Cyclometalated Dicarbonyl Ruthenium Catalysts for Transfer Hydrogenation and Hydrogenation of Carbonyl Compounds

The dicarbonyl complex RuCl2(L)(2)(CO)(2) (l) was easily prepared by reaction of ruthenium chloride hydrate with formic acid and L (L = (2,6-Me2C6H3)PPh2) in ethanol at reflux, via the [RuCl2(CO)(2)](n) intermediate. Alternatively, 1 was obtained from [RuCl2(CO)(3)](2) and L by CO elimination. Reaction of 1 with NEt3 in toluene at reflux afforded the cyclometalated derivative RuCl\(2-CH2-6-MeC6H3)PPh2\-(L)(CO)(2) (2). A simple one-pot synthesis of 2 was achieved by treatment of RuCl3 hydrate with formic acid, L, and NEt3. The cyclometalated dicarbonyl complexes [Ru\(2-CH2-6-MeC6H3)PPh2\(NN)(CO)(2)]Cl (NN = ethylenediamine, 3; 2-(aminomethyl)pyridine, 4; (R,R)-l,2-diphenylethane-l,2-diamine, 5) were isolated by reaction of 2 with the corresponding dinitrogen ligand in methanol at reflux. Complexes 1-4 catalyze the transfer hydrogenation (TH) of acetophenone in 2-propanol at reflux (S/C = 1000 and TOF up to 30 000 h(-1)) with alkali base (l-5 mol \%), whereas 5 leads to (S)-1-phenylethanol with 68\% ee. The derivatives 1-5 catalyze the hydrogenation (HY) of several ketones (H-2, 30 bar) at 70 degrees C in MeOH and EtOH with KOtBu (2 mol \%) (S/C and TOF up to 25 000 and 14 000 h(-1)). Addition of NN ligands to 1 and 2 in situ increases both the TH and HY activity, with ampy displaying the better performance. Heating of the cationic complex 3 in solid state and in solution leads to decarbonylation, affording the neutral monocarbonyl compound RuCl\(2-CH2-6-MeC6H3)PPh2\(en)(CO) (6) which was found active in the ketone HY.