Photolytic activation of Ni^(II)X₂L explains how Ni-mediated cross coupling begins

Nickel photocatalysis has recently become vital to organic synthesis, but how the Ni^(II)X₂L pre-catalyst (X = Cl, Br; L = bidentate ligand) becomes activated to Ni^(I)XL has remained puzzling and is typically addressed on a case-by-case basis. Here, we reveal a general mechanism where light induces photolysis of the Ni^(II)-X bond, either via direct excitation or triplet energy transfer. Photolysis produces Ni^(I)XL and a halogen radical, X^•. Subsequent hydrogen atom abstraction, often from the solvent, produces a C(sp³) radical, R^•, that recombines with Ni^(I) to form organonickel(II) complexes, Ni^(II)XRL. Rather than acting as a loss pathway, Ni^(II)XRL behaves as a light-activated reservoir of Ni^(I) via photolysis of the Ni^(II)-C bond. These results explain the role of the solvent in protecting the catalyst from off-cycle dimerization, demonstrate that two photons are often required to drive the reaction, and show how tuning the ligand can control the concentration of active Ni^(I) species.