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Lookup NU author(s): Dr Nataliya Maksimchuk, Dr John ErringtonORCiD
This is the authors' accepted manuscript of an article that has been published in its final definitive form by American Chemical Society, 2021.
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Zr-monosubstituted Lindqvist-type polyoxometalates (Zr-POM), (Bu4N)2[W5O18Zr(H2O)3] (1) and (Bu4N)6[{W5O18Zr(µ-OH)}2] (2) have been employed as molecular models to unravel the mechanism of hydrogen peroxide activation over Zr(IV) sites. Compounds 1 and 2 are hydrolytically stable and catalyze epoxidation of C=C bonds in unfunctionalized alkenes and α,β-unsaturated ketones, as well as sulfoxidation of thioethers. Monomer 1 is more active than dimer 2. Acid additives greatly accelerate the oxygenation reactions and, oppositely, suppress H2O2 unproductive decomposition in the presence of 1 and 2, thereby increasing oxidant utilization efficiency up to >99%. Product distributions are indicative of heterolytic oxygen transfer mechanism that involves electrophilic oxidizing species formed upon interaction of Zr-POM and H2O2. The interaction of 1 and 2 with H2O2, and the resulting peroxo derivatives, have been investigated by UV-vis, FTIR and Raman spectroscopy, HR-ESI-MS, and combined HPLC-ICP-AES technique. Also the interaction between an 17O-enriched dimer, (Bu4N)6[{W5O18Zr(µ-OCH3)}2] (2'), and H2O2 was analyzed by 17O NMR spectroscopy. Combining these experimental studies with DFT calculations suggested the existence of dimeric peroxo species [(μ-η2:η2-O2){ZrW5O18}2]6-, as well as monomeric Zr-hydroperoxo [W5O18Zr(η2-OOH)]3- and Zr-peroxo [HW5O18Zr(η2-O2)]3- species. Reactivity studies revealed that the dimeric peroxo is inert toward alkenes but is able to transfer oxygen atom to thioethers while the monomeric peroxo intermediate is capable of epoxidizing C=C bonds. DFT characterization of the reaction mechanism identifies monomeric Zr-hydroperoxo intermediate as the real epoxidizing species, and the corresponding 𝛼-oxygen transfer to the substrate as the rate-determining step. Calculations also showed that protonation of Zr-POM reduces significantly the free-energy barrier of the key oxygen-transfer step because of the higher electrophilicity of the catalyst, and that dimeric species hampers the approach of alkene substrates due to steric repulsions reducing its reactivity. The improved performance of Zr(IV)-catalyst relatively to Ti(IV) and Nb(V) catalysts is respectively due to a flexible coordination environment and to a low tendency to form energy deep-well, low-reactive Zr-peroxo intermediates.
Author(s): Maksimchuk NV, Evtushok VY, Zalomaeva OV, Maksimov GM, Ivanchikova ID, Chesalov YA, Eltsov IV, Abramov PA, Glazneva TS, Yanshole YA, Kholdeeva OA, Errington RJ, Solé-Daura A, Poblet JM, Carbó JJ
Publication type: Article
Publication status: Published
Journal: ACS Catalysis
Year: 2021
Volume: 11
Issue: 16
Pages: 10589-10603
Print publication date: 20/08/2021
Online publication date: 11/08/2021
Acceptance date: 30/07/2021
Date deposited: 02/08/2021
ISSN (electronic): 2155-5435
Publisher: American Chemical Society
URL: https://doi.org/10.1021/acscatal.1c02485
DOI: 10.1021/acscatal.1c02485
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