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Artificial Water Splitting: Ruthenium Complexes for Water Oxidation
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry. (Licheng Sun's group)ORCID iD: 0000-0003-1662-5817
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis concerns the development and study of Ru-based water oxidation catalysts (WOCs) which are the essential components for solar energy conversion to fuels. The first chapter gives a general introduction about the field of homogenous water oxidation catalysis, including the catalytic mechanisms and the catalytic activities of some selected WOCs as well as the concerns of catalyst design. The second chapter describes a family of mononuclear Ru complexes [Ru(pdc)L3] (H2pdc = 2,6-pyridinedicarboxylic acid; L = pyridyl ligands) towards water oxidation. The negatively charged pdc2 dramatically lowers the oxidation potentials of Ru complexes, accelerates the ligand exchange process and enhances the catalytic activity towards water oxidation. A Ru aqua species [Ru(pdc)L2(OH2)] was proposed as the real catalyst. The third chapter describes the analogues of [Ru(terpy)L3]2+ (terpy = 2,2′:6′,2′′-terpyridine). Through the structural tailor, the ligand effect on the electrochemical and catalytic properties of these Ru complexes was studied. Mechanistic studies suggested that these Ru-N6 complexes were pre-catalysts and the Ru-aqua species were the real WOCs. The forth chapter describes a family of fast WOCs [Ru(bda)L2] (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid). Catalytic mechanisms were thoroughly investigated by electrochemical, kinetic and theoretical studies. The main contributions of this work to the field of water oxidation are (i) the recorded high reaction rate of 469 s−1; (ii) the involvement of seven-coordinate Ru species in the catalytic cycles; (iii) the O-O bond formation pathway via direct coupling of two Ru=O units and (iv) non-covalent effects boosting up the reaction rate. The fifth chapter is about visible light-driven water oxidation using a three component system including a WOC, a photosensitizer and a sacrificial electron acceptor. Light-driven water oxidation was successfully demonstrated using our Ru-based catalysts.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , 80 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011:48
Keyword [en]
water oxidation, ruthenium, electrochemistry, DFT calculation, photoelectrochemistry, negatively charged ligand, catalyst
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-40848ISBN: 978-91-7501-083-0 (print)OAI: oai:DiVA.org:kth-40848DiVA: diva2:442476
Public defence
2011-10-07, E3, Osquars backe 14, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110922Available from: 2011-09-22 Created: 2011-09-21 Last updated: 2011-09-22Bibliographically approved
List of papers
1. Chemical and Photochemical Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes with a Negatively Charged Tridentate Ligand
Open this publication in new window or tab >>Chemical and Photochemical Water Oxidation Catalyzed by Mononuclear Ruthenium Complexes with a Negatively Charged Tridentate Ligand
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2010 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 16, no 15, 4659-4668 p.Article in journal (Refereed) Published
Abstract [en]

Two mononuclear ruthenium complexes [RuL(pic)(3)] (1) and [RuL(bpy)(pic)] (2) (H2L = 2,6-pyridinedicarboxylic acid, pic=4-picoline, bpy = 2,2'-bipyridine) have been synthesized and fully characterized. Both complexes could promote water oxidation chemically and photochemically. Compared with other known ruthenium-based water oxidation catalysts using [Ce(NH4)(2)(NO3)(6)] (Ce-IV) as the oxidant in solution at pH 1.0, complex 1 is one of the most active catalysts yet reported with an initial rate of 0.23 turnovers(-1). Under acidic conditions, the equatorial 4-picoline in complex 1 dissociates first. In addition, ligand exchange in 1 occurs when the Rum state is reached. Based on the above observations and MS measurements of the intermediates during water oxidation by 1 using Ce-IV as oxidant, [RuL(pic)(2)(H2O)](+) is proposed as the real water oxidation catalyst.

Keyword
electrochemistry, homogeneous catalysis, oxidation, ruthenium, water splitting
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-27911 (URN)10.1002/chem.200902603 (DOI)000277331000027 ()2-s2.0-77950824222 (Scopus ID)
Note
QC 20110104Available from: 2011-01-04 Created: 2011-01-03 Last updated: 2017-12-11Bibliographically approved
2. The Ru complexes containing pyridine-dicarboxylate ligand: electronic effect on their catalytic activity toward water oxidation
Open this publication in new window or tab >>The Ru complexes containing pyridine-dicarboxylate ligand: electronic effect on their catalytic activity toward water oxidation
2011 (English)In: Faraday discussions (Online), ISSN 1359-6640, E-ISSN 1364-5498, Vol. 155, 267-275 p.Article in journal (Refereed) Published
Abstract [en]

