Synthesis, coordination chemistry and reactivity of transition metal complexes supported by a chelating benzimidazolylidene carboxylate ligand

• A bidentate anionic benzimidazolylidene-carboxylate ligand scaffold was developed.
• An Ag–NHC complex was synthesized and underwent transmetallation with Ru.
• A dodecametallic macrocycle with mono- and dinuclear sites was observed for Ag–NHC.
• Transfer hydrogenation of CO, CN and CC bonds with iPrOH was catalyzed by Ru–NHC.

To construct the framework for a chelating, anionic benzimidazole-based N-heterocyclic carbene (NHC) ligand (1−), N-(3,5-dimethylphenyl)benzimidazole was reacted with 2-bromoacetic acid to yield the corresponding benzimidazolium [1H2][Br]. Double deprotonation followed by metallation of [1H2][Br] with Ag2O produced the Ag–NHC complex [Ag(1)]n (2), which exhibited good solubility in organic solvents despite its multinuclear nature. Crystallographic analysis of 2 revealed a dodecametallic structure (n = 12) comprising mononuclear [Ag(1)] and dinuclear [Ag2(1)2] subunits, the latter of which featured close Ag⋯Ag contacts. Transmetallation of 1− from Ag to Ru via the reaction of [RuCl(η6-cymene)(μ–Cl)]2 with 2 afforded the Ru–NHC complex [RuCl(1)(η6-cymene)] (3), which demonstrated catalytic activity for transfer hydrogenation of CO, CN and CC bonds using isopropanol as the H2 source.

A chelating benzimidazolylidene carboxylate ligand (1−) afforded a dodecametallic cyclic silver–NHC complex (2) comprising mononuclear [Ag(1)] and dinuclear [Ag2(1)2] subunits. Transmetallation from 2 to [RuCl(η6-cymene)(μ–Cl)]2 produced a ruthenium–NHC complex (3) that catalyzed transfer hydrogenation of CO, CN and CC bonds.Figure optionsDownload as PowerPoint slide