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

• Published in: International journal of mass spectrometry 2019-01, Vol.435, p.163-167
• Main Authors: Fan, YanXuan, Lu, ShiFang, Cao, Jie
• Format: Article
• Language: English
• Subjects: Electrospray tandem mass spectrometry; Non-covalent interactions; Silicotungstic acid; γ-Cyclodextrin
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/j.ijms.2018.09.030

[Display omitted] •The mixed solution consisting of silicotungstic acid (SiW12 for short) and γ-cyclodextrin (γ-CD) to form a novel non-covalent hybrid complex SiW12/γ-CD was characterized by electrospray ionization tandem mass spectrometry (ESI-MS/MS) and proton nuclear magnetic resonance spectroscopy (1H NMR) with an aim to elucidate the nature of the intermolecular interactions involved in the complex.•The solid 1:1 SiW12/γ-CD complex was obtained by vapor diffusion method and characterized by Fourier Transform Infrared (FT-IR) Spectroscopy.•The solid complex was then used as a catalyst in the epoxidation of cyclooctene with H2O2 as oxidant. The reaction efficiency was evaluated in comparison with the single-component catalyst SiW12. 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.