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Water Oxidation Catalysis: Influence of Anionic Ligands upon the Redox Properties and Catalytic Performance of Mononuclear Ruthenium Complexes
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.ORCID iD: 0000-0003-1662-5817
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry.
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2012 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 51, no 6, 3388-3398 p.Article in journal (Refereed) Published
Abstract [en]

Aiming at highly efficient molecular catalyts for water oxidation, a mononuclear ruthenium complex Ru-II(hqc)(pic)(3) (1; H(2)hqc = 8-hydroxyquinoline-2-carboxylic acid and plc = 4-picoline) containing negatively charged carboxylate and phenolate donor groups has been designed and synthesized. As a comparison, two reference complexes, Ru-II(pdc)(pic)(3) (2; H(2)pdc = 2,6-pyridine-dicarboxylic acid) and Ru-II(tpy)(pic)(3) (3; tpy = 2,2':6',2 ''-terpyridine), have also been prepared. All three complexes are fully characterized by NMR, mass spectrometry (MS), and X-ray crystallography. Complex 1 showed a high efficiency toward catalytic water oxidation either driven by chemical oxidant (Ce-IV in a pH 1 solution) with a initial turnover number of 0.32 s(-1), which is several orders of magnitude higher than that of related mononuclear ruthenium catalysts reported in the literature, or driven by visible light in a three-component system with [Ru(bpy)(3)](2+) types of photosensitizers. Electrospray ionization MS results revealed that at the Rum state complex 1 undergoes ligand exchange of 4-picoline with water, forming the authentic water oxidation catalyst in situ. Density functional theory (DFT) was ernployed to explain how anionic ligands (hqc and pdc) facilitate the 4-picoline dissociation compared with a neutral ligand (tpy). Electrochemical measurements show that complex 1 has a much lower E(Ru-III/Ru-II) than that of reference complex 2 because of the introduction of a phenolate ligand. DFT was further used to study the influence of anionic ligands upon the redox properties of mononuclear aquaruthenium species, which are postulated to be involved in the catalysis cycle of water oxidation.

Place, publisher, year, edition, pages
2012. Vol. 51, no 6, 3388-3398 p.
Keyword [en]
Molecular Catalysts, Excited-States, Density, Thermochemistry, Solvation, Dioxygen, Kinetics, Energy, Cells, Dimer
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-95121DOI: 10.1021/ic201348uISI: 000301624500008Scopus ID: 2-s2.0-84863338735OAI: oai:DiVA.org:kth-95121DiVA: diva2:526827
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20120515Available from: 2012-05-15 Created: 2012-05-14 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Theoretical Studies on Artificial Water Splitting-Water Oxidation and Proton Transfer
Open this publication in new window or tab >>Theoretical Studies on Artificial Water Splitting-Water Oxidation and Proton Transfer
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis is concerned with the theoretical studies on artificial water splitting process. As the quick development of research on utilizing of solar energy, which is a green, clean, and renewable energy source, many research groups focus their attention on artificial photo-synthesis systems inspired by the photosystem I and II. The overall reaction in these artificial systems is water splitting to oxygen and hydrogen. Artificial water splitting can generally be divided into two half reactions, catalytic water oxidation and catalytic proton reduction. There is an increasing interest and demand to understand the detailed mechanism of these two key parts. Since DFT (density functional theory) in particular, has proven to be a powerful and popular tool in exploring reaction mechanisms, B3LYP and M06 functionals were employed to provide a theoretical explanation of these two important reactions in this thesis.

For water oxidation reaction, many efficient Water Oxidation Catalysts (WOCs) based on Ru, Ir, etc., have been reported over the last several years. The discovery of mononuclear ruthenium WOCs carrying anionic ligands is one of the major breakthroughs recently. WOCs bearing anionic ligands are able to efficiently drive catalytic water oxidation with relatively higher Turnover Numbers (TON) and Turnover Frequencies (TOF). Therefore the influence of anionic ligands gained our attention. We decided to carry out a detailed investigation on this effect, and try to propose a full mechanism of this catalytic water oxidation as well. We found that 1) The anionic ligands exert a promoting influence on the ligand exchange between picoline and water, which facilitates the formation of aqua-Ru complex, 2) The anionic ligands facilitate the complex access to higher oxidation states, which is necessary for the OO bond formation, and 3) The work of OO bond formation is in progress.

