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 Ru^II₄^Zn^II₂ 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 Ru^II₄^Zn^II₂ 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.

中文翻译：

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均相多金属钌（ii）^锌（ii）配合物：将乙酰丙酸有效氢化为γ-戊内酯的坚固催化剂†

合成了新的4-（二苯基膦基）苯甲酸（L1）和3-（二苯基膦基）丙酸（L2）的Ru（II）配合物（1和2）并与Zn（OAc）₂反应形成六核配合物（3和4）包含四个Ru（II）和两个Zn（II）中心。四个钌络合物是通过Ru与膦配位而形成的，而膦的侧链羧酸根基团在络合物3和4中附近的其他Ru和Zn中心之间形成了μ2-桥。。所有化合物已被充分表征通过NMR和红外光谱，质谱和烧毛晶体的X射线衍射（对络合物1，2和3）。配合物3和4在低催化剂负载量（0.05–0.10 mol％）和温和条件下，在乙酰丙酸（LA）加氢成γ-戊内酯（GVL）的过程中表现出出色的催化活性。催化剂负载量为0.1 mol％时，六核Ru ^II ₄ ^ Zn ^II ₂配合物的周转频率高达540 h ^-1（在使用甲酸加氢LA的过程中，均相钌络合物的最高周转频率）。反应（i）除反应物外不需要其他溶剂，（ii）利用甲酸作为氢源（并且更安全），并且（iii）可在低温下进行。这些多金属Ru ^II ₄ ^ Zn ^II ₂催化剂非常坚固，稳定，可循环使用七次而活性没有明显损失。根据反应过程中获得的原位NMR光谱，提出了合理的反应机理。