Synthesis, structural characterization, computational study, and catalytic activity of metal complexes based on tetradentate pyridine/N-heterocyclic carbene ligand

The ligand precursor [LH₂]²⁺·2Br^- (1) for the bis(NHC) ligand with N-picolyl moieties is synthesized in 83% yield. Complexation of 1 with palladium and nickel acetates produced [PdL] ²⁺·2Br^- (2) and [NiL]²⁺·2Br ^- (3), respectively, in quantitative yields. Metathesis reactions of 2 and 3 with AgPF₆ give the corresponding bromide-free complexes [PdL]²⁺·2PF₆^- (4) and [NiL] ²⁺·2PF₆^- (5). Complexes 2-4 were characterized by X-ray structural determinations, which reveal a highly twisted helical coordination of L around the metal ions. In solution, however, the tetradentate chelate ring undergoes a rapid fluxional process of ring twisting. A theoretical study confirms that the tetradentate coordination of L in 2 and 3 is energetically more favorable than the bidentate chelation mode with dangling picolyl groups. A preliminary application of [NiL]²⁺·2Br ^- in Suzuki coupling of aryl halides with phenylboronic acid shows effective activity.