Effect of chelating ring size in catalytic ketone hydrogenation: facile synthesis of ruthenium(ii) precatalysts containing an N-heterocyclic carbene with a primary amine donor for ketone hydrogenation and a DFT study of mechanisms

Abstract

A half-sandwich ruthenium(II) complex, [Ru(η⁶-p-cymene)(C–NH₂)Cl]PF₆ (4·PF₆), containing an N-heterocyclic carbene (NHC) with a primary amine donor (C–NH₂) which chelates through the carbene carbon and the amine nitrogen to form a 6-membered ring was synthesized in a one-pot reaction starting from a primary-amine functionalized imidazolium salt 2. Complex 4·PF₆ catalyzed the hydrogenation of ketones using 2-propanol or H₂ as the reductant. A maximum turnover frequency of 1062 h⁻¹ and a turnover number of 1140 at 5 h were achieved for the transfer hydrogenation of 3′-chloroacetophenone in 2-propanol at 75 °C. A cationic hydride–amine complex 5, [Ru(η⁶-p-cymene)(C–NH₂)H]PF₆, was synthesized, and this reacted very slowly with acetophenone unless first activated by an alkoxide base. Computational studies by DFT methods suggested that the poor reactivity of the hydride–amine complex 5 was attributed to a large barrier for the transfer of its H⁺/H⁻ couple to a ketone for bifunctional catalysis. An inner-sphere mechanism, which involves a decoordinated amine group of the C–NH₂ ligand, was computed to be a feasible energetic pathway in comparison to the computed outer-sphere bifunctional mechanism. This explains the catalytic activity and selectivity that is observed for the newly synthesized ruthenium(II) catalysts.