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

Abstract

A series of Ru(III)–NHC complexes, identified as [Ru^III(PyNHC^R)(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. The best results were obtained with azolium salts having the 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. Furthermore, these benchtop stable Ru(III)–NHC complexes were shown to be excellent metal precursors for the synthesis of new [Ru^II(PyNHC^R)(Cl)₂(PPh₃)₂] (2a–c) and [Ru^II(PyNHC^R)(CNC^Me)I]PF₆ (3a–c) complexes. All the complexes have been characterised using spectroscopic methods, and the 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.