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

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 LNiBr ₂ -type precursor [Ni ₃ (μ ² -Br) ₃ (μ ³ -Br) ₂ (L ^Me-2,4 ) ₃ ]·Br (1; L ^Me-2,4 = [(2,4-Me ₂ C ₆ H ₃ )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 C═N 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.