DFT mechanistic insights into the formation of the metal-dioxygen complex [Co(12-TMC)O2]+ using H2O2 as an [O2] unit source

Abstract

The reaction of [M(L)]ⁿ with H₂O₂ as an [O₂] unit source and NEt₃ as a base is a widely used biomimetic transition metal-peroxo and -superoxo complex [M(L)O₂]ⁿ⁻¹ synthesis method, but the mechanism and accurate stoichiometry of the synthesis remain elusive. In this study, we performed DFT calculations to deeply understand the mechanism, using the synthesis of the cobalt-peroxo complex [Co^III(12-TMC)O₂]⁺ (12-TMC = (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane)) from the reaction of [Co^II(12-TMC)]²⁺ and H₂O₂ in the presence of NEt₃ as an example. The study found that cobalt-peroxo complex formation proceeds via three stages: (Stage I) the conversion of [Co^II(12-TMC)]²⁺ and H₂O₂ to [Co^III(12-TMC)OH]²⁺ and OOH˙ radical, (Stage II) the coordination of OOH˙ to [Co^II(12-TMC)]²⁺ to give [Co^III(12-TMC)OOH]²⁺, followed by deprotonation with NEt₃, affording [Co^III(12-TMC)O₂]⁺, and (Stage III) the transformation of [Co^III(12-TMC)OH]²⁺ which is generated in Stage I to [Co^III(12-TMC)O₂]⁺. The overall stoichiometry of the synthesis is 2*[Co(12-TMC)]²⁺ + 3*H₂O₂ + 2*NEt₃ → 2*[Co(12-TMC)O₂]⁺ + 2*HNEt₃⁺ + 2*H₂O. In addition, compared to its analog [Co^III(TBDAP)O₂]⁺ (TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane) which is synthesized by the same method and has the same Co(III) oxidation state exhibits dioxygenase-like reactivity to nitriles, [Co^III(12-TMC)O₂]⁺ could be inactive towards acetonitrile because the reaction severely deteriorates the coordination of the 12-TMC ligand to the Co center, which results in high reaction barriers.