An air and moisture stable boat shaped hydroxo-bridged ruthenium(II) binuclear complex for the catalytic hydrogenation of CO₂

Abstract

A new hydroxo-bridged ruthenium (II) complex [{Ru(η⁶-p-cymene)(ⁱPr₂P−O)}₂(μ-OH)]Cl (2) has fortuitously been isolated from the reaction between the known ruthenium(II) complex [RuCl₂(η⁶-p-cymene)(ⁱPr₂P-OH)] (1) in either CDCl₃ or CHCl₃ solutions in the presence of excess DBU (1,8-diazabicyclo(5.4.0)undec-7-ene) at room temperature. Complex 2 is only formed under specific conditions as an orange-colored air and moisture stable binuclear complex. It is formed via the deprotonation of the P−OH unit in [RuCl₂(η⁶-p-cymene)(ⁱPr₂P-OH)]. Complex 2 has been fully characterized by spectroscopic and analytical methods. Its structure was determined by single crystal X-ray diffraction which reveals a boat-shaped motif containing a bridging hydroxide unit which undergoes hydrogen bonding with a chloride counterion. Complex 2 was utilized as a catalyst for the hydrogenation of CO₂ to formic acid salt using molecular hydrogen. The catalytic reactions were performed at 100 °C in THF in the presence of DBU as a base to isolate [DBU+H][OC(O)H] salt as the final product. Using catalyst loadings down to 0.01 mol%, it was possible to hydrogenate gaseous CO₂ with TONs up to 9900.

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