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Metal vs. ligand protonation and the alleged proton-shuttling role of the azadithiolate ligand in catalytic H-2 formation with FeFe hydrogenase model complexes
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström. Univ Geneva, Dept Phys Chem, 30 Quai Ernest Ansermet, CH-1211 Geneva, Switzerland.ORCID iD: 0000-0003-0216-6920
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.ORCID iD: 0000-0001-7919-2444
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics. CNRS, LCC, 205 Route Narbonne,BP 44099, F-31077 Toulouse 4, France.
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2019 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 10, no 21, p. 5582-5588Article in journal (Refereed) Published
Abstract [en]

Electron and proton transfer reactions of diiron complexes [Fe(2)adt(CO)(6)] (1) and [Fe(2)adt(CO)(4)(PMe3)(2)] (4), with the biomimetic azadithiolate (adt) bridging ligand, have been investigated by real-time IR- and UV-vis-spectroscopic observation to elucidate the role of the adt-N as a potential proton shuttle in catalytic H-2 formation. Protonation of the one-electron reduced complex, 1(-), occurs on the adt-N yielding 1H and the same species is obtained by one-electron reduction of 1H(+). The preference for ligand vs. metal protonation in the Fe-2(i,0) state is presumably kinetic but no evidence for tautomerization of 1H to the hydride 1Hy was observed. This shows that the adt ligand does not work as a proton relay in the formation of hydride intermediates in the reduced catalyst. A hydride intermediate 1HHy(+) is formed only by protonation of 1H with stronger acid. Adt protonation results in reduction of the catalyst at much less negative potential, but subsequent protonation of the metal centers is not slowed down, as would be expected according to the decrease in basicity. Thus, the adtH(+) complex retains a high turnover frequency at the lowered overpotential. Instead of proton shuttling, we propose that this gain in catalytic performance compared to the propyldithiolate analogue might be rationalized in terms of lower reorganization energy for hydride formation with bulk acid upon adt protonation.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2019. Vol. 10, no 21, p. 5582-5588
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-390686DOI: 10.1039/c9sc00876dISI: 000474412700015PubMedID: 31293742OAI: oai:DiVA.org:uu-390686DiVA, id: diva2:1343438
Funder
Swedish Research Council, 621-2014-5670Swedish Research Council, 2016-04271Swedish Research Council Formas, 213-2014-880Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2019-08-16Bibliographically approved
In thesis
1. Probing Catalytic Reaction Mechanisms of Biomimetic Diiron Complexes through Time-resolved Absorption Spectroscopy
Open this publication in new window or tab >>Probing Catalytic Reaction Mechanisms of Biomimetic Diiron Complexes through Time-resolved Absorption Spectroscopy
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Directed design of improved molecular catalysts for hydrogen evolution reactions relies on rational benchmarking based on a detailed understanding about the mechanism of catalysis. Specifically, investigation of multi-electron redox catalysis, with structural characterization of catalytic intermediates, combined with the kinetics of their transformations, can reveal the rate-limiting step of the overall reaction, possible degradation pathways and the function of structural motives. However, direct spectroscopic observation of catalytic intermediates is in most cases not available due to the rapid turnover of efficient catalysts.

In this thesis, time-resolved absorption spectroscopy with UV-Vis and mid-IR detection was used to identify catalytic reaction intermediates and account for kinetics relevant to elementary reactions steps of H2 formation on a nanosecond to second time scale. For a class of FeIFeI (S-R-S)(CO)6-n(PMe3)n complexes (R = propyl, benzyl or azapropyl), inspired by the active site of FeFe-hydrogenase, the key intermediates formed in different catalytic pathways have been characterized. These complexes typically feature very similar coordination geometry, but show different structural rearrangements upon reduction. This could be applied to rationalize their differences in protonation dynamics. Protonation kinetics of singly reduced species, forming a bridging hydride, indicate a direct proton transfer step in the FeIFe0 state, in contrast to that of the neutral complex (FeIFeI state) with phosphine ligands (PMe3) in which the hydride formation is likely mediated by one of the CO-ligands, as had been proposed. In catalysis of FeFe-hydrogenase, the amine function of the bridgehead is known to assist enzymatic H2 formation by proton shuttling. The same role in catalysis by the synthetic diiron complex with the azapropyl bridgehead had been proposed. However, our results show that for the synthetic complex, the aza-group has no role as a proton shuttle in the hydride formation in the FeIFe0 state. Instead, the effect of nitrogen protonation is to lower the catalyst overpotential, without substantially slowing down the hydride formation with external protons. The amine acting as a proton shuttle in the hydride formation could be expected in the Fe0Fe0 level. However, slower second reduction of FeIFeI (S-azapropyl-S)(CO)6 complex impedes observation of the doubly reduced species under the catalytic conditions. For the benzyldithiolate complex, on the other hand, the rigid and unsaturated bridging ligand generally leads to less negative potentials and prevent the reduced forms from rapid degradation. This allows characterization of the later intermediates of the catalytic processes, and to obtain direct kinetic information on the turnover step.  

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 78
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1787
Keywords
Artificial photosynthesis, Biomimetic catalysts, H2 formation, Catalytic intermediates
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-380279 (URN)978-91-513-0610-0 (ISBN)
Public defence
2019-05-10, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
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Supervisors
Available from: 2019-04-23 Created: 2019-03-26 Last updated: 2019-08-16
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Aster, AlexanderWang, ShihuaiMirmohades, MohammadEsmieu, CharlèneBerggren, GustavHammarström, LeifLomoth, Reiner

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