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.

Graphical abstractArene 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 X-ray crystal structure analyses show these complexes to have a piano-stool geometry with the arene ligand, the chelating oxinato ligand and the chloro or the aqua ligand surrounding the ruthenium center in a pseudo-tetrahedral fashion.Figure optionsDownload full-size imageDownload as PowerPoint slide