Arene ruthenium oxinato complexes: Synthesis, molecular structure and catalytic activity for the hydrogenation of carbon dioxide in aqueous solution

Two families of arene ruthenium oxinato complexes of the types [(η6-arene)Ru(η2-N,O-L)Cl] and [(η6-arene)Ru(η2-N,O-L)(OH2)]+ have been synthesized from the dinuclear precursors [(η6-arene)RuCl2]2 (arene = para-cymeme or hexamethylbenzene) and the corresponding oxine LH (LH = 8-hydroxyquinoline, 5-chloro-8-hydroxyquinoline, 5,7-dichloro-8-hydroxyquinoline, 5-nitro-8-hydroxyquinoline, 5,7-dimethyl-8-hydroxyquinoline, 5,7-dichloro-2-methyl-8-hydroxyquinoline). The molecular structures of the neutral chloro complexes [(η6-C6Me6)Ru(η2-N,O-L)Cl] (LH = 8-hydroxyquinoline, 5,7-dichloro-2-methyl-8-hydroxyquinoline) and [(η6-MeC6H4Pri)Ru(η2-N,O-L)Cl] (LH = 5,7-dichloro-2-methyl-8-hydroxyquinoline) as well as those of the cationic aqua derivatives [(η6-MeC6H4Pri)Ru(η2-N,O-L)(OH2)]+ (LH = 8-hydroxyquinoline, 5,7-dimethyl-8-hydroxyquinoline), isolated as the tetrafluoroborate salts, show in all cases a piano-stool arrangement with the arene ligand, the chelating oxinato ligand and the chloro or the aqua ligand surrounding the ruthenium center in a pseudo-tetrahedral fashion. The analogous reaction of [(η6-MeC6H4Pri)RuCl2]2 with other N,O-chelating ligands such as 2-pyridinemethanol or tetrahydrofurfurylamine did not give the expected analogs but resulted in the formation of the complexes [(η6-MeC6H4Pri)Ru(η2-NC5H4CH2OH)Cl]+ and [(η6-MeC6H4Pri)Ru(η1-NHCH2C4H3O)Cl2]. The neutral and cationic complexes of the types [(η6-arene)Ru(η2-N,O-L)Cl] and [(η6-arene)Ru(η2-N,O-L)(OH2)]+ have been found to catalyze the hydrogenation of carbon dioxide to give formate in alkaline aqueous solution with catalytic turnovers up to 400.