For the proton reduction reaction, the transport or movement of protons is vital and interesting in many biological and chemical processes, including the hydrogen uptake/production, the reduction of CO2 to formate, and the reduction of O2 to water. It is often related to energy storage and utilization. However, the details of these processes are still ambiguous. In most natural hydrogenase enzymes or synthetic catalysts based on iron or nickel, the incorporation of a pendant amine is a frequently occurring feature. This internal amine base seems to facilitate this proton transfer by acting as a proton relay. Our calculated results showed that the internal base allows for a splitting of one high enthalpy-high entropy barrier into two: one with a high enthalpy-low entropy barrier and the other with a low enthalpy-high entropy barrier, resulting in a low free energy of activation for proton transfer. Our results can serve as a guideline in the development of new catalysts, not only for proton reduction catalysts, but also for any process that involves proton transfer from a metal hydride to an external base, such as C-H activation and functionalization catalysts.

A thorough understanding on the mechanism of water splitting can help generate a strategy to enhance the catalytic performance on both water oxidation and proton reduction. We can tune or modify the synthetic complex by accelerating the slow step (rate-determining step) in the overall catalytic cycle, and can construct artificial water splitting systems with improved performance.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. x, 50 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2012:20
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-102566 (URN)
Presentation
2012-09-26, RB15, Albanova Universitetscentrum, Roslagstullsbacken, Stockholm, 11:00 (English)
Opponent
Supervisors
Note

QC 20120920

Available from: 2012-09-20 Created: 2012-09-20 Last updated: 2012-09-20Bibliographically approved
2. Mononuclear Ruthenium Complexes that Catalyze Water to Dioxgen Oxidation
Open this publication in new window or tab >>Mononuclear Ruthenium Complexes that Catalyze Water to Dioxgen Oxidation
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The theme of this thesis is the development of mononuclear Ru-based complexes that are capable of catalyzing the water oxidation (or O2-evolving) reaction, e.g. 2 H2O → O2 + 4 H+ + 4 e. Several families of mononuclear Ru water oxidation catalysts were designed and prepared. They feature with anionic ancillary ligands that contain carboxylate or phenolate donors. The properties of the catalysts were investigated in various aspects including coordination geometry, electrochemical behavior, and ligand exchange. All catalysts showed outstanding catalytic activity towards water oxidation in the presence of cerium(IV) ammonium nitrate as a sacrificial oxidant. High-valent Ru intermediates involved in the reactions were characterized both experimentally and theoretically. The kinetics of catalytic water oxidation was examined based on one catalyst and a prevailing catalytic pathway was proposed. The catalytic cycle involved a sequence of oxidation steps from RuII−OH2 to RuV=O species and O−O bond formation via water-nucleophilic-attack to the RuV=O intermediate. By comparing properties and catalytic performance of Ru catalysts herein with that of previously reported examples, the effect of anionic ancillary ligands was clearly elucidated in the context of catalytic water oxidation. Aiming to further application in an envisaged artificial photosynthesis device, visible light-driven water oxidation was conducted and achieved primarily in a homogeneous three-component system containing catalyst, photosensitizer, and sacrificial electron acceptor. Moreover, one model Ru catalyst was successfully immobilized on ordinary glass carbon surface through a facile and widely applicable method.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. 101 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:55
Keyword
Ruthenium complex, Homogeneous catalysis, Water oxidation, O2 evolution, anionic ligand, Molecular catalyst, Electrocatalysis, Kinetics, Artificial photosynthesis, Light-driven, Immobilization of catalyst
National Category
Organic Chemistry Inorganic Chemistry Energy Systems
Identifiers
urn:nbn:se:kth:diva-104765 (URN)978-91-7501-517-0 (ISBN)
Public defence
2012-11-30, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20121112

Available from: 2012-11-12 Created: 2012-11-12 Last updated: 2012-11-13Bibliographically approved
3. Theoretical Studies on Water Splitting Using Transition Metal Complexes
Open this publication in new window or tab >>Theoretical Studies on Water Splitting Using Transition Metal Complexes
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xii, 66 p.
Series
TRITA-BIO-Report, ISSN 1654-2312 ; 2014:6
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-145191 (URN)978-91-7595-069-3 (ISBN)
Public defence
2014-06-13, Albanova FB53, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140526

Available from: 2014-05-26 Created: 2014-05-14 Last updated: 2014-05-26Bibliographically approved

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