Nickel-based photoredox catalysis has seen wide use in synthetic chemistry; these catalysts have often been proposed to operate via entry into Ni(I)/Ni(III) cycles, made possible by light-induced, excited state Ni–C bond homolysis. However, the mechanism of this critical step is not well understood. Herein we report experimental and theoretical analyses of a wide library of nickel-bipyridine aryl halide complexes, Ni(Rbpy)(R′-aryl)Cl (R = MeO, t-Bu, H, MeOOC; R′ = CH3, H, OMe, F, CF3), to illuminate the mechanism of excited state bond homolysis. Kinetic interrogation of this step reveals photochemical rates spanning two orders of magnitude across these structures, demonstrating a correlation between experimental rates and the energies of both the metal-to-ligand (MLCT) charge transfer and ligand-to-metal charge transfer (LMCT) excited state potential energy surfaces. Varying the incident light energy (390 nm – 525 nm) revealed a minimum energy threshold for the initial excitation, and temperature-dependent rates have provided an experimental activation energy. From these data, we find Ni–C bond homolysis to be the result of high energy Ni(II)-to-bipyridine charge transfer followed by aryl-to-nickel LMCT, which grants entry into repulsive triplet excited state potential energy surfaces. Time-dependent density functional theory (TDDFT) and multireference ab initio methods (CASSCF/QD-NEVPT2) were both used to identify these key repulsive states. This report presents the first experimental evidence of repulsive excited state surfaces in catalytically relevant nickel-complexes and provides a framework with which to rationalize and employ excited state bond homolysis processes in synthetic applications.

Elucidating the mechanism of excited state bond homolysis in nickel–bipyridine photoredox catalysts

ACS Spring 2022

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