Multidentate Pyridyl-Aminophosphinite and Pyridyl-Phosphoramidite Ruthenium(II) Complexes: Synthesis, Structure and Application as Levulinic Acid Hydrogenation Pre-Catalysts

Novel multidentate pyridyl-aminophosphinite (L1) and pyridyl-phosphoramidite (L2) ligands of N^{^}P^{^}P^{^}N-donor system have been synthesized via a series of simple steps. The ligands are symmetrical and as a result, their reactions with [Ru(p-cymene)Cl₂]₂ and [Ru(benzene)Cl₂]₂ lead to the formation of four monodentate bimetallic complexes (1–4) that retain the symmetry of the ligands. Meso and racemic mixtures (rac) of bidentate bimetallic complexes 5–8 were formed from the monodentate complexes through coordination of the pyridine nitrogen atoms to the two metal centers. The isomerism occurs at each metal center, which was evidenced by ³¹P{¹H}, ¹H NMR spectroscopy and single-crystal X-ray diffraction. The complexes were active towards hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) using formic acid as the hydrogen source. The complexes are active at relatively low temperatures and are able to perform the hydrogenation in the absence of any additional solvent apart from the reagents to give high TON of 3 600. The catalysts are recyclable up to the fourth cycle, following which 20 % loss of activity is seen.