Redox Activity and Potentials of Bidentate N-Ligands Commonly Applied in Nickel-Catalyzed Cross-Coupling Reactions

Bidentate N-ligands are paramount to recent advances in nickel-catalyzed cross-coupling reactions. Through comprehensive organometallic, spectroscopic, and computational studies on bi-oxazoline and imidazoline ligands, we reveal that a square planar geometry enables redox activity of these ligands in stabilizing nickel radical species. This finding contrasts with the prior assumption that bi-oxazoline lacks redox activity due to strong mesomeric donation. Moreover, we conducted systematic cyclic voltammetry (CV) analyses of bidentate pyridyl, oxazoline, and imidazoline nitrogen ligands, along with their corresponding nickel complexes. Complexation with nickel shifts the reduction potentials to a more positive region and narrows the differences in redox potentials among the ligands. Additionally, various ligands led to different degrees of bromide dissociation from singly reduced (L)Ni(Ar)(Br) complexes, reflecting varying reactivity in the subsequent activation of alkyl halides, a crucial step in cross-electrophile coupling. These insights highlight the significant electronic effects of ligands on the stability of metalloradical species and their redox potentials, which interplay with coordination geometry. Quantifying the electron-donating, π-accepting properties of these ligands, as well as their effect on catalyst speciation, provides crucial benchmarks for controlling catalytic activity and enhancing catalyst stability.