Inverted piano-stool dimers of half-sandwich Ru(II) complexes with (R)-phenylglycine methylester and (S)-phenylalanineamide: An X-ray structural study and preliminary catalytic results

The two new complexes [Ru(η6-p-cymene)(κ1-N-(rac)-phenylglycine methylester)Cl2] (1) and [Ru(η6-p-cymene)(κ2-N,N′-(S)-phenylalanineamido)Cl] (2) have been synthesized by reacting [Ru(p-cymene)Cl2]2 with the HCl-free corresponding ligands. The complexes have been fully characterized by spectroscopic and analytical methods, and their solid structures have been determined by single crystal X-ray analysis. Both complexes have a pseudo-tetrahedral geometry: in 1 ruthenium is bound to the nitrogen, to the η6-coordinated p-cymene molecule and to two chloride ligands. In 2 the ligand has lost an amide proton binding ruthenium in a N,N′ bidentate fashion, the coordination geometry being completed by a η6-coordinated p-cymene molecule and a chloride ligand. The X-ray structure of 1 has revealed the racemization of the ligand, while in 2 the ligand has retained its configuration but, interestingly, the two diasteromers RRuSC and SRuSC have co-crystallized in the same single crystal. The crystal architecture of 2 is characterized by the presence of two opposite helices of akin diastereoisomers, connected through strong intermolecular hydrogen bonds between the amine and carbonyl groups. ESI-MS of an i-PrOH solution of 2 points out that the dimers are maintained also in solution. Complex 2 is an active catalyst for the homogeneous transfer hydrogenation of acetophenone and cyclohexanone in an i-PrOH/t-BuOK mixture, with TOFs up to 800 and 1000 h−1, respectively.