Phosphine-Catalyzed C(sp2)–H Bond Activation with a Heterometallic Nickel---Aluminum Complex

Herein we present the discovery of a ligand-catalyzed bond breaking and making processes that occurs in the coordination sphere of a novel Ni–Al heterometallic complex. While combinations of Ni pre-catalysts and Al additives are known for site-selective C–H functionalisation, detailed studies of such systems are rare. Combining [Ni(COD)2] and a molecular aluminum dihydride [(BDI)AlH2] (1; BDI = 2,6-diisopropylphenyl-beta-methyldiketiminate), results in facile formation of a Ni–Al heterometallic complex [(COD)Ni{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 an exogenous phosphine ligand, PCy3. Intimate steps of the mechanism of C(sp2)–H bond activation and H2 elimination were studied through kinetics, kinetic isotope effects (KIEs), isotope labelling studies, and computational modeling. The reaction is 1st order in PCy3 and proceeds with a low KIE of 0.9-1.1. Support is provided for a mechanism involving stepwise oxidative addition and reductive elimination processes that require the synergistic action of both metals, promoted by reversible coordination of the phosphine. These data strongly suggest that both C–H activation and H2 reductive elimination steps in this system are low energy and readily accessible and are not rate limiting. This finding has implications for future catalyst design using the synergistic behavior of Ni and Al metals and suggests that choice of ligand and control of ligand exchange steps, may well be the most important consideration in determining the rate of reaction.