Synthesis, structural characterization of benzimidazole-functionalized Ni(II) and Hg(II) N-heterocyclic carbene complexes and their applications as efficient catalysts for Friedel–Crafts alkylations

[Ni(L1)2](PF6)2 (3a, L1 = 3-(1-ethyl-1H-benzimidazol-2-yl)methyl)-1-((6-methylpyridin-2-yl)methyl)imidazolyl-idene), [Ni(L2)2(CH3CN)](PF6)2 (3b, L2 = 3-(1-ethyl-1H-benzimidazol-2-yl)methyl)-1-((6-methylpyridin-2-yl)-methyl)benzimidazolylidene), and [Hg(L1)2(CH3CN)2](PF6)2 (4) have been successfully prepared and fully characterized by NMR, ESI-MS spectroscopy, and X-ray diffraction analysis. The nickel complexes reveal a square-planar structure with two carbene ligands and benzimidazole groups at the cis configuration. The nickel complex 3b has been proved to be a highly efficient catalyst for Friedel–Crafts alkylation of indoles with β-nitrostyrenes at room temperature in moderate-to-excellent yields. The crystal packing structure of 4 shows that double-stranded 1D supramolecular chains are formed by inter-chain benzimidazole rings and pyridine rings face-to-face π-π stacking interactions.