Benzotriazole-Functionalized Rh(I), Ir(I), and Ru(II) N-Heterocyclic Carbene Complexes: Synthesis, Structural Characterization, and Catalytic Hydrosilylation of Terminal Alkynes

A benzotriazole-functionalized benzimidazolium salt together with its corresponding Rh(I) and Ir(I) N-heterocyclic carbene (NHC) complexes was synthesized and comprehensively characterized using elemental analysis together with 1D and 2D NMR spectroscopy. A previously reported Ru(II)-NHC complex bearing the same ligand framework was also employed to enable a systematic comparison of the influence of the metal center on catalytic behavior. The catalytic performances of the Rh(I), Ir(I), and Ru(II) complexes were evaluated by in situ time-resolved 1H NMR monitoring of the hydrosilylation of terminal alkynes. Under the optimized reaction conditions, the Ir(I) complex exhibited the highest catalytic activity, affording 99% conversion in the hydrosilylation of phenylacetylene with dimethylphenylsilane. In contrast, the Rh(I) complex showed lower activity (75% conversion) but provided the highest β-(E) selectivity (60%) without detectable styrene formation. In contrast, the Ru(II) complex displayed the highest β-(Z) selectivity (80%) despite its limited catalytic activity (20% conversion). Additional experiments employing different terminal alkynes and hydrosilanes indicated that variation of the substrates did not substantially improve the catalytic performance of the benzotriazole-functionalized Ir(I)-NHC catalyst. The benzotriazole functionality was selected because its pendant nitrogen donor sites provide additional coordination capability and offer a means of modifying the coordination environment and electronic properties of NHC-metal complexes. The observed catalytic behavior, together with comparison with previously reported NHC systems, indicates that both the metal center and the nature of the NHC framework are important factors governing catalytic activity and product distribution. These findings provide further insight into the roles of both the metal center and ligand design in terminal alkyne hydrosilylation and may provide useful guidance for the future design of functionalized NHC ligands for selective hydrosilylation catalysis.