Abstract: Zr-monosubstituted Lindqvist-type polyoxometalates (Zr-POMs), (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 the 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 increase oxidant utilization efficiency up to >99%. Product distributions are indicative of a heterolytic oxygen transfer mechanism that involves electrophilic oxidizing species formed upon the 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, Raman spectroscopy, HR-ESI-MS, and combined HPLC-ICP-atomic emission spectroscopy techniques. The interaction between an 17O-enriched dimer, (Bu4N)6[{W5O18Zr(μ-OCH3)}2] (2′), and H2O2 was also 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 atoms to thioethers, while the monomeric peroxo intermediate is capable of epoxidizing C=C bonds. DFT analysis of the reaction mechanism identifies the monomeric Zr-hydroperoxo intermediate as the real epoxidizing species and the corresponding α-oxygen transfer to the substrate as the rate-determining step. The calculations also showed that protonation of Zr-POM significantly reduces the free-energy barrier of the key oxygen-transfer step because of the greater 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 the Zr(IV) catalyst relative to Ti(IV) and Nb(V) catalysts is respectively due to a flexible coordination environment and a low tendency to form energy deep-well and low-reactive Zr-peroxo intermediates.

Cite: Maksimchuk N.V. , Evtushok V.Y. , Zalomaeva O.V. , Maksimov G.M. , Ivanchikova I.D. , Chesalov Y.A. , Eltsov I.V. , Abramov P.A. , Glazneva T.S. , Yanshole V.V. , Kholdeeva O.A. , Errington R.J. , Solé-Daura A. , Poblet J.M. , Carbó J.J.
Activation of H2O2 over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates
ACS Catalysis. 2021. V.11. N16. P.10589−10603. DOI: 10.1021/acscatal.1c02485 WOS Scopus РИНЦ AN OpenAlex

Dates:

Submitted:	Jun 3, 2021
Accepted:	Jul 30, 2021
Published print:	Aug 11, 2021
Published online:	Aug 11, 2021

Identifiers:

Web of science:	WOS:000687845800050
Scopus:	2-s2.0-85113845356
Elibrary:	47062272
Chemical Abstracts:	2021:1809143
OpenAlex:	W3195304685

Citing:

DB	Citing
Scopus	28
Web of science	29
Elibrary	19
OpenAlex	28

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