Nickel-Mediated Radical Capture: Evidence for a Concerted Inner-Sphere Mechanism

Nickel catalysis enables cross-coupling of a broad scope of C(sp³) moieties by mediating carbon-carbon bond formation from carbon-centered radicals. A widely proposed mechanism involves stepwise radical capture by a nickel(II) complex that forms a nickel(III) intermediate. The alternative pathway, a concerted radical capture and carbon-carbon bond formation, has been largely overlooked. This study investigates the ligand effect and kinetics of nickel-mediated radical capture and reductive elimination, which provide evidence to distinguish between stepwise and concerted pathways. Through radical clock experiments, spectroscopic investigation, electrochemical studies, and multivariate linear regression analysis of a series of [(pybox)Ni(Ar)]BAr^F₄ complexes, we established a strong correlation between the rate of radical capture and HOMO and LUMO energies, along with positive charge stabilization at nickel and the aryl actor ligand. These data rule out the stepwise formation of a nickel(III) intermediate and support a concerted pathway. Redox-active <i>nitrogen</i> ligands and nonredox-active <i>phosphine</i> ligands exhibit contrasting reactivity, with only redox-active ligands facilitating radical capture and carbon-carbon bond formation. This critical role of ligand redox activity can be attributed to the participation of the LUMO in bond cleavage and formation. Among redox-active ligands, bidentate and tridentate ligands exhibit similar rates, suggesting a consistent mechanism with relatively minimal ancillary ligand effect. Our results highlight the critical interplay between ligand electronics, sterics, and orbital contributions, offering valuable design principles for nickel-catalyzed cross-coupling reactions involving radical intermediates.