Cyclometalated Ru(II)-NHC Complexes as Effective Catalysts for Transfer Hydrogenation: Influence of Wingtip Group on Catalytic Outcome

Cyclometalated N-heterocyclic carbene (NHC) based ruthenium benzene complexes of general formula [(eta(6)-C6H6)Ru(CC)Cl] (CC = 1-butyl-3-phenylimidazol-2-ylidene 1a, 1-isopropyl-3-phenylimidazol-2-ylidene 1b and 1-benzyl-3-phenylimidazol-2-ylidene 1c) have been synthesised via C-H activation of 1-alkyl-3-phenylimidazolium salts with [(eta(6)-C6H6)RuCl2](2) under silver transmetallation condition. All the complexes have been characterized by elemental analysis and spectral methods. The solid-state structures of the complexes 2a-c have been established by a X-ray single crystal diffraction study which revealed that NHC ligands behaved as mono anionic bidentate CC donors to the Ru(II) center via carbenic carbon and cyclometalated carbon. The newly synthesised ruthenium(II) NHC complexes 2a-c have been exploited as potential homogeneous catalysts to the transfer hydrogenation reactions for a broad scope of ketones to alcohols using 2-propanol as a hydrogen source. The titled complexes worked well with low catalyst loading of 0.2 mol% with maximum conversion of 100% under optimized condition with reference to base, temperature, time, catalyst loading and substrate scope. Besides, the influence of the nature of wingtip group of the NHC ligands on the transfer hydrogenation reaction has also been reported to demonstrate the efficacy of catalyst.