Benzimidazole-based N-heterocyclic carbene silver complexes as catalysts for the formation of carbonates from carbon dioxide and epoxides

Abstract

A series of left-to-right inequivalent 1,3-disubstituted benzimidazolium halide pro-ligands having the general formula [RBNHCCH2OxMe][X] (R = 3-Me-Bn, 3,5-Me2-Bn, 2,4,6-Me3-Bn, 2,3,5,6-Me4-Bn, 2,3,4,5,6-Me5-Bn, 3,4,5-(OMe)3-Bn, or 4-tBu-Bn; X = Cl, Br) were synthesized by the alkylation of 1-((3-methyloxetan-3-yl)methyl)benzimidazole. The corresponding Ag complexes, (RBNHCCH2OxMe)AgX, were prepared following pro-ligand addition to Ag2O. These compounds were characterized using spectroscopic techniques such as FT-IR, NMR spectroscopy, and single-crystal X-ray diffraction. The solid-state structure of (3-Me-BnBNHCCH2OxMe)AgCl revealed a linear monomer while [(2,4,6-Me3-BnBNHCCH2OxMe)AgCl]2 was found to exist as a dimer with pseudo trigonal planar geometry about each metal center. The synthesized (RBNHCCH2OxMe)AgX complexes were found to be efficient for the addition of carbon dioxide to epoxides to yield value-added cyclic carbonates at ambient pressure. Amongst the investigated complexes, the bimetallic complex [(2,4,6-Me3-BnBNHCCH2OxMe)AgCl]2 was found to be the most active for CO2 insertion, exhibiting favorable activity when compared to known NHC complexes.