Reactivity of Transition-Metal Complexes in Excited States: C–O Bond Coupling Reductive Elimination of a Ni(II) Complex Is Elicited by the Metal-to-Ligand Charge Transfer State

Abstract

Reactivity of electronically excited base transition metals is an emerging frontier wherein mechanistic understanding is highly desired but mostly lacking. To reveal how C–O bond coupling reductive elimination (RE) is stimulated by excited NiII [Welin, E. R.; Le, C.; Arias-Rotondo, D. M.; McCusker, J. K.; MacMillan, D. W. C. Science 2017, 355, 380], we report here high-level theoretical modellings based on a combined ab initio protocol (CASSCF, CASPT2, DLPNO–CCSD(T)). In contrast to the experimental proposal of the d-d excited state, we find that the metal-to-ligand charge transfer (MLCT) excited state is most likely to stimulate the C–O coupling RE. This unprecedented assignment of the reactive excited state not only obviates the known thermodynamic prohibition of C–O coupling by ground state NiII, but also matches the experimental triplet energy requisite for energy transfer. In addition, the enhanced RE reactivity in excited NiII can be well rationalized by the NiIII character of the MLCT state. The resolution of this intriguing mechanistic puzzle in the excited-state chemistry of a NiII complex underscores the potential of multireference methods in this field.