16-Electron pentadienyl- and cyclopentadienyl-ruthenium half-sandwich complexes with bis(imidazol-2-imine) ligands and their use in catalytic transfer hydrogenation

Abstract

Bis(η⁵-2,4-dimethylpentadienyl)ruthenium(II), [(η⁵-C₇H₁₁)₂Ru] (1, “open ruthenocene”), which has become accessible in high yield and large quantities via an isoprene-derived diallyl ruthenium(IV) complex, can be converted into the protonated open ruthenocene 2 by treatment with HBF₄ and subsequently into the protonated half-open ruthenocene 3 by reaction with cyclopentadiene. The electronic structure of 3 was studied by DFT methods, revealing that the CH-agostic complex [(η⁵-C₅H₅)Ru{(1-4η)-C₇H₁₂-η²-C⁵,H⁵}]BF₄ (A) represents the global minimum, which is 3.7 kcal mol⁻¹ lower in energy than the hydride complex [(η⁵-C₅H₅)RuH(η⁵-C₇H₁₁)]BF₄ (B). 2 and 3 were treated with the ligands N,N′-bis(1,3,4,5-tetramethylimidazolin-2-ylidene)-1,2-ethanediamine (BL^Me) and N,N′-bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,2-ethanediamine (BL^iPr) to afford the cationic 16-electron pentadienyl and cyclopentadienyl complexes [(η⁵-C₇H₁₁)Ru(BL^R)]BF₄ (4a, R = Me; 4b, R = iPr) and [(η⁵-C₅H₅)Ru(BL^R)]BF₄ (5a, R = Me; 5b, R = iPr). All complexes catalyse the transfer hydrogenation of acetophenone in isopropanol, and the most active complex 4a in this reaction was employed for the hydrogenation of a broader range of aliphatic and aromatic ketones.