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

摘要

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 H-1 and O-17 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 "MOOR", 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 beta-oxygen, whereas for the Nb catalyst the transference of the more electrophilic alpha-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.