Mechanistic Insights into H2O2 Activation over Nb(V) Using Nb-Substituted Lindqvist Tungstates
43rd International Conference on Coordination Chemistry
30 Jul - 4 Aug 2018
|| Kholdeeva Oxana Anatolʹevna
, Maksimchuk Nataliya Vladimirovna
, Maksimov Gennadii Mikhailovich
, Evtushok Vasilij Yurʹevich
, Ivanchikova Irina Dmitrievna
, Zalomaeva Olga Vadimovna
, Carbo Jorge J.
, Poblet Josep M.
Boreskov Institute of Catalysis SB RAS
Novosibirsk State University
Rovira i Virgili University
In recent years, Nb-containing single-site catalysts have attracted significant attention. Nb-substituted
tungstates of the Lindqvist structure, (Bu4N)4[(NbW5O18)2O) (1) and
(Bu4N)3[NbW5O19] (2), mimic well the catalytic performance of heterogeneous Nb-catalysts
in H2O2-based oxidations and have been used as tractable molecular models to gain insights into the structure and reactivity of active Nb(V) species using various spectroscopic (UV-vis, IR, Raman, 17O, 93Nb and 183W NMR) and computational techniques. While dimer 1 possesses high catalytic activity in alkene epoxidation, monomer 2 reveals a similar catalytic performance only in the presence of acid additives. The product selectivity and reaction kinetics are consistent with a mechanism that involves
hydrolysis of 1 to produce (Bu4N)2[Nb(OH)W5O18] (3), interaction of 3 with H2O2 leading to
the formation of a protonated niobium peroxo complex, (Bu4N)2[HNb(O2)W5O18] (4), followed by oxygen transfer from 4 to a C=C bond in alkene. The protonated peroxo complex 4 has been synthesized via interaction of 1 with an excess of 30% aqueous H2O2 in CH3CN and characterized by using elemental analysis, UV-vis, FT-IR, Raman, 93Nb, 17O and 183W NMR spectroscopy, potentiometric titration, cyclic voltammetry, and DFT calculations. A compilation of all the techniques supports a monomeric Lindqvist type structure of 4 bearing one peroxo ligand attached to Nb(V) in a η2-coordination mode. 4 readily reacts with cyclohexene at 50 °C in CH3CN to give cyclohexene epoxide and 1,2-trans-cyclohexane diol in a nearly quantitative yield, indicating a heterolytic oxidation
mechanism. The IR, Raman, UV-vis, and 17O NMR experiments along with theoretical calculations carried out at the DFT level showed that the activating proton in 4 is presumably localized at a Nb–O–W bridging oxygen rather than at the peroxo group. However, DFT calculations revealed that a hydroperoxo niobium species ‘NbOOH’ has a lower activation barrier of oxygen transfer to alkene relative to the peroxo species ‘HNb(O2)’.