Two series of mononuclear ruthenium complexes [Ru(pdc)L-3] (H(2)pdc = 2,6-pyridinedicarboxylic acid; L = 4-methoxypyridine, 1; pyridine, 2; pyrazine, 3) and [Ru(pdc)L-2(dmso)] (dmso = dimethyl sulfoxide; L = 4-methoxypyridine, 4; pyridine, 5) were synthesized and spectroscopically characterized. Their catalytic activity toward water oxidation has been examined using Ce-IV (Ce(NH4)(2)(NO3)(6)) as the chemical oxidant under acidic conditions. Complexes 1, 2 and 3 are capable of catalyzing Ce-IV-driven water oxidation while 4 and 5 are not active. Electronic effects on their catalytic activity were illustrated: electron donating groups increase the catalytic activity.

National Category
Organic Chemistry
Identifiers
urn:nbn:se:kth:diva-40905 (URN)10.1039/c1fd00101a (DOI)000299793100018 ()2-s2.0-84857067133 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
Original title: The Ru-pdc Complexes: Electronic Effect on Their Catalytic Activity toward CeIV-Driven Water Oxidation QC 20120227Available from: 2011-09-22 Created: 2011-09-22 Last updated: 2017-12-08Bibliographically approved
3. Ce-IV- and Light-Driven Water Oxidation by [Ru(terpy)(pic)(3)](2+) Analogues: Catalytic and Mechanistic Studies
Open this publication in new window or tab >>Ce-IV- and Light-Driven Water Oxidation by [Ru(terpy)(pic)(3)](2+) Analogues: Catalytic and Mechanistic Studies
2011 (English)In: CHEMSUSCHEM, ISSN 1864-5631, Vol. 4, no 2, 238-244 p.Article in journal (Refereed) Published
Abstract [en]

A series of mononuclear ruthenium polypyridyl complexes [Ru(Mebimpy)(pic)(3)](PF6)(2) (2; Mebimpy=2,6-bis(1-methylbenzimidazol-2-yl)pyridine; pic=4-picoline), Ru(bimpy)(pic)(3) (3; H(2)bimpy=2,6-bis(benzimidazol-2-yl)pyridine), trans-[Ru(terpy)-(pic)(2)Cl](PF6) (4; terpy=2,2';6',2 ''-terpyridine), and trans-[Ru(terpy)(pic)(2)(OH2)](ClO4)(2) (5) are synthesized and characterized as analogues of the known Ru complex, [Ru(terpy)(pic)(3)](PF6)(2) (1). The effect of the ligands on electronic and catalytic properties is studied and discussed. The negatively charged ligand, bimpy(2-), has a remarkable influence on the electrochemical events due to its strong electron-donating ability. The performance in light- and Ce-IV-driven (Ce-IV=Ce(NH4)(2)(NO3)(6)) water oxidation is successfully demonstrated. We propose that ligand exchange between pic and H2O occurs to form the real catalyst, a Ru-aqua complex. The synthesis and testing of trans[Ru(terpy)(pic)(2)(OH2)](ClO4)(2) (5) confirmed our proposal. In addition, complex 5 possesses the best catalytic activity among these five complexes.

Keyword
electrochemistry, homogeneous catalysis, photochemistry, ruthenium, water chemistry
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-31641 (URN)10.1002/cssc.201000313 (DOI)000288043400016 ()2-s2.0-79951807046 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20110321Available from: 2011-03-21 Created: 2011-03-21 Last updated: 2011-09-22Bibliographically approved
4. Isolated Seven-Coordinate Ru(IV) Dimer Complex with HOHOH (-) Bridging Ligand as an Intermediate for Catalytic Water Oxidation
Open this publication in new window or tab >>Isolated Seven-Coordinate Ru(IV) Dimer Complex with HOHOH (-) Bridging Ligand as an Intermediate for Catalytic Water Oxidation
2009 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 131, no 30, 10397-+ p.Article in journal (Refereed) Published
Abstract [en]

