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-<i>N</i>,O-L)Cl] and [(η⁶-arene)Ru(η²-<i>N</i>,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-<i>N</i>,O-L)Cl] (LH = 8-hydroxyquinoline, 5,7-dichloro-2-methyl-8-hydroxyquinoline) and [(η⁶-MeC₆H₄Pr<i>ⁱ</i>)Ru(η²-<i>N</i>,O-L)Cl] (LH = 5,7-dichloro-2-methyl-8-hydroxyquinoline) as well as those of the cationic aqua derivatives [(η⁶-MeC₆H₄Prⁱ)Ru(η²-<i>N</i>,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-MeC₆H₄Pr<i>ⁱ</i>)RuCl₂]₂ with other <i>N</i>,O-chelating ligands such as 2-pyridinemethanol or tetrahydrofurfurylamine did not give the expected analogs but resulted in the formation of the complexes [(η⁶-MeC₆H₄Pr^<i>i</i>)Ru(η²-NC₅H₄CH₂OH)Cl]⁺ and [(η⁶-MeC₆H₄Pr^<i>i</i>)Ru(η¹-NHCH₂C₄H₃O)Cl₂]. The neutral and cationic complexes of the types [(η⁶-arene)Ru(η²-<i>N</i>,O-L)Cl] and [(η⁶-arene)Ru(η²-<i>N</i>,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.