant but decays via a unimolecular rate-determining step to unmask a powerful oxidant. The latter is shown by density functional theory calculations to be an oxoiron(V) species that oxidises substrate without a barrier. This study provides a mechanistic scenario for understanding catalyst reactivity and selectivity as well as a basis for improving catalyst design.
Bibliographical noteFunding Information:
This work was supported by the US Department of Energy (grant DE-FG02-03ER15455 to L.Q.), the US National Science Foundation (grants CHE1012485 and CHE1305111 to E.M.), the National Science Foundation of China (grants 21003116 and 21173211 to Y.W.), and the Israel Science Foundation (grant 1183/13 to S.S.). K.K.M. wishes to thank Dr E.L. Bominaar for providing valuable insight into the DFT calculations of the acyl-peroxoiron(III) complex.