Ruthenium(II) complexes with 6,6′-disubstituted bipyridine ligands and their reactivity towards CO and H2 for homogeneous syngas conversion

Bipyridine ligands with proximate H-bonding substituents in the 6,6′ positions, specifically 6,6′-dihydroxybipyridine (dhbipy, L1) and 6,6′-diaminobipyridine (dabipy, L2), have been used to prepare Ru complexes for potential application in the homogeneous conversion of syngas to higher alcohols. Reaction of dhbipy and dabipy with RuCl3 results in the formation of RuII complexes, [Ru(dhbipy)2Cl2] (2) and [Ru(dabipy)2Cl(CO)]Cl (7). At higher pressures of CO, reduced multinuclear complexes are formed, a dinuclear complex [Ru2(μ2-dhbipy-H)2(CO)4] (4) in the case of dhbipy, whereas a hexanuclear complex [Ru6(μ3-H)2(μ5-dabipy-H2)(μ-CO)3(CO)12] (8) was isolated from the reaction with dabipy. Ligand deprotonation was investigated for complex (4) and the addition of nBu4NOH resulted in the formation of a dianionic complex [Ru2(μ2-dhbipy-H2)2(CO)4]2– (5). Attempts to activate H2 with complexes 4 and 5 and to observe the formation of formyl complexes from these ruthenium carbonyl complexes were unsuccessful under the conditions used.