Catalytic Hydrogenation of CO2 to Methanol Using Multinuclear Iridium Complexes in a Gas-Solid Phase Reaction.

We report a novel approach toward the catalytic hydrogenation of CO2 to methanol performed in the gas-solid phase using multinuclear iridium complexes at low temperature (30-80 °C). Although homogeneous CO2 hydrogenation in water catalyzed by amide-based iridium catalysts provided only a negligible amount of methanol, the combination of a multinuclear catalyst and gas-solid phase reaction conditions led to the effective production of methanol from CO2. The catalytic activities of the multinuclear catalyst were dependent on the relative configuration of each active species. Conveniently, methanol obtained from the gas phase could be easily isolated from the catalyst without contamination with CO, CH4, or formic acid (FA). The catalyst can be recycled in a batchwise manner via gas release and filling. A final turnover number of 113 was obtained upon reusing the catalyst at 60 °C and 4 MPa of H2/CO2 (3:1). The high reactivity of this system has been attributed to hydride complex formation upon exposure to H2 gas, suppression of the liberation of FA under gas-solid phase reaction conditions, and intramolecular multiple hydride transfer to CO2 by the multinuclear catalyst.