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C(sp2)─H Bond Activation with a Heterometallic Nickel-Aluminium Complex
Joseph A Zurakowski1,2, Benedek Stadler3, Marcus W Drover1
1Department of Chemistry, Western University, 1151 Richmond Street, London, ON, N8K 3G6, Canada.
None:
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 [(η4-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 1st 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.
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