Carbon-centered radical capture at nickel(II) complexes: Spectroscopic evidence, rates, and selectivity

The capture of carbon-centered radicals at a nickel(II) center is commonly featured in recent cross-coupling and metallaphotoredox catalytic reactions. Despite its widespread application in catalysis, this fundamental step lacks experimental characterization. This report portrays radical capture at catalytically relevant nickel(II) centers from several aspects, including the structure-activity relationships of the ligands, the mechanism, the kinetics, and the stereoselectivity. Spectroscopic data provide evidence for the formation of a nickel(III) intermediate. Strikingly different reactivity between nickel-aryl and nickel-alkyl complexes implies different rate-determining steps for C(sp³)-C(sp³) and C(sp²)-C(sp³) bond formation. Kinetic data benchmark the capture rates on the scale of 10⁷ M^-1s^-1 and 10⁶ M^-1s^-1 for primary and secondary radicals, respectively. Overall, the activation energy is higher than that of previous computational estimations. Finally, stoichiometric experiments with well-defined chiral nickel complexes demonstrate that the radical trapping step can confer diastereoselectivity and enantioselectivity with a drastic ligand effect.