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Direct Observation of Key Catalytic Intermediates in a Photoinduced Proton Reduction Cycle with a Diiron Carbonyl Complex
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.
Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
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2014 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 136, no 50, 17366-17369 p.Article in journal (Refereed) Published
Abstract [en]

The structure and reactivity of intermediatesin the photocatalytic cycle of a proton reductioncatalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), wereinvestigated by time-resolved spectroscopy. The singlyreduced catalyst [Fe2(bdt)(CO)6]−, a key intermediate inphotocatalytic H2 formation, was generated by reactionwith one-electron reductants in laser flash-quench experimentsand could be observed spectroscopically on thenanoseconds to microseconds time scale. From UV/visand IR spectroscopy, [Fe2(bdt)(CO)6]− is readilydistinguished from the two-electron reduced catalyst[Fe2(bdt)(CO)6]2− that is obtained inevitably in theelectrochemical reduction of [Fe2(bdt)(CO)6]. For thedisproportionation rate constant of [Fe2(bdt)(CO)6]−, anupper limit on the order of 107 M−1 s−1 was estimated,which precludes a major role of [Fe2(bdt)(CO)6]2− inphotoinduced proton reduction cycles. Structurally [Fe2-(bdt)(CO)6]− is characterized by a rather asymmetricallydistorted geometry with one broken Fe−S bond and sixterminal CO ligands. Acids with pKa ≤ 12.7 protonate[Fe2(bdt)(CO)6]− with bimolecular rate constants of 4 ×106, 7 × 106, and 2 × 108 M−1 s−1 (trichloroacetic,trifluoroacetic, and toluenesulfonic acids, respectively).The resulting hydride complex [Fe2(bdt)(CO)6H] istherefore likely to be an intermediate in photocatalyticcycles. This intermediate resembles structurally andelectronically the parent complex [Fe2(bdt)(CO)6], withvery similar carbonyl stretching frequencies.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014. Vol. 136, no 50, 17366-17369 p.
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-240552DOI: 10.1021/ja5085817ISI: 000346682600003OAI: oai:DiVA.org:uu-240552DiVA: diva2:776699
Available from: 2015-01-07 Created: 2015-01-07 Last updated: 2017-12-05
In thesis
1. Insight into Catalytic Intermediates Relevant for Water Splitting
Open this publication in new window or tab >>Insight into Catalytic Intermediates Relevant for Water Splitting
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Catalysis is an important part of chemistry. This is also reflected in the chemical industry where 85-90 % of all products are made catalytically. Also nature employs catalysts, i.e. enzymes, for its reactions.

To improve on the already existing catalysts one can learn a lot from nature which often uses earth-abundant elements in the enzymes which have also been optimized and finely tuned for billions of years. To gain a deeper understanding of both enzymatic and artificial catalysis one needs to investigate the mechanism of the catalytic process. But for very efficient catalysts with turnover frequencies of several thousand per second this is not easy, since an investigation of the mechanism involves resolving intermediates in the catalytic cycle. The intermediates in these instances are short-lived corresponding to their turnover frequencies. A maximum turnover frequency of 1,000 s-1 e.g. means that each catalyst goes through the whole catalytic cycle in 1 ms. Therefore time-resolved techniques are necessary that have a faster detection speed than the turnover frequency of the catalyst.

Flash photolysis is a spectroscopic technique with an instrument response function down to 10 ns.  Coupling this technique with mid-infrared probing yields an excellent detection system for probing different redox and protonation states of carbonyl metal complexes. Since many catalysts as well as natural enzymes involved in water splitting are metal carbonyl complexes this is an ideal technique to monitor the intermediates of these catalysts.

Chapter 3 covers the investigation of [FeFe] hydrogenases, enzymes that catalyze the reduction of protons to hydrogen in nature. Chapter 4 investigates the intermediates of biomimetic complexes, resembling the active site of natural [FeFe] hydrogenases. Chapter 5 covers the insights gained from investigating other catalysts which are also involved in water splitting and artificial photosynthesis.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 81 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1359
Keyword
Catalysis, Artificial photosynthesis, Molecular biomimetics
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Physical Chemistry; Chemistry with specialization in Chemical Physics
Identifiers
urn:nbn:se:uu:diva-281447 (URN)978-91-554-9526-8 (ISBN)
Public defence
2016-06-03, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2016-05-11 Created: 2016-03-24 Last updated: 2016-06-01
2. Mimicking the Outer Coordination Sphere in [FeFe]-Hydrogenase Active Site Models: From Extended Ligand Design to Metal-Organic Frameworks
Open this publication in new window or tab >>Mimicking the Outer Coordination Sphere in [FeFe]-Hydrogenase Active Site Models: From Extended Ligand Design to Metal-Organic Frameworks
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biomimetic catalysis is an important research field, as a better understanding of nature´s powerful toolbox for the conversion of molecules can lead to technological progress. [FeFe]-hydrogenases are very efficient catalysts for hydrogen production. These enzymes play a crucial role in the metabolism of green algae and certain cyanobacteria. Their active site consists of a diiron complex that is embedded in an interactive protein matrix.

In this thesis, two pathways for mimicking the outer coordination sphere effects resulting from the protein matrix are explored.

The first is the construction of model complexes containing phosphine ligands that are coordinated to the iron center as well as covalently linked to the bridging ligand of the complex. The effect of such linkers is an increased energy barrier for the rotation of the Fe(CO2)(PL3)-subunit, which potentially could stabilize a terminal hydride that is an important intermediate in the proton reduction cycle.

The second pathway follows the incorporation of [FeFe]-hydrogenase active site model complexes into metal-organic frameworks (MOFs). Resulting MOF-catalysts exhibit increased photocatalytic activity compared to homogenous references due to a stabilizing effect on catalytic intermediates by the surrounding framework. Catalyst accessibility within the MOF and the influence of the framework on chemical reactivity are examined in the work presented. Furthermore, an initial step towards application of MOF-catalysts in a device was made by interfacing them with electrodes.

The work of this thesis highlights strategies for the improvement of biomimetic model catalysts and the knowledge gained can be transferred to other systems mimicking the function of enzymes. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 115 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1498
Keyword
[FeFe]-hydrogenases, outer coordination sphere, model complexes, biomimetic catalysis, artificial photosynthesis, metal-organic frameworks
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-318975 (URN)978-91-554-9878-8 (ISBN)
Public defence
2017-05-19, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2017-04-28 Created: 2017-03-30 Last updated: 2017-05-05

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