Synthesis and Characterization of Multinuclear Manganese-Containing Polyoxotungstates for Homogeneous Water-Oxidation Catalysis

  • Polyoxometalates are anionic, nano-sized metal-oxygen clusters of early transition metals in high oxidation states such as Mo6+, W6+, and V5+. The structural and compositional versatility of polyoxometalates is remarkable and is complemented by other tunable properties such as high stability, solubility and redox activity. Such properties render polyoxometalates highly desirable for catalytic studies such as homogeneous water-oxidation catalysis. Our group was able for the past thirteen years to synthesize a variety of polyanions containing multinuclear transition metal centers and clusters in acidic aqueous media. In particular, manganese-containing heteropolyoxotungstates are of special interest owed to the discovery of the active center responsible for the production of molecular oxygen from water in Photosystem II. The tetranuclear manganese oxygen-evolving center has motivated numerous attempts directed towards mimicking its structural or functional properties by designing a variety of manganese organic-based complexes. However have only resulted in poor activity being affected by thermal and oxidative degradation, unlike the robustness of polyoxometalates making them ideal frameworks for developing novel artificial photosystems. Along these guidelines inspired by nature, the reaction of [MnIII8MnIV4O12(CH3COO)16(H2O)4] with [A-α-PW9O34]9- in a 1:2 molar ratio in aqueous, acidic medium resulted in a trimanganese(III)-containing, sandwich-type 18-tungsto-2-phosphate [MnIII3(H2O)5(A-α-PW9O34)2]9-. The polyanion crystallizes as a mixed potassium/cesium/guanidinium salt in the triclinic space group P-1, with cell parameters a = 13.1718(19) Å, b = 16.505(2) Å, c = 22.842(4) Å, α = 104.141(7)°, β = 106.267(8)°, γ = 101.369(6)°, V = 4430.6(12) Å3, and Z = 2. The polyanion comprises two [A-α-PW9O34]9- Keggin fragments linked via three MnIII ions, two of which are 6-ccordinated and one is 5-coordinated. The obtained polyanion salt was further characterized by IR, TGA, elemental analysis and magnetic measurements in the solid state. Electrochemical studies showed that the polyanion is both stable in solution and interesting for the elaboration of a water oxidation electrocatalyst, due to its MnIII oxidation wave governed by a mixed adsorption-diffusion regime, accompanied by electrode activation. Five other manganese-containing Keggin-based tungstosilicates [MnII3(OH)3(H2O)3(A-α-SiW9O34)]7-, [MnIII3(OH)3(H2O)3(A-α-SiW9O34)]4-, [MnIII3(OH)3(H2O)3(A-β-SiW9O34)]4-, [MnIII3MnIVO3(CH3COO)3(A-α-SiW9O34)]6-, and [MnIII3MnIVO3(CH3COO)3(A-β-SiW9O34)]6- were also synthesized in aqueous medium by interaction of [A-α-SiW9O34]10- or [A-β-SiW9O34H]9- with either MnCl2·4H2O or [MnIII8MnIV4O12(CH3COO)16(H2O)4] under various reaction conditions. The obtained salts of these polyanions were further characterized in the solid state by IR, TGA, elemental analysis and magnetic studies, as well as in solution by electrochemistry. The tetranuclear manganese-containing 9-tungstosilicate, with its mixed-valence [MnIII3MnIVO3(CH3COO)3]4+ manganese assembly reminiscent of the oxygen evolving complex in Photosystem II, revealed high activity in photo-induced water splitting reactions. These results were further supported by several analytical techniques such as FT-IR, UV-Vis, electrochemical and conductiometric titration experiments. Additionally, in a similar fashion, the interaction of the same manganese coordination complex however with 9-tungstoarsenate(V) [A-α-AsVW9O34]9-, resulted into a novel mixed-valent decamanaganese-containing 36-tungsto-4aresenate(V) [MnIII6MnIV4O4(OH)12(H2O)12(A-β-AsVW9O34)4]22-, which might show interesting magnetic properties and photocatalytic activity in water-splitting due to the structurally appealing and electrocatalytically active [MnIII6MnIV4O4(OH)12(H2O)12]14+ core stabilized within the tungstoarsenate framework.

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Publishing Institution:IRC-Library, Information Resource Center der Jacobs University Bremen
Granting Institution:Jacobs Univ.
Author:Rami Al-Oweini
Referee:Ulrich Kortz, Gerd-Volker Röschenthaler, Marcella Bonchio, Thomas Heine, Bassem Bassil
Advisor:Ulrich Kortz
Persistent Identifier (URN):urn:nbn:de:gbv:579-opus-1002288
Document Type:PhD Thesis
Language:English
Date of Successful Oral Defense:2013/05/06
Year of Completion:2013
Date of First Publication:2013/08/12
Full Text Embargo Until:2014/08/31
PhD Degree:Chemistry
School:SES School of Engineering and Science
Other Countries Involved:Italy
Library of Congress Classification:Q Science / QD Chemistry / QD450-801 Physical and theoretical chemistry / QD474 Complex compounds
Call No:Thesis 2013/18

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