Abstract: Ti(IV)- and Nb(V)-containing catalysts are able to activate hydrogen peroxide and catalyze a range of selective oxidations, but their catalytic performance differs significantly [1,2]. In particular, Nb-catalysts demonstrate higher heterolytic pathway selectivity in epoxidation of alkenes than their Ti-counterparts. The reasons for this distinction still remain under debate. Transition-metal-substituted polyoxometalates (POM) can be employed as tractable molecular models to gain insights into the structure and reactivity of active peroxo species using various experimental and computational techniques [3-6]. Recently, we found that Ti- and Nb-substituted tungstates of the Lindqvist structure (shortly, MW5, M = Ti and Nb) mimic well the catalytic performance of heterogeneous M-silicates. The interaction of NbW5 with H2O2 has been studied and the structure of the corresponding peroxo species has been established using elemental analysis, UV-vis, FT-IR, Raman, 93Nb, 17O and 183W NMR spectroscopy, potentiometric titration, and DFT calculations. Studies on reactivity of the peroxo species under turnover and stoichiometric conditions revealed a crucial role of protons in heterolytic activation of H2O2 over Nb(V). The IR, Raman, UV-vis, and 17O NMR experiments together with DFT calculations showed that proton is presumably localized at a Nb–O–W bridging oxygen rather than at the peroxo group. On the other hand, DFT calculations revealed that hydroperoxo species ‘NbOOH’ has a lower activation barrier of oxygen transfer to alkene relative to peroxo species ‘HNb(O2)’. The higher epoxidation selectivity of Nb-catalysts is, most likely, due to higher energy cost of homolytic O–O bond breaking in NbOOH intermediate relative to TiOOH.

Cite: Kholdeeva O.A.
Selective Oxidations with H2O2 Catalyzed by Ti(IV) and Nb(V): Mechanistic Insights from Model Studies Using Polyoxometalates
International Simposium on Activation of Dioxygen and Homogeneous Catalytic Oxidation (ADHOC 2018) 24-29 Jun 2018