Cyclotrimerization of alkynes catalyzed by a self-supported cyclic tri-nuclear nickel(0) complex with α-diimine ligands

Abstract

A cyclic tri-nuclear α-diimine nickel(0) complex [{Ni(μ-L^Me-2,4)}₃] (2) was synthesized from a “pre-organized”, trimerized trigonal LNiBr₂-type precursor [Ni₃(μ²-Br)₃(μ³-Br)₂(L^Me-2,4)₃]·Br (1; L^Me-2,4 = [(2,4-Me₂C₆H₃)NC(Me)]₂). In complex 2, the α-diimine ligands not only exhibit the normal N,N′-chelating mode, but they also act as bridges between the Ni atoms through an unusual π-coordination of a CN bond to Ni. Complex 2 is able to catalyze the cyclotrimerization of alkynes to form substituted benzenes in good yield and regio-selectivity for the 1,3,5-isomers, which is found to vary with the nature of the alkyne employed. This complex represents a convenient self-supported nickel(0) catalyst with no need for additional ligands and reducing agent.