Half-Sandwich Ruthenium(II) Picolyl-NHC Complexes: Synthesis, Characterization, and Catalytic Activity in Transfer Hydrogenation Reactions

Pentamethylcyclopentadienyl ruthenium(II) complexes with picolyl-functionalized N-heterocyclic carbenes [(eta(5)-C5Me5)-Ru(L)(CH3CN)][PF6] (L = 3-methyl-1-(2-picolyl)imidazol-2-ylidene (1a), 3-isopropyl-1-(2-picolyl)imidazol-2-ylidene (1b), 3-phenyl-1-(2-picolyl)imidazol-2-ylidene (1c), 3-mesityl-1-(2-picolyl)imidazol-2-ylidene (1d), 3-methyl-1-(2-picolyl)benzoimidazol-2-ylidene (1e), 3-methyl-1-(2-picolyl)-4,5-dichloroimidazol-2-ylidene (1f)) have been synthesized and characterized. Compounds 1a,b were recrystallized as BAr4F salts (anion BAr4F- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate, giving 2a,b. X-ray crystal structures of the acetonitrile adduct 2a and the dioxygen compound 2b are also reported. Furthermore, carbonyl derivatives 3a-f have been prepared, characterized, and used to study the donor properties of the picolylcarbene ligands (L) via infrared spectroscopy. Compounds 1a-f show catalytic activity in transfer hydrogenation of ketones. Notably, complex 1a was found to be a very efficient and versatile catalyst toward transfer hydrogenation of a wide range of ketones and imines.