High-valence metal-oxo (M-O, M = Fe, Mn, etc.) species are well-known reaction intermediates that are responsible for a wide range of pivotal oxygenation reactions and water oxidation reactions in metalloenzymes. Although extensive efforts have been devoted to synthesizing and identifying such complexes in biomimetic studies, the structure-function relationship and related reaction mechanisms of these reaction intermediates remain elusive, especially for the cobalt-oxygen species. In the present manuscript, the calculated results demonstrate that the tetraamido macrocycle ligated cobalt complex, Co(O)(TAML) (1), behaves like a chameleon: the electronic structure varies from a cobalt(iii)-oxyl species to a cobalt(iv)-oxo species when a Lewis acid Sc3+ salt coordinates or an acidic hydrocarbon attacks 1. The dichotomous correlation between the reaction rates of C-H bond activation by 1 and the bond dissociation energy (BDE) vs. the acidity (pKa) was rationalized for the first time by different reaction mechanisms: for normal C-H bond activation, the Co(iii)-oxyl species directly activates the C-H bond via a hydrogen atom transfer (HAT) mechanism, whereas for acidic C-H bond activation, the Co(iii)-oxyl species evolves to a Co(iv)-oxo species to increase the basicity of the oxygen to activate the acidic C-H bond, via a novel PCET(PT) mechanism (proton-coupled electron transfer with a PT(proton-transfer)-like transition state). These theoretical findings will enrich the knowledge of biomimetic metal-oxygen chemistry.
Bibliographical noteFunding Information:
This work is supported by the National Natural Science Foundation of China (no. 21873052), the National Research Foundation (NRF) of Korea (no. NRF-2021R1A3B1076539), the Natural Science Foundation of Zhejiang Province (no. LQ20B030004), the Ningbo Natural Science Foundation (No. 202003N4079), the Scientific Research Grant of Ningbo University (no. 215-432000282) and the Ningbo Top Talent Project (no. 215-432094250).
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