Insights into the effect of contact ion-pairs on C[sbnd]H bond activation for the synthesis of Ru(III)-NHC complexes: A combined experimental and computational study

Abstract

The effect of contact ion pairs and comparison of the three halide ions on the electrophilic CH activation during the base-free synthesis of Ru(III)-NHC complexes [RuIII(PyNHCR)(Cl)3(H2O)] (1a; R = Me, and 1b; R = iPr) have been investigated. DFT modelling of two pathways for the formation of Ru(III)-NHC complexes, viz. halide coordination pathway and ion-pair pathway, reveals the effect of halide ions during electrophilic CH activation for the generation of carbene. The ion-pair path is preferred for azolium salts with Cl− and I−, whereas with Br−, the reaction follows the halide coordination route leading to the mixed halide complex. The synthetic route follows concerted-metalation-deprotonation (CMD) mechanism for ion pair pathway. The higher activation barrier for the reaction with Cl− ions can be explained in terms of greater stabilization of the intermediates, compared to the transition states, due to halide coordination or hydrogen bonding with Cl− ions. Furthermore, the successful synthesis of 1b from the reaction of RuCl3·3H2O and the corresponding imidazolium salt precursor with Cl− in dioxane at 100 °C validates the conclusions of the DFT studies.