Half-Sandwich Osmium(II) Complexes with Bidentate N, N-Chelating Ligands and Their Use in the Transfer Hydrogenation of Ketones

The reaction of appropriate N, N-bidentate ligands with the [(eta(6)-p-cymene)Os(mu-Cl) Cl](2) dimer (p-cymene = C10H14), followed by metathesis reaction with NH4PF6, gave the new osmium(II) arene complex salts [(eta(6)-p-cymene) OsCl(C5H4N-2-CH=N-R)]PF6, where [R = tert-butyl (1), isopropyl (2), 2,6-di-methylphenyl (3), or 2,6-di-isopropylphenyl (4)]. The dimer was also reacted with the N, N'-bidentate ligands di-(2-pyridyl)amine (5), 4-phenyl-3,6-di(2-pyridyl) pyridazine (6), 4,4'-di-tert-butyl-2, 2'-bipyridine (7), and 5,5'-dimethyl-2,2'-bipyridine (8). In addition, the reaction of the precursor [(eta(6)-C6H6)Os(mu-Cl)Cl](2) with the N, N'-bidentate ligands gave [(eta(6)-C6H6) OsCl(N, N)](2) where N, N = 4,4'-di-tert-butyl-2,2'-bipyridine (9), 5,5'-dimethyl-2,2'-bi-pyridine (10), 3,6-bis(2-pyridyl)-4-phenyl pyridazine (11), or di-( 2-pyridyl) amine (12). The compounds were characterized by using H-1 and C-13 NMR, UV/Vis, FTIR spectroscopy and elemental analysis. The single-crystal X-ray structures for compounds 1, 4, 8, 10, 11, and 12 showed that the osmium(II) complexes adopted the classical three-legged piano stool geometry. These osmium(II) compounds were found to be effective catalysts for the transfer hydrogenation of ketones into alcohols with NaOH as base and 2-propanol as the solvent and hydrogen source. A range of cyclic, aromatic, and aliphatic ketones was studied and good turnover numbers achieved.