Despite the prevalence of Ni/Al catalysts for pyridine C─H functionalization, mechanistic details of such systems remain scarce. Herein, we present the discovery of PCy₃-catalyzed bond-breaking and making processes that occur in the coordination sphere of a novel Ni─Al heterometallic complex. The reaction is 1^st order in PCy₃ and proceeds with a low KIE of 0.9–1.1. These data suggest that both C─H activation and H₂ reductive elimination steps in this system are low energy, are readily accessible, and are not rate limiting.

Abstract

Herein we present a ligand-catalyzed bond-breaking and making process that occurs with a Ni–Al heterometallic complex. While combinations of Ni pre-catalysts and Al additives are known for site-selective C─H functionalization, detailed studies of such systems are rare. Combining [Ni(COD)₂] and [(BDI)AlH₂] (1; BDI = 2,6-diisopropylphenyl-β-methyldiketiminate) results in facile formation of a Ni–Al heterometallic complex [(η ⁴-COD)Ni{(μ-H)₂Al(BDI)}] (2; COD = 1,5-cyclooctadiene). Once generated, this species can effect the C(sp²)─H activation of 4-dimethylaminopyridine (DMAP) or pyridine with concomitant H₂ evolution, a process that is accelerated through addition of PCy₃. The mechanism was studied through kinetics, kinetic isotope effects (KIEs), isotope labeling studies, and modeling. The reaction is 1^st order in PCy₃ and proceeds with a KIE of 0.9–1.1. Support is provided for a pathway involving the synergistic action of both metals, promoted by reversible coordination of the phosphine. The data suggest that both C─H oxidative addition and H₂ reductive elimination steps in this system are readily accessible and are not rate limiting. This finding has implications for future catalyst design using combinations of Ni and Al metals and suggests that control of ligand exchange steps may be the most important consideration in determining the rate of reaction.