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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

Lookup NU author(s): Dr Nataliya Maksimchuk, Daniel Lebbie, Dr John ErringtonORCiD

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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 Nb-containing 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 Н₂О₂ 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 С=С 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 Ti catalyst, which has a rigid and hindered metal center, the hydroperoxo species transfers preferentially the nondistorted -oxygen whereas for the Nb catalysts the transference of the more electrophilic -oxygen is favored. Moreover, upon increasing the oxidation state from Ti(IV) to Nb(V) the reaction accelerates and selectivity towards 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.

Publication metadata

Author(s): Maksimchuk NV, Ivanchikova ID, Maksimov GM, Eltsov IV, Evtushok VY, Kholdeeva OA, Lebbie D, Errington RJ, Solé-Daura A, Poblet JM, Carbó JJ

Publication type: Article

Publication status: Published

Journal: ACS Catalysis

Year: 2019

Volume: 9

Pages: 6262-6275

Print publication date: 14/06/2019

Online publication date: 30/05/2019

Acceptance date: 04/05/2019

Date deposited: 30/05/2019

ISSN (electronic): 2155-5435

Publisher: American Chemical Society

URL: https://doi.org/10.1021/acscatal.9b01326

DOI: 10.1021/acscatal.9b01326

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