An Air and Moisture Stable Boat Shaped Hydroxo-Bridged Ruthenium(II) Binuclear Complex for the Catalytic Hydrogenation of CO2

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.