Epoxidation at Isolated Titanium Site Modeled by Ti-Siloxy-Polyoxometalates Built on [α-A-XW9O34]n- and [α-B-YW9O33]9-: Comparative Study of Their Hydrolytic Stability

Identifier:  imarina:9432958

Handle: https://hdl.handle.net/20.500.11797/imarina9432958

Authors:  Bidi, Ludivine K; Sole-Daura, Albert; Zhang, Teng; Desjonqueres, Alix; Poblet, Josep M; Proust, Anna; Carbo, Jorge J; Guillemot, Geoffroy

Abstract:
This report investigates the structural differences in a series of titanium complexes constructed from silanol functionalized polyoxometalate (SiloxPOMs) derivatives, designed to create a constrained coordination site for titanium (IV) cations, namely (THA)3[PW9O34(tBuSiO)3Ti(OiPr)] and (THA)3[SbW9O33(tBuSiO)3Ti(OiPr)]. The complexes serve as structural and functional models for titanium silicates, facilitating the epoxidation of allylic alcohols and alkenes by aqueous hydrogen peroxide solutions. The different activity and selectivity observed between the two derivatives are attributed to variations in the polyoxotungstic platform used, A-type-[XW9O34]n- versus B-type -[YW9O33]3-. A combined experimental and theoretical investigation highlights the influence of these structural differences on water interaction and hydrolytic stability, with A-type structures proving more susceptible to hydrolysis. In addition, the study also delves into the nuclearity of the active sites, a monomeric titanium (IV)-hydroperoxide [Ti]-(OOH) active species evidenced by diffusion NMR spectroscopy, and the influence of the presence of water on catalytic performance in epoxidation reaction, thus shedding light on the relationship between catalyst stability, intermediates formed and reaction pathway. The study finally demonstrates the suitability of B-type SiloxPOM derivatives as models for titanium silicates, offering insights into their stability and catalytic activity for epoxidation reactions.