A novel inorganic-organic hybrid complex between polyoxometalate and cyclodextrin: Synthesis, structure and catalytic activity

A novel non-covalent hybrid complex formed between silicotungstic acid (H₄SiW₁₂O₄₀, SiW₁₂, an abbreviation also for [SiW₁₂O₄₀]⁴⁻) and γ-cyclodextrin (γ-CD) was investigated by means of electrospray ionization tandem mass spectrometry (ESI-MS/MS) and proton nuclear magnetic resonance spectroscopy (¹H NMR). The experimental results showed that stable noncovalent complexes existing in equilibrium, [SiW₁₂ + (γ-CD)_m]⁴⁻ (m = 1, 2, and 3), can be formed in solution where the predominant species is the 1:1 complex. The significant chemical shift changes for the inner-cavity H-5 protons and the H-6/6′ protons on the primary faces of the CD torus in ¹H NMR titration revealed that SiW₁₂ probably interacts with the primary faces of γ-CD through intermolecular hydrogen-bonding interactions. The gas-phase fragmentation of [SiW₁₂ + (γ-CD)_m]⁴⁻ showed that a proton can be transferred from γ-CD to SiW₁₂ within the complex upon dissociation, leading to a pair of anionic products, [HSiW₁₂ + (γ-CD)_m-1]^3- and [γ-CD – H]-, providing further evidence for the nature of the intermolecular interactions. The solid 1:1 adduct catalyst SiW₁₂/γ-CD obtained by vapor diffusion method was then used in the epoxidation of cyclooctene to cyclooctene oxide with H₂O₂ as oxidant as a comparison with SiW₁₂ alone. The result showed that the catalytic performance of SiW₁₂/γ-CD is superior to SiW₁₂ (SiW₁₂/γ-CD is about 10 times better than SiW₁₂), serving as the best example to demonstrate the synergistic catalysis arising from the supramolecular catalyst.