Synthesis, structures and catalytic activities of ruthenium(II) carbonyl chloride complexes containing pyridine-functionalised N-heterocyclic carbenes

A series of ruthenium(II) carbonyl chloride complexes with pyridine-functionalised N-heterocyclic carbenes, trans(Cl)- and cis(Cl)-[Ru(Py-NHC)(CO)(2)Cl-2] [Py-NHC = 3-tert-butyl-1-(2-pyridyl)imidazol-2-ylidene, 1a and 1b; 3-n-butyl-1-(2-pyridyl)imidazol-2-ylidene, 2a and 2b; 3-tert-butyl-1-picolylimidazol-2-ylidene, 3a and 3b; 3-n-butyl-1-picolylimidazol-2-ylidene, 4a and 4b; 3-benzyl-1-picolylimidazol-2-ylidene, 5a and 5b] have been prepared by transmetallation from the corresponding silver carbene complexes and characterised by elemental analyses, infrared and H-1 and C-13 NMR spectroscopies. The yields of trans(Cl)- and cis(Cl)-isomers could be controlled by altering the reaction conditions: the trans-isomers were kinetically formed as the main products in 60-70% yield in CH2Cl2 at ambient temperature while the thermodynamic cis-isomers were obtained as the major products in 70-80% yield in toluene at refluxing temperature. The trans-isomer was transformed into the thermodynamic cis-isomer by heating in CH2Cl2 in a Teflon-lined stainless autoclave for 24 h at 120 degrees C. The molecular structures of 1a, 1b, 2b, and 4b have been determined by single-crystal X-ray diffraction. These complexes showed catalytic activities in hydrogen transfer reactions of ketones.