Cationic ruthenium(II)-NHC pincer complexes with hemilabile COD: Solid-state structural characterization and theoretical study of an η2-(E,Z)-COD ligand

Abstract

Cationic Ru(II)-CNC pincer complexes (CNC = 2,6-bis(1-methylimidazol-2-ylidene)pyridine), [Ru(CNC)(η2:η2-COD)Cl]Cl (1a) (COD = 1,5-cyclooctadiene), [Ru(CNC)(η2:η2-COD)Cl]PF6 (1b), [Ru(CNC)(η2-COD)Cl2] (2), [Ru(CNC)(DMSO)2Cl]Cl (3a), and [Ru(CNC)(DMSO)2Cl]PF6 (3b) have been synthesised and characterised. Single-crystal x-ray diffraction studies of the complexes reveal that the COD ligand remains coordinated to the Ru in η2:η2- and η2-fashion in 1a/b and 2, respectively. The solid-state structure of complex 2 shows an (E,Z) configuration for the η2-COD ligand, instead of the expected (Z,Z) configuration, with the free alkene having an E-configuration. DFT study of isomeric forms of 2 with E,Z- and Z,Z-COD indicates that the complex with Z,Z-COD ligand is thermodynamically more stable and should be favoured in solution. The complexes were investigated for the transfer hydrogenation reaction with 2-propanol and appear to be efficient catalyst precursors with no sign of hydrogenation of the COD ligand during the catalysis.