Unveiling the Active Surface Sites in Heterogeneous Titanium-Based Silicalite Epoxidation Catalysts: Input of Silanol-Functionalized Polyoxotungstates as Soluble Analogues

We report on a site-isolated model for Ti(IV) by reacting [Ti(ⁱPrO)₄] with the silanol-functionalized polyoxotungstates [XW₉O_34-x(^tBuSiOH)₃]^3- (X = P, x = 0, 1; X = Sb, x = 1, 2) in tetrahydrofuran. The resulting titanium(IV) complexes [XW₉O_34-x(^tBuSiO)₃Ti(OⁱPr)]^3- (X = P, 3; X = Sb, 4) were obtained in monomeric forms both in solution and in the solid state, as proved by diffusion NMR experiments and by X-ray crystallographic analysis. Anions 3 and 4 represent relevant soluble models for heterogeneous titanium silicalite epoxidation catalysts. The POM scaffolds feature slight conformational differences that influence the chemical behavior of 3 and 4 as demonstrated by their reaction with H₂O. In the case of 3, the hydrolysis reaction of the isopropoxide ligand is only little shifted toward the formation of a monomeric [PW₉O₃₄(^tBuSiO)₃Ti(OH)]^3- (5) species [log K = -1.96], whereas 4 reacted readily with H₂O to form a μ-oxo bridged dimer {[SbW₉O₃₃(^tBuSiO)₃Ti]₂O}^6- (6). The more confined was the coordination site, the more hydrophobic was the metal complex. By studying the reaction of 3 and 4 with hydrogen peroxide using NMR and Raman spectroscopies, we concluded that the reaction leads to the formation of a titanium-hydroperoxide Ti-(η¹-OOH) moiety, which is directly involved in the epoxidation of the allylic alcohol 3-methyl-2-buten-1-ol. The combined use of both spectroscopies also led to understanding that a shift of the acid-base equilibrium toward the formation of Ti(η²-O₂) and H₃O⁺ correlates with the partial hydrolysis of the phosphotungstate scaffold in 3. In that case, the release of protons also catalyzed the oxirane opening of the in situ formed epoxide, leading to an increased selectivity for 1,2,3-butane-triol. In the case of the more stable [SbW₉O₃₃(^tBuSiO)₃Ti(OⁱPr)]^3- (4), the evolution to Ti(η²-O₂) peroxide was not detected by Raman spectroscopy, and we performed reaction progress kinetic analysis by NMR monitoring the 3-methyl-2-buten-1-ol epoxidation to assess the efficiency and integrity of 4 as precatalyst.