Mixed heteropentadienyl and N-heterocyclic carbene ruthenium(II) complexes: Synthesis and transfer hydrogenation catalysis

Abstract

Two synthetic methodologies for obtaining novel mixed heteropentadienyl and N-heterocyclic carbenes (NHC) ruthenium(II) complexes have been explored. A synthetic strategy involving the addition of heteropentadienyl precursors to [RuII]-NHC complexes proved to be a successful route for the isolation of four novel mixed ligand complexes, namely stereoisomers [(p-cymene)RuII(BuImMe)(η3-exo-syn-CH2CHCHCHO)]BF4 (3 and 3′, BuImMe = 1-butyl-3-methylimidazolyl-2-idene) and [(η1-CH2CHCRCHSO2)RuII(PyImMe)(CH3CN)3](A) (PyImMe = 3-methyl-1-(2-pyridyl)imidazolyl-2-idene, R = H, A = BF4, 6; R = CH3, A = PF6, 7). Moreover complex 6 has been characterised by X-ray crystallography confirming the sulphur-bound monodentate nature of CH2CHCHCHSO2 ligand adopting an S-conformation. Complexes 6 and 7 were tested as pre-catalysts in the transfer hydrogenation reaction of benzophenone, showing moderate and good activities, respectively.