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

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posted on 2016-02-22, 15:44 authored by Francys E. Fernández, M. Carmen Puerta, Pedro Valerga
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 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 3af have been prepared, characterized, and used to study the donor properties of the picolylcarbene ligands (L) via infrared spectroscopy. Compounds 1af 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.