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 [XW9O34–x(tBuSiOH)3]3– (X = P, x = 0, 1; X = Sb, x = 1, 2) in tetrahydrofuran. The resulting
titanium­(IV) complexes [XW9O34–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.