Catalytic Dinuclear Nickel Spin Crossover Mechanism and Selectivity for Alkyne Cyclotrimerization

Homodinuclear transition-metal catalysts with a direct metal metal bond have the potential to induce novel reaction mechanisms and selectivity compared with mononuclear catalysts. The dinuclear (i-PrNDI)Ni-2(C6H6) (NDI = naphthyridine-diimine) complex catalyzes selective cyclotrimerization of monosubstituted alkynes, whereas mononuclear Ni catalysts generally give cyclotetramerization of alkynes. Density functional theory calculations reveal that the homodinuclear Ni-Ni catalyst induces a spin crossover mechanism that involves metallacyclopentadiene and nonclassical bridging metallacycloheptatriene intermediates. The cis configuration of the nonclassical bridging metallacycloheptatriene Ni-vinyl bonds results in alkyne cyclotrimerization by fast reductive elimination. This dinuclear mechanism differs from previously reported mononuclear Ni mechanisms and provides an explanation for cyclotrimerization versus cyclotetramerization selectivity and arene regioselectivity.