A Comprehensive Multireference Study of Excited-State Ni-Br Bond Homolysis in (dtbbpy)Ni^II(aryl)(Br)

The mechanism of visible light-driven Ni-C(aryl) bond homolysis in (2,2′-bipyridine)Ni^II(aryl)(halide) complexes, which play a crucial role in metallaphotoredox catalysis for cross-coupling reactions, has been well studied. Differently, the theoretical understanding of Ni-halide bond homolysis remains limited. In this study, we introduce a novel electronic structural framework to elucidate the mechanisms underlying photoinduced Ni-Br bond rupture in the (dtbbpy)Ni^II(aryl)(Br) complex. Using multireference ab initio calculations, we characterized the excited state potential energy surfaces corresponding to metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT). Our calculations reveal that the Ni-Br dissociation, triggered by an external photocatalyst, begins with the promotion of Ni(II) to a ¹MLCT excited state. This state undergoes intersystem crossing with repulsive triplet surfaces corresponding to the ³MLCT and Br-to-Ni ³LMCT states, resulting in Ni-Br bond breaking via the Dexter energy transfer mechanism. In the absence of a photocatalyst, the photoexcited Ni(II) favors Ni-C(aryl) homolysis, whereas the presence of a photocatalyst promotes Ni-Br dissociation. The Ni(III) species, resulting from the oxidation of Ni(II) by the photocatalyst, was found to be unproductive toward Ni-Br or Ni-C(aryl) activation.