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Direct Spectroscopic Detection of Key Intermediates and Turnover Process in Catalytic H2 Formation by a Biomimetic Diiron Catalyst
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, Molecular Biomimetics. TU Dortmund Univ, Fac Chem & Chem Biol, Otto Hahn Str 6, D-44227 Dortmund, Germany.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
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2019 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 25, no 47, p. 11135-11140Article in journal (Refereed) Published
Abstract [en]

[FeFe(Cl-2-bdt)(CO)(6)] (1; Cl-2-bdt=3,6-dichlorobenzene-1,2-dithiolate), inspired by the active site of FeFe-hydrogenase, shows a chemically reversible 2 e(-) reduction at -1.20 V versus the ferrocene/ferrocenium couple. The rigid and aromatic bdt bridging ligand lowers the reduction potential and stabilizes the reduced forms, compared with analogous complexes with aliphatic dithiolates; thus allowing details of the catalytic process to be characterized. Herein, time-resolved IR spectroscopy is used to provide kinetic and structural information on key catalytic intermediates. This includes the doubly reduced, protonated complex 1H(-), which has not been previously identified experimentally. In addition, the first direct spectroscopic observation of the turnover process for a molecular H-2 evolving catalyst is reported, allowing for straightforward determination of the turnover frequency.

Place, publisher, year, edition, pages
2019. Vol. 25, no 47, p. 11135-11140
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-380278DOI: 10.1002/chem.201902100ISI: 000479841700001PubMedID: 31210385OAI: oai:DiVA.org:uu-380278DiVA, id: diva2:1299009
Funder
Swedish Research Council, 2016-04271Stiftelsen Olle Engkvist Byggmästare, 2016/3Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2019-10-31Bibliographically 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)
Opponent
Supervisors
Available from: 2019-04-23 Created: 2019-03-26 Last updated: 2019-08-16

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Wang, ShihuaiPullen, SonjaLiu, TianfeiOtt, SaschaLomoth, ReinerHammarström, Leif

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