title: Unravelling the Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed by Ru-PNP Pincer Complexes.
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 H2and 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 H2gas 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.
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.|Istituto di Chimica Biomolecolare, Consiglio Nazionale delle Ricerche , tr. La Crucca 3, 07100 Sassari, Italy.
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Alberico
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Alastair J J
authors.initials: AJ
authors.lastname: Lennox
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Lydia K
authors.initials: LK
authors.lastname: Vogt
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Haijun
authors.initials: H
authors.lastname: Jiao
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Baumann
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Drexler
authors.affiliation: Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark , Kemitorvet 207, 2800 Kgs. Lyngby, Denmark.
authors.forename: Martin
authors.initials: M
authors.lastname: Nielsen
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Anke
authors.initials: A
authors.lastname: Spannenberg
authors.affiliation: CreativeQuantum GmbH , Wegedornstraße 32, 12524 Berlin, Germany.
authors.forename: Marek P
authors.initials: MP
authors.lastname: Checinski
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Henrik
authors.initials: H
authors.lastname: Junge
authors.affiliation: Leibniz Institute for Catalysis, University of Rostock , Albert Einstein-Straße 29a, 18059 Rostock, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Beller