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

摘要

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 Ni-II [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 Ni-II, but also matches the experimental triplet energy requisite for energy transfer. In addition, the enhanced RE reactivity in excited Ni-II can be well rationalized by the Ni-III character of the MLCT state. The resolution of this intriguing mechanistic puzzle in the excited-state chemistry of a Ni-II complex underscores the potential of multireference methods in this field.