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Which Oxidation State Leads to O-O Bond Formation in Cp*Ir(bpy)CI-Catalyzed Water Oxidation, Ir(V), Ir(VI), or Ir(VII)?
Stockholm University, Faculty of Science, Department of Organic Chemistry.ORCID iD: 0000-0002-8989-6928
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Number of Authors: 22014 (English)In: ACS Catalysis, ISSN 2155-5435, E-ISSN 2155-5435, Vol. 4, no 11, p. 3937-3949Article in journal (Refereed) Published
Abstract [en]

Density functional calculations are used to revisit the reaction mechanism of water oxidation catalyzed by the Cp*Ir(bpy)Cl (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine) complex. One of the experimentally suggested active species [(bpy)Ir(H2O)(2)(HCOO)Cl](+) can undergo very facile intramolecular formate oxidation at higher oxidation state even though it can also promote OO bond formation. Therefore, [(bpy)Ir(H2O)(2)(CH3COO)Cl](+) is here proposed to be the most likely precatalyst as acetate was also experimentally observed after Cp* oxidation. OO bond formation takes place at the high formal oxidation states of IrVI and IrVII, rather than that of IrV, as suggested before. Three sequential proton-coupled electron transfer oxidations result in the formation of a highly oxidized intermediate, [(bpy)IrVIO(OH)(CH3COO)Cl](+). From this formal IrVI intermediate, OO bond formation takes place by a water attack on the IrVI=O moiety assisted by the acetate ligand, which abstracts a proton during the attack. The barrier was calculated to be very facile, being 14.7 kcal/mol, in good agreement with experimental kinetic results, which gave a barrier of around 18 kcal/mol. The attack leads to the formation of an IrIV-peroxide intermediate, which undergoes proton-coupled electron transfer to form an IrIIIO2 intermediate. Finally, O2 can be released, coupled with the binding of another water molecule, to regenerate the catalytic Ir-III species. Water oxidation at IrVII has a slightly higher barrier, but it may also contribute to the activity. However, water oxidation at IrV has a significantly higher barrier. Acetate oxidation by CH activation was found to have a much higher barrier, suggesting that [(bpy)Ir(H2O)(2)(CH3COO)Cl](+) is a remarkably stable catalyst. The possible catalytic species [(bpy-dc)IrIII(H2O)(3)Cl](2+) without acetate coordination has also been considered and also gave a reasonably feasible barrier for the water oxidation. OO bond formation at IrVII is slightly preferred compared with at IrVI, which is different from the case with acetate.

Place, publisher, year, edition, pages
2014. Vol. 4, no 11, p. 3937-3949
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Chemical Sciences
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URN: urn:nbn:se:su:diva-160581DOI: 10.1021/cs501160xISI: 000344639300017OAI: oai:DiVA.org:su-160581DiVA, id: diva2:1254149
Available from: 2018-10-08 Created: 2018-10-08 Last updated: 2018-10-08Bibliographically approved

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Liao, Rong-ZhenSiegbahn, Per E. M.
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