A highly efficient homogeneous catalyst system for the production of CH3OH from CO2 using pentaethylenehexamine and Ru-Macho-BH (1) at 125-165 °C in an ethereal solvent has been developed (initial turnover frequency = 70 h(-1) at 145 °C). Ease of separation of CH3OH is demonstrated by simple distillation from the reaction mixture. The robustness of the catalytic system was shown by recycling the catalyst over five runs without significant loss of activity (turnover number > 2000). Various sources of CO2 can be used for this reaction including air, despite its low CO2 concentration (400 ppm). For the first time, we have demonstrated that CO2 captured from air can be directly converted to CH3OH in 79% yield using a homogeneous catalytic system.

Mn(I)-PNP pincer catalyzed sequential one-pot homogeneous CO 2 hydrogenation to CH 3 OH by molecular H 2 is demonstrated. The hydrogenation consists of two partsN-formylation of an amine utilizing CO 2 and H 2 , and subsequent formamide reduction to CH 3 OH, regenerating the amine in the process. A reported air-stable and welldefined Mn-PNP pincer complex was found active for the catalysis of both steps. CH 3 OH yields up to 84% and 71% (w.r.t amine) were obtained, when benzylamine and morpholine were used as amines, respectively; and a TON of up to 36 was observed. In our opinion, this study represents an important development in the nascent field of base-metal-catalyzed homogeneous CO 2 hydrogenation to CH 3 OH.

Amine-assisted homogeneous hydrogenation of CO2 to methanol is one of the most effective approaches to integrate CO2 capture with its subsequent conversion to CH3OH. The hydrogenation typically proceeds in two steps. In the first step the amine is formylated via an in situ formed alkylammonium formate salt (with consumption of 1 equiv of H2). In the second step the generated formamide is further hydrogenated with 2 more equiv of H2 to CH3OH while regenerating the amine. In the present study, we investigated the effect of molecular structure of the ruthenium pincer catalysts and the amines that are critical for a high methanol yield. Surprisingly, despite the high reactivity of several Ru pincer complexes [RuHClPNP R (CO)] (R = Ph/i-Pr/Cy/t-Bu) for both amine formylation and formamide hydrogenation, only catalyst Ru-Macho (R = Ph) provided a high methanol yield after both steps were performed simultaneously in one pot. Among various amines, only (di/poly)­amines were effective in assisting Ru-Macho for methanol formation. A catalyst deactivation pathway was identified, involving the formation of ruthenium biscarbonyl monohydride cationic complexes [RuHPNP R (CO)2]+, whose structures were unambiguously characterized and whose reactivities were studied. These reactivities were found to be ligand-dependent, and a trend could be established. With Ru-Macho, the biscarbonyl species could be converted back to the active species through CO dissociation under the reaction conditions. The Ru-Macho biscarbonyl complex was therefore able to catalyze the hydrogenation of in situ formed formamides to methanol. Complex Ru-Macho-BH was also highly effective for this conversion and remained active even after 10 days of continuous reaction, achieving a maximum turnover number (TON) of 9900.

Conversion of carbon dioxide (CO2) captured from industrial sources (e.g.flue gas of power plants) or even from ambient air to formate through CO2capture and utilization (CCU) as a possible strategy to mitigate anthropogenic CO2emissions to the atmosphere is proposed.

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