Transfer Hydrogenation (pH Independent) of Ketones and Aldehydes in Water with Glycerol: Ru, Rh, and Ir Catalysts with a COOH Group near the Metal on a (Phenylthio)methyl-2-pyridine Scaffold

The reactions of 2-(pyridine-2-ylmethylsulfanyl)benzoic acid (L) with [(eta(5)-Cp*/eta(6)-benzene)MCl(mu-Cl)](2), (benzene, M = Ru; Cp*, M = Rh, Ir) at room temperature followed by treatment with NH4PF6 result in a new class of water-soluble half-sandwich complexes [(eta Cp-5*/eta(6)-benzene)M(L)Cl][PF6] (1-3, respectively, for M = Ru, Rh, Ir). Their characteristic HR-MS and H-1 and C-13{H-1} NMR spectra have been found. The single-crystal structures of 1-3 have been established with X-ray crystallography. The Ru S, Rh S, and Ir-S bond lengths are 2.4079(6), 2.3989(10), and 2.3637(14) angstrom, respectively. Complexes 1-3 have been found to be efficient for catalytic transfer hydrogenation (TH) of carbonyl compounds in water with glycerol as a hydrogen donor. Glycerol has been explored for TH in water for the first time. The efficiency in water of other hydrogen sources, viz. HCOOH, citric acid, ascorbic acid, and 2-propanol, is less and/or is pH dependent. Catalysis with glycerol as a hydrogen source is pH independent and appears to be homogeneous. Higher reactivity for the Rh complex in comparison to the Ru and Ir species has been observed. DFT calculations are generally consistent with the experimental values of bond lengths and angles and catalytic reactivity order.