Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO<sub>2</sub> to Methanol
2019-02-12T17:33:40Z (GMT) by
Amine-assisted homogeneous hydrogenation of CO<sub>2</sub> to methanol is one of the most effective approaches to integrate CO<sub>2</sub> capture with its subsequent conversion to CH<sub>3</sub>OH. 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 H<sub>2</sub>). In the second step the generated formamide is further hydrogenated with 2 more equiv of H<sub>2</sub> to CH<sub>3</sub>OH 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<sup><i>R</i></sup>(CO)] (R = Ph/<i>i-</i>Pr/Cy/<i>t-</i>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<sup><i>R</i></sup>(CO)<sub>2</sub>]<sup>+</sup>, 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.