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 [(η⁵-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 BAr₄^F salts (anion BAr₄^F– = 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.