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Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO₂ to Methanol

Kar, Sayan, Sen, Raktim, Kothandaraman, Jotheeswari, Goeppert, Alain, Chowdhury, Ryan, Munoz, Socrates B., Haiges, Ralf, Prakash, G. K. Surya
Journal of the American Chemical Society 2019 v.141 no.7 pp. 3160-3170
amines, carbon dioxide, catalysts, chemical structure, dissociation, formates, hydrogen, hydrogenation, methanol, ruthenium
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 [RuHClPNPᴿ(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 [RuHPNPᴿ(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.