Hydrocondensation of carbon dioxide with methanol catalyzed by anionic ruthenium complexes: Isolation, structural characterization, and catalytic implications of the dinuclear anion [Ru-2(CO)(4)(mu(2)-eta(2)-CO2CH3)(2)(mu(2)-OCH3)Cl-2](-)

The anionic ruthenium complex [Ru(CO)(3)Cl-3](-) (1) was found to catalyze the formation of methyl formate from CO2, H-2, and CH3OH in the presence of a strong base. From the basic methanol solution, the dinuclear anion [Ru-2(CO)(4)(mu(2)-eta(2)-CO2CH3)(2)(mu(2)-OCH3)Cl-2](-) (2) has been isolated as the bis(triphenylphosphoranylidene)ammonium salt and characterized by a single-crystal X-ray diffraction analysis. In anion 2, two Ru(CO)(2)Cl units are held together by a mu(2)-OCH3 ligand and by two mu(2)-eta(2)-OCOCH3 bridges; apart from the three bridging ligands there is no direct Ru-Ru interaction (distance 3.380 Angstrom). Complex 2 was found to read with 2 equiv of [OCH3](-) to give two reactive intermediates 3 and 4, the latter one being considered as the catalytically active species in the hydrocondensation reaction. On the basis of the spectroscopic data and the isolation of aromatic analogues from the reaction of 2 with various phenolates, the constitution of [Ru-2(CO)(6)(OCH3)(4)(mu(2)-OCH3)](-) is proposed for the active species 4. The elimination of water as an inherent side product of the methyl formate formation from CO2, H-2, and CH3OH was found to be the limiting factor of this catalytic reaction: It hydrolyses the methoxide employed as cocatalyst and thus deactivates the catalytic system. In the presence of trimethyl orthoformate as a water trap, the catalytic turnover numbers observed are considerably higher.