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

Abstract

Ruthenium PNP complex 1a (RuH(CO)Cl(HN(CHPi-Pr))) represents a state-of-the-art catalyst for low-temperature (<100 °C) aqueous methanol dehydrogenation to H and CO. 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 H 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.

Catalytic Dehydrogenation of Formic Acid with Ruthenium‐PNP‐Pincer Complexes: Comparing N‐Methylated and NH‐Ligands

“…Previously, the proposed mechanism for methanol dehydrogenation, which proceeds via formate intermediate, highlighted the importance of base in the catalytic cycle ,. Consequently, it was assumed that catalyst reactivity is much better under basic conditions than under neutral, and more over than at acidic pH.…”

“…Interestingly, Prakash and co‐workers reported the catalytic activity for formate dehydrogenation (DH) being independent of the N−H moiety of the pincer complex . Independently, our group presented a detailed mechanistic investigation of the Ru‐catalyzed methanol reforming . In this case, apart from Ru complex 7 with free N−H moiety, the N‐methylated Ru‐PNP pincer complex [Ru(H)(Cl)(CO)(CH 3 N{CH 2 CH 2 P(iPr) 2 } 2 )] 8 displayed lower but still noteworthy activity.…”

Cobalt‐Catalyzed Aqueous Dehydrogenation of Formic Acid

“…Apart from the O‐coordination, we located the complex of C−H coordination 3 A2 , which is responsible for the activation and dissociation of the C−H bond, and higher in energy than 3 A1 by 25.28 kcal mol −1 . In 3 A2 , the H−C distance is 1.235 Å, the Co−H distance is 1.805 Å and the H‐to‐O H‐bonding distance is 1.822 Å.…”

Formic Acid Dehydrogenation by Ruthenium Catalyst – Computational and Kinetic Analysis with the Energy Span Model

“…[10][11][12][13][14][15][16] For that, not only we have to undertake the computation of the full network, but we also have to consider the concentrations of reactants and products that will affect the rates. NMR, MS and other characterization tools are commonly used in order to gain some understanding of the catalytic mechanism through the detection of the most stable intermediates.…”