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.. Authors: Kudisch, Max [National Renewable Energy Laboratory (NREL), Golden, CO (United States)] (ORCID:0000000219350049); Hooper, Reagan X. [SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States). Photon Ultrafast Laser Science and Engineering Institute (PULSE)]; Valloli, Lakshmy K. [Brookhaven National Laboratory (BNL), Upton, NY (United States)]; Earley, Justin D. [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Univ. of Colorado, Boulder, CO (United States)] (ORCID:0000000304924692); Zieleniewska, Anna [National Renewable Energy Laboratory (NREL), Golden, CO (United States)] (ORCID:0000000199877520). DOE Contract: AC02-06CH11357; AC02-76SF00515; AC36-08GO28308; SC0012704; SC0019370. Subjects: 08 HYDROGEN; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; C-H activation; EPR; Ni; Ni-C; Ni-Cl cleavage; Ni-X cleavage; TA; XAS; catalysis; energy transfer; hydrogen atom transfer; nickel; organometallic; photocatalysis; photochemistry; photolysis; photolytic; transient absorption