Abstract: Ti- and Nb-monosubstituted tungstates of the Lindqvist structure, (Bu4N)3[(CH3O)TiW5O18] (TiW5) and (Bu4N)2[(CH3O)NbW5O18] (NbW5), display catalytic reactivity analogous to that of heterogeneous Ti- and Nbcontaining catalysts in alkene oxidation with aqueous hydrogen peroxide. In this work, we make an attempt to rationalize the differences observed in the catalytic performance of Ti and Nb single-site catalysts for alkene epoxidation with H2O2 using MW5 (M = Ti and Nb) as tractable molecular models. In the oxidation of cyclohexene, NbW5 reveals higher catalytic activity and heterolytic pathway selectivity than its Ti counterpart, while TiW5 is more active for decomposition of H2O2. The heterolytic and homolytic oxidation pathways have been investigated by means of kinetic and computational tools. The kinetic trends established for MW5-catalyzed epoxidation, comparative spectroscopic studies (IR, Raman, UV−vis, and 1H and 17O NMR) of the reaction between MW5 and hydrogen peroxide, and DFT calculations implemented on cyclohexene epoxidation over MW5 strongly support a mechanism that involves interaction of either MW5 or its hydrolyzed form “MOH” with H2O2 to afford a protonated peroxo species “HMO2” that is present in equilibrium with a hydroperoxo species “MOOH”, followed by electrophilic oxygen atom transfer from “MOOH” to the CC bond to give epoxide and “MOH”. For both Ti and Nb, the peroxo species “HMO2” is more stable than the hydroperoxo species “MOOH”, but the latter is more reactive toward alkenes. For the Ti catalyst, which has a rigid and hindered metal center, the hydroperoxo species transfers preferentially the nondistorted β-oxygen, whereas for the Nb catalyst the transference of the more electrophilic α-oxygen is favored. Moreover, upon increasing the oxidation state from Ti(IV) to Nb(V), the reaction accelerates and selectivity toward electrophilic products increases. Calculations showed that the Nb(V) catalyst reduces significantly the free-energy barrier for the heterolytic oxygen transfer because of the higher electrophilicity of the metal center. The improved performance of the Nb(V) single site is due to a combination of a flexible coordination environment with a higher metal oxidation state.

Cite: Maksimchuk N.V. , Ivanchikova I.D. , Maksimov G.M. , Eltsov I.V. , Evtushok V.Y. , Kholdeeva O.A. , Lebbie D. , Errington R.J. , Solé-Daura A. , Poblet J.M. , Carbó J.J.
Why does Nb(V) Show Higher Heterolytic Pathway Selectivity than Ti(IV) in Epoxidation with H2O2? Answers from Model Studies on Nb- and Ti-Substituted Lindqvist Tungstates
ACS Catalysis. 2019. V.9. P.6262−6275. DOI: 10.1021/acscatal.9b01326 WOS Scopus РИНЦ ANCAN OpenAlex

Dates:

Submitted:	Apr 1, 2019
Accepted:	May 27, 2019
Published online:	May 30, 2019
Published print:	Jul 5, 2019

Identifiers:

Web of science:	WOS:000474812400047
Scopus:	2-s2.0-85067993431
Elibrary:	41695592
Chemical Abstracts:	2019:1063666
Chemical Abstracts (print):	171:82501
OpenAlex:	W2947058098

Citing:

DB	Citing
Scopus	41
Web of science	39
Elibrary	37
OpenAlex	43

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