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

Abstract

Ti- and Nb-monosubstituted tungstates of the Lindqvist structure, (Bu4N)3[(CH3O)­TiW5O18] (TiW 5 ) and (Bu4N)2[(CH3O)­NbW5O18] (NbW 5 ), 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 MW 5 (M = Ti and Nb) as tractable molecular models. In the oxidation of cyclohexene, NbW 5 reveals higher catalytic activity and heterolytic pathway selectivity than its Ti counterpart, while TiW 5 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 MW 5 -catalyzed epoxidation, comparative spectroscopic studies (IR, Raman, UV–vis, and 1H and 17O NMR) of the reaction between MW 5 and hydrogen peroxide, and DFT calculations implemented on cyclohexene epoxidation over MW 5 strongly support a mechanism that involves interaction of either MW 5 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.

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

“…A sequential intramolecular electron transfer (ET) from Mn2 to Mn1 takes place (2a → 3a), followed by a proton transfer (PT) from the coordinated water molecule to the acetate ligand (3a → 4a), resulting in the formation of a Mn–OH group. This PT reaction is not water mediated (unlike analogous PT reactions reported by Maksimchuk et al 69 ) as the second water molecule does not actively participate in it. The second acetate ligand exchange at Mn1 3+ (4a → 5a) represents the rate determining step of this path, with an activation Gibbs free energy of 8.0 kcal mol −1 (TS2a).…”

Reaction Pathway Discrimination in Alkene Oxidation Reactions by Designed Ti‐Siloxy‐Polyoxometalates

“…In order to rationalize the experimental results and to understand how the substituents of Ti‐SiloxPOM catalysts affect the oxidation activity and selectivity with TBHP, we next performed computational investigations (see Computational Details). Based on our previous computational studies on the epoxidation with H 2 O 2 by Ti‐SiloxPOM catalysts, [13] as well as related studies on other Ti‐ and transition metal‐substituted polyoxometallates, [20,28–34] we can define 3 possible pathways for the epoxidation of allylic alcohols with TBHP (see Scheme 2). Pathway (i) consists in an outer sphere O‐transfer from the [Ti]−( η 2 ‐OO t Bu) species; whereas mechanisms (ii) and (iii) involve inner‐sphere transition states in which both the oxidant and the substrate are coordinated to the Ti center after promoting the protolytic cleavage of one of the Ti‐OSi bonds.…”

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

“…A sequential intramolecular electron transfer (ET) from Mn2 to Mn1 takes place (2a / 3a), followed by a proton transfer (PT) from the coordinated water molecule to the acetate ligand (3a / 4a), resulting in the formation of a Mn-OH group. This PT reaction is not water mediated (unlike analogous PT reactions reported by Maksimchuk et al 69 ) as the second water molecule does not actively participate in it. The second acetate ligand exchange at Mn1 3+ (4a / 5a) represents the rate determining step of this path, with an activation Gibbs free energy of 8.0 kcal mol À1 (TS2a).…”