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

The effect of contact ion pairs and comparison of the three halide ions on the C-H activation during the synthesis of Ru(III)-NHC complexes 1a and 1b have been investigated. DFT modeling of two pathways for the formation of Ru(III)-NHC complexes, viz. halide coordination pathway and ion-pair pathway, reveals the effect of ion pairing. 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 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.