Base-free synthesis of benchtop stable Ru(III)-NHC complexes from RuCl3·3H2O and their use as precursors for Ru(II)-NHC complexes

A series of Ru(III)-NHC complexes, identified as [RuIII(PyNHCR)(Cl)3(H2O)] (1a-c), have been prepared, starting from RuCl3·3H2O following a base-free route. The Lewis acidic Ru(III) centre operates via a halide-assisted, electrophilic C-H activation for carbene generation. Best results were obtained with azolium salts having I- anion while ligand precursors with Cl-, BF4-, and PF6- gave no complex formation and those with Br- gave a product with mixed halides. The structurally simple, air and moisture-stable complexes represent rare examples of paramagnetic Ru(III)-NHC complexes. Further, these benchtop stable Ru(III)-NHC complexes were shown to be excellent metal precursors for the synthesis of new [RuII(PyNHCR)(Cl)2(PPh3)2] (2a-c) and [RuII(PyNHCR)(CNCMe)I]PF6 (3a-c) complexes. All the complexes have been characterised using spectroscopic methods, and structures of 1a, 1b, 2c and 3a have been determined using the single-crystal X-ray diffraction technique. This work allows easy access to new Ru-NHC complexes for the study of new properties and novel applications.