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Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol
journal contribution
posted on 2019-02-12, 17:33 authored by Sayan Kar, Raktim Sen, Jotheeswari Kothandaraman, Alain Goeppert, Ryan Chowdhury, Socrates B. Munoz, Ralf Haiges, G. K. Surya PrakashAmine-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.