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Probing Catalytic Reaction Mechanisms of Biomimetic Diiron Complexes through Time-resolved Absorption Spectroscopy
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.ORCID iD: 0000-0001-7919-2444
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 [en]
Artificial photosynthesis, Biomimetic catalysts, H2 formation, Catalytic intermediates
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-380279ISBN: 978-91-513-0610-0 (print)OAI: oai:DiVA.org:uu-380279DiVA, id: diva2:1299011
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
List of papers
1. Direct Spectroscopic Detection of Key Intermediates and Turnover Process in Catalytic H2 Formation by a Biomimetic Diiron Catalyst
Open this publication in new window or tab >>Direct Spectroscopic Detection of Key Intermediates and Turnover Process in Catalytic H2 Formation by a Biomimetic Diiron Catalyst
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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.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-380278 (URN)10.1002/chem.201902100 (DOI)000479841700001 ()31210385 (PubMedID)
Funder
Swedish Research Council, 2016-04271Stiftelsen Olle Engkvist Byggmästare, 2016/3
Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2019-10-31Bibliographically approved
2. Spectroscopic Observation of Two-electron Reduced Diiron Azadithiolate Catalyst related to the active site of FeFe-hydrogenase
Open this publication in new window or tab >>Spectroscopic Observation of Two-electron Reduced Diiron Azadithiolate Catalyst related to the active site of FeFe-hydrogenase
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-380277 (URN)
Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2019-04-23
3. Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst
Open this publication in new window or tab >>Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst
2018 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 57, no 2, p. 768-776Article in journal (Refereed) Published
Abstract [en]

One-electron reduction and subsequent protonation of a biomimetic proton-reduction catalyst [FeFe(μ-pdt)(CO)6] (pdt = propanedithiolate), 1, were investigated by UV-vis and IR spectroscopy on a nano- to microsecond time scale. The study aimed to provide further insight into the proton-reduction cycle of this [FeFe]-hydrogenase model complex, which with its prototypical alkyldithiolate-bridged diiron core is widely employed as a molecular, precious metal-free catalyst for sustainable H2 generation. The one-electron-reduced catalyst was obtained transiently by electron transfer from photogenerated [Ru(dmb)3]+ in the absence of proton sources or in the presence of acids (dichloro- or trichloroacetic acid or tosylic acid). The reduced catalyst and its protonation product were observed in real time by UV-vis and IR spectroscopy, leading to their structural characterization and providing kinetic data on the electron and proton transfer reactions. 1 features an intact (μ22-pdt)(μ-H)Fe2 core in the reduced, 1-, and reduced-protonated states, 1H, in contrast to the Fe-S bond cleavage upon the reduction of [FeFe(bdt)(CO)6], 2, with a benzenedithiolate bridge. The driving-force dependence of the rate constants for the protonation of 1- (kpt = 7.0 × 105, 1.3 × 107, and 7.0 × 107 M-1 s-1 for the three acids used in this study) suggests a reorganization energy >1 eV and indicates that hydride complex 1H is formed by direct protonation of the Fe-Fe bond. The protonation of 1- is sufficiently fast even with the weaker acids, which excludes a rate-limiting role in light-driven H2 formation under typical conditions.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-343862 (URN)10.1021/acs.inorgchem.7b02687 (DOI)000422810900029 ()29297686 (PubMedID)
Funder
Swedish Research Council, 2016-04271Stiftelsen Olle Engkvist Byggmästare, 2016/3
Available from: 2018-03-02 Created: 2018-03-02 Last updated: 2019-03-26
4. Metal vs. ligand protonation and the alleged proton-shuttling role of the azadithiolate ligand in catalytic H-2 formation with FeFe hydrogenase model complexes
Open this publication in new window or tab >>Metal vs. ligand protonation and the alleged proton-shuttling role of the azadithiolate ligand in catalytic H-2 formation with FeFe hydrogenase model complexes
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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
National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-390686 (URN)10.1039/c9sc00876d (DOI)000474412700015 ()31293742 (PubMedID)
Funder
Swedish Research Council, 621-2014-5670Swedish Research Council, 2016-04271Swedish Research Council Formas, 213-2014-880
Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2019-08-16Bibliographically approved

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