Formation and crystallographic elucidation of stable [4 + 2]-coordinate nickel(ii) N,S-heterocyclic carbene (NSHC) complexes

Abstract

A series of square-planar N,S-heterocyclic carbene (NSHC) complexes trans-[NiX2(N-RBzTh)2] (BzTh = benzothiazolin-2-ylidene) (R/X = Bz/Br; Me/I; Et/I; Pri/I; Bui/I) have been synthesized and characterized by X-ray single-crystal diffraction analysis. These are the first crystallographically established NSHC complexes of nickel in the literature. The N,S-heterocyclic carbene (NSHC) rings invariably twist away from co-planarity with the metal coordination plane such that the N-substituent moves on top and below the metal to facilitate electrostatic γ-hydride interaction, thus giving an essentially [4 + 2] coordination at the Ni(ii) center. These compounds are active toward reductive Ullmann-type coupling reactions in Bu4NBr showing higher activities towards bromoanisole or bromotoluene than bromobenzene. The complex trans-[NiI2(N-Pr iBzTh)2] with the shortest Ni⋯H anagostic separation and a near-ideal orthogonal orientation between the carbene and metal planes gives the highest yields. © 2009 The Royal Society of Chemistry.