Comparison of Alkyl-Bridged Bis(N-Heterocyclic Carbene) Nickel Precatalysts: Structure and Catalytic Activity in the Reductive Cleavage and Suzuki–Miyaura Reactions

Four new nickel(II) precatalysts featuring bidentate chelating ethylene-bridged bisbenzimidazolin-2-ylidenes (3e–3h) were synthesized and structurally characterized by single crystal X-ray analysis. The catalytic activity of the ethylene-bridged complexes (3e–3h) and the previously reported propylene-bridged analogs (3a–3d) are compared for the reductive cleavage and Suzuki–Miyaura coupling (SMC) of aryl sulfamates. The propylene-bridged chelate complexes (3a–3d) generally exhibit higher catalytic activity relative to their ethylene counterparts. Experimental findings reveal a notable impact of the alkyl bridge length on the synthesis outcomes and catalytic performance of the Ni(II) complexes. The ligand parameters, percent buried volume (% VBur), bite angle and estimated σ-donor properties based on 1H NMR measurements of the precursor benzimidazolium salts are reported. The X-ray crystal structure of a rare well-defined bis(NHC) nickel(I) chelate complex (5) is reported and its catalytic activity in the reductive cleavage and SMC reactions demonstrated.