Unravelling the Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed by Ru-PNP Pincer Complexes

Elisabetta Alberico, Alastair J.J. Lennox, Lydia K. Vogt, Haijun Jiao, Wolfgang Baumann, Hans Joachim Drexler, Martin Nielsen, Anke Spannenberg, Marek P. Checinski, Henrik Junge, Matthias Beller*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

96 Citations (Scopus)
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Abstract

Ruthenium PNP complex 1a (RuH(CO)Cl(HN(C2H4Pi-Pr2)2)) represents a state-of-the-art catalyst for low-temperature (<100 °C) aqueous methanol dehydrogenation to H2 and CO2. Herein, we describe an investigation that combines experiment, spectroscopy, and theory to provide a mechanistic rationale for this process. During catalysis, the presence of two anionic resting states was revealed, Ru-dihydride (3-) and Ru-monohydride (4-) that are deprotonated at nitrogen in the pincer ligand backbone. DFT calculations showed that O- and CH- coordination modes of methoxide to ruthenium compete, and form complexes 4- and 3-, respectively. Not only does the reaction rate increase with increasing KOH, but the ratio of 3-/4- increases, demonstrating that the "inner-sphere" C - H cleavage, via C - H coordination of methoxide to Ru, is promoted by base. Protonation of 3- liberates H2 gas and formaldehyde, the latter of which is rapidly consumed by KOH to give the corresponding gem-diolate and provides the overall driving force for the reaction. Full MeOH reforming is achieved through the corresponding steps that start from the gem-diolate and formate. Theoretical studies into the mechanism of the catalyst Me-1a (N-methylated 1a) revealed that C - H coordination to Ru sets-up C - H cleavage and hydride delivery; a process that is also promoted by base, as observed experimentally. However, in this case, Ru-dihydride Me-3 is much more stable to protonation and can even be observed under neutral conditions. The greater stability of Me-3 rationalizes the lower rates of Me-1a compared to 1a, and also explains why the reaction rate then drops with increasing KOH concentration.

Original languageEnglish
Pages (from-to)14890-14904
Number of pages15
JournalJournal of the American Chemical Society
Volume138
Issue number45
Early online date4 Nov 2016
DOIs
Publication statusPublished - 16 Nov 2016

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