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

Research output: Contribution to journalJournal articleResearchpeer-review

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 4and 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
JournalAmerican Chemical Society. Journal
Volume138
Issue number45
Pages (from-to)14890–14904
Number of pages15
ISSN0002-7863
DOIs
Publication statusPublished - 2016

Cite this

Alberico, E., Lennox, A. J. J., Vogt, L. K., Jiao, H., Baumann, W., Drexler, H-J., ... Beller, M. (2016). Unravelling The Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed By Ru-PNP Pincer Complexes. American Chemical Society. Journal, 138(45), 14890–14904. https://doi.org/10.1021/jacs.6b05692
Alberico, Elisabetta ; Lennox, Alastair J. J. ; Vogt, Lydia K. ; Jiao, Haijun ; Baumann, Wolfgang ; Drexler, Hans-Joachim ; Nielsen, Martin ; Spannenberg, Anke ; Checinski, Marek P. ; Junge, Henrik ; Beller, Matthias. / Unravelling The Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed By Ru-PNP Pincer Complexes. In: American Chemical Society. Journal. 2016 ; Vol. 138, No. 45. pp. 14890–14904.
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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 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.",
author = "Elisabetta Alberico and Lennox, {Alastair J. J.} and Vogt, {Lydia K.} and Haijun Jiao and Wolfgang Baumann and Hans-Joachim Drexler and Martin Nielsen and Anke Spannenberg and Checinski, {Marek P.} and Henrik Junge and Matthias Beller",
year = "2016",
doi = "10.1021/jacs.6b05692",
language = "English",
volume = "138",
pages = "14890–14904",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
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Alberico, E, Lennox, AJJ, Vogt, LK, Jiao, H, Baumann, W, Drexler, H-J, Nielsen, M, Spannenberg, A, Checinski, MP, Junge, H & Beller, M 2016, 'Unravelling The Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed By Ru-PNP Pincer Complexes', American Chemical Society. Journal, vol. 138, no. 45, pp. 14890–14904. https://doi.org/10.1021/jacs.6b05692

Unravelling The Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed By Ru-PNP Pincer Complexes. / Alberico, Elisabetta; Lennox, Alastair J. J.; Vogt, Lydia K.; Jiao, Haijun; Baumann, Wolfgang; Drexler, Hans-Joachim; Nielsen, Martin; Spannenberg, Anke; Checinski, Marek P.; Junge, Henrik; Beller, Matthias.

In: American Chemical Society. Journal, Vol. 138, No. 45, 2016, p. 14890–14904.

Research output: Contribution to journalJournal articleResearchpeer-review

TY - JOUR

T1 - Unravelling The Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed By Ru-PNP Pincer Complexes

AU - Alberico, Elisabetta

AU - Lennox, Alastair J. J.

AU - Vogt, Lydia K.

AU - Jiao, Haijun

AU - Baumann, Wolfgang

AU - Drexler, Hans-Joachim

AU - Nielsen, Martin

AU - Spannenberg, Anke

AU - Checinski, Marek P.

AU - Junge, Henrik

AU - Beller, Matthias

PY - 2016

Y1 - 2016

N2 - 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.

AB - 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.

U2 - 10.1021/jacs.6b05692

DO - 10.1021/jacs.6b05692

M3 - Journal article

VL - 138

SP - 14890

EP - 14904

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

SN - 0002-7863

IS - 45

ER -