Photochemical Mechanism of Co–C Bond Activation via Triplet Energy Transfer in Ethyl(aqua)cobaloxime

The photochemical decomposition of ethyl(aqua)cobaloxime, [EtCo^III(dmgH)₂H₂O], a vitamin B₁₂ derivative model complex, was investigated to understand the mechanism of the Co-C_Et bond scission induced by light. Upon irradiation of [EtCo^III(dmgH)₂H₂O], the Co-C_Et bond undergoes homolytic scission, resulting in Et/Co(II) radical pair (RP) formation in a similar fashion observed in alkylcobalamins. The [EtCo^III(dmgH)₂H₂O] complex acts as a potent quencher of a wide variety of excited states in the presence of organic molecules such as benzophenone. It has been proposed that the reaction mechanism involves Dexter energy transfer, resulting in the bond dissociation process. Two issues associated with the proposed mechanism have been investigated, namely, (i) how the energy transfer occurs from benzophenone to cobaloxime and (ii) how the Co-C_Et bond is activated and cleaved. Both TD-DFT and CASSCF/NEVPT2 methods have been applied to show the feasibility of the energy transfer reaction via the triplet pathway from photocatalyst to substrate.