Homogeneous polymetallic ruthenium(ii)^zinc(ii) complexes: Robust catalysts for the efficient hydrogenation of levulinic acid to γ-valerolactone

New Ru(ii) complexes (1 and 2) of 4-(diphenylphosphino)benzoic acid (L1) and 3-(diphenylphosphino)propanoic acid (L2) have been synthesized and reacted with Zn(OAc)₂ to form hexanuclear complexes (3 and 4) containing four Ru(ii) and two Zn(ii) centres. The four ruthenium complexes were formed through coordination of Ru to a phosphine, while the phosphines' pendant carboxylate group forms μ2-bridges between other nearby Ru and Zn centres in complexes 3 and 4. All compounds have been fully characterized by NMR and infrared spectroscopy, mass spectrometry and singe crystal X-ray diffraction (for complexes 1, 2 and 3). Complexes 3 and 4 showed outstanding catalytic activities in the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) at low catalyst loadings (0.05-0.10 mol%) and under mild conditions. With catalyst loading of 0.1 mol%, the hexanuclear RuII4-ZnII2 complexes gave turnover frequencies as high as 540 h^-1 (the highest turnover frequency for homogeneous ruthenium complexes in the hydrogenation of LA using formic acid). The reactions (i) require no additional solvent other than the reactant, (ii) make use of formic acid as a hydrogen source (and are safer) and (iii) can be run at low temperatures. These multi-metallic RuII4-ZnII2 catalysts are very robust, stable and can be recycled up to seven times without significant loss in activity. A plausible mechanism of the reaction is proposed based on in situ NMR spectra obtained during the reaction.