Multireference Description of Nickel–Aryl Homolytic Bond Dissociation Processes in Photoredox Catalysis
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Multireference electronic structure calculations consistent with known experimental data have elucidated a novel mechanism for photo-triggered Ni(II)–C homolytic bond dissociation in Ni 2,2’-bipyridine (bpy) photoredox catalysts. Previously, a thermally assisted dissociation from the lowest energy triplet ligand field excited state was proposed and supported by density functional theory (DFT) calculations that reveal a barrier of ~30 kcal mol-1. In contrast, multireference ab initio calculations suggest this process is disfavored, with barrier heights of ~70 kcal mol-1, and highlight important ligand noninnocent contributions to excited state relaxation and bond dissociation processes that are not captured with DFT. In the multireference description, photo-triggered Ni(II)–C homolytic bond dissociation occurs via initial population of a singlet Ni(II)-to-bpy metal-to-ligand charge transfer (1MLCT) excited state followed by intersystem crossing and aryl-to-Ni(III) charge transfer, overall a formal two-electron transfer process driven by a single photon. This results in repulsive triplet excited states from which spontaneous homolytic bond dissociation can occur, effectively competing with relaxation to the lowest energy, nondissociative triplet Ni(II) ligand field excited state. These findings guide important electronic structure considerations for the experimental and computational elucidation of the mechanisms of ground and excited state cross-coupling catalysis mediated by Ni heteroaromatic complexes.