Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel-Bipyridine Photoredox Catalysts

Ni 2,2 '-bipyridine (bpy) complexes are commonlyemployed photoredox catalysts of bond-forming reactions inorganic chemistry. However, the mechanisms by which theyoperate are still under investigation. One potential mode ofcatalysis is via entry into Ni(I)/Ni(III) cycles, which can be madepossible by light-induced, excited-state Ni(II)-C bond homolysis.Here, we report experimental and computational analyses of alibrary of Ni(II)-bpy aryl halide complexes, Ni(Rbpy)(R ' Ph)Cl (R= MeO,t-Bu, H, MeOOC; R '=CH3, H, OMe, F, CF3), toilluminate the mechanism of excited-state bond homolysis. At givenexcitation wavelengths, photochemical homolysis rate constants span 2 orders of magnitude across these structures and correlatelinearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge-transfer(MLCT) and ligand-to-metal charge-transfer (LMCT) excited-state potential energy surfaces (PESs). Temperature- andwavelength-dependent investigations reveal moderate excited-state barriers (Delta H double dagger similar to 4 kcal mol-1) and a minimum energy excitationthreshold (similar to 55 kcal mol-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism inwhich repulsive triplet excited-state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited-state PESs provides a rational approach to utilize photonic energy and leverage excited-state bond homolysis processes in syntheticchemistry.