With the inspiration from an oxygen evolving complex (OEC) in Photosystern II (PSII), a mononuclear Ru(II) complex with a tetradentate ligand containing two carboxylate groups has been synthesized and structurally characterized. This Ru(II) complex showed efficient catalytic properties toward water oxidation by the chemical oxidant cerium(IV) ammonium nitrate. During the process of catalytic water oxidation, Ru(III) and Ru(IV) species have been successfully isolated as intermediates. To our surprise, X-ray crystallography together with HR-MS revealed that the Ru(IV) species is a seven-coordinate Ru(IV) dimer complex containing a [HOHOH](-) bridging ligand. This bridging ligand has a short O center dot center dot center dot O distance and is hydrogen bonded to two water molecules. The discovery of this very uncommon seven-coordinate Ru(IV) dimer together with a hydrogen bonding network may contribute to a deeper understanding of the mechanism for catalytic water oxidation. It will also provide new possibilities for the design of more efficient catalysts for water oxidation, which is the key step for solar energy conversion into hydrogen by tight-driven water splitting, the ultimate challenge in artificial photosynthesis.

Identifiers
urn:nbn:se:kth:diva-18657 (URN)10.1021/ja9034686 (DOI)000268644400031 ()2-s2.0-68049108505 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
5. The First Replication of the Water Oxidation Activity of Photosystem-II by a Molecular Ruthenium Catalyst
Open this publication in new window or tab >>The First Replication of the Water Oxidation Activity of Photosystem-II by a Molecular Ruthenium Catalyst
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(English)Manuscript (preprint) (Other academic)
National Category
Organic Chemistry
Identifiers
urn:nbn:se:kth:diva-40901 (URN)
Note
QS 2011Available from: 2011-09-22 Created: 2011-09-22 Last updated: 2011-09-22Bibliographically approved
6. Visible Light-Driven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System
Open this publication in new window or tab >>Visible Light-Driven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System
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2010 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 49, no 1, 209-215 p.Article in journal (Refereed) Published
Abstract [en]

Discovery of an efficient catalyst bearing low overpotential toward water oxidation is a key step for light-driven water splitting into dioxygen and dihydrogen. A mononuclear ruthenium complex, Ru(II)L(pic)(2) (1) (H2L = 2,2'-bipyridine-6,6'-dicarboxylic acids pic = 4-picoline), was found capable of oxidizing water eletrochemically at a relatively low potential and promoting light-driven water oxidation using a three-component system composed of a photosensitizer, sacrificial electron acceptor, and complex 1. The detailed electrochemical properties of 1 were studied, and the onset potentials of the electrochemically catalytic curves in pH 7.0 and pH 1.0 solutions are 1.0 and 1.5 V, respectively. The low catalytic potential of 1 under neutral conditions allows the use of [Ru(bpy)(3)](2+) and even [Ru(dmbpy)(3)](2+) as a photosensitizer for photochemical water oxidation. Two different sacrificial electron acceptors, [Co(NH3)(5)Cl]Cl-2 and Na2S2O8, were used to generate the oxidized state of ruthenium tris(2,2'-bipyridyl) photosensitizers. In addition, a two-hour photolysis of I in a pH TO phosphate buffer did not lead to obvious degradation, indicating the good photostability of our catalyst. However, under conditions of light-driven water oxidation, the catalyst deactivates quickly. In both solution and the solid state under aerobic conditions, complex 1 gradually decomposed via oxidative degradation of its ligands, and two of the decomposed products, sp(3) C-H bond oxidized Ru complexes, were identified. The capability of oxidizing the sp(3) C-H bond implies the presence of a highly oxidizing Ru species, which might also cause the final degradation of the catalyst.

Keyword
O-O BOND, ARTIFICIAL PHOTOSYNTHESIS, FUNCTIONAL-MODEL, EXCITED-STATE, RU(II) COMPLEXES, REDOX PROPERTIES, PHOTOSYSTEM-II, O-2 EVOLUTION, OXYGEN, LIGAND
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-19065 (URN)10.1021/ic9017486 (DOI)000272935800023 ()2-s2.0-73349134347 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20100525

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
7. Visible light-driven water oxidation catalyzed by a highly efficient dinuclear ruthenium complex
Open this publication in new window or tab >>Visible light-driven water oxidation catalyzed by a highly efficient dinuclear ruthenium complex
Show others...
2010 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 46, no 35, 6506-6508 p.Article in journal (Refereed) Published
Abstract [en]

Visible light-driven water oxidation has been achieved by the dinuclear ruthenium complex 1 with a high turnover number of 1270 in a homogeneous system in the presence of a Ru polypyridine complex photosensitizer.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-26833 (URN)10.1039/c0cc01250e (DOI)000281237600022 ()2-s2.0-77956042575 (Scopus ID)
Note
QC 20101206Available from: 2010-12-06 Created: 2010-11-29 Last updated: 2017-12-11Bibliographically approved

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