Immobilization of a Molybdenum Complex on Bipyridine-Based Periodic Mesoporous Organosilica and Its Catalytic Activity for Epoxidation of Olefins

Satoshi Ishikawa, Yoshifumi Maegawa, Minoru Waki, Shinji Inagaki

Index: 10.1021/acscatal.8b00809

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Abstract

The dichlorodioxomolybdenum(VI) complex (MoO2Cl2) is an efficient and low-cost homogeneous catalyst for a variety of organic reactions, but its activity usually decreases after immobilization on a solid support. This report describes the synthesis of heterogeneous Mo complex catalysts using a bipyridine-periodic mesoporous organosilica (BPy-PMO) as a solid chelating ligand and MoO2Cl2 as a precursor, and their catalysis in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP). The MoO2Cl(OH) complex could be immobilized on trimethylsilylated BPy-PMO (BPy-PMO-TMS), which was confirmed by Fourier-transform infrared spectroscopy, Raman spectroscopy, and X-ray absorption fine-structure analysis. The Mo complex immobilized on BPy-PMO-TMS exhibited greater catalytic activity for the epoxidation of cis-cyclooctene compared with conventional heterogeneous Mo complex catalysts using mesoporous silica, polystyrene, and naked BPy-PMO as supports. A large amount (up to 0.72 mmol g–1) of the Mo complex could be loaded on BPy-PMO-TMS, which resulted in 37% of the exposed bipyridine ligands on the surface forming Mo complexes. The turnover frequency relative to Mo was nearly constant, even with a high density of Mo complex on the pore surface. The Mo-BPy-PMO-TMS catalyst exhibited a solvent effect on the catalysis, and the presence of water in the reaction medium was detrimental for the catalysis. Under anhydrous conditions, Mo-BPy-PMO-TMS showed good catalytic activity for at least three reuse cycles in the epoxidation of cis-cyclooctene. A variety of olefins including aliphatic and aromatic olefins were successfully oxidized by Mo-BPy-PMO-TMS to give the desired epoxides in high yield.