Epoxidation of cyclooctene using soluble or MCM-41-supported molybdenum tetracarbonyl–pyridylimine complexes as catalyst precursors

The ligand N-(n-propyl)-2-pyridylmethanimine (pyim) and an immobilised analogue of this ligand (MCM-41-pyim) were prepared by the condensation reaction of 2-pyridinecarboxaldehyde with either propylamine or aminopropyl groups covalently attached to the ordered mesoporous silica MCM-41. Free and immobilised tetracarbonyl complexes of the type cis-[Mo(CO)4(L)] (L = pyim (1), MCM-41-pyim) were then prepared by microwave-assisted heating of a mixture of Mo(CO)6 and the organic ligand or ligand-silica in toluene at 110 °C for 30–45 min. The products were characterised by NMR spectroscopy (1H, 13C and 29Si, in solution and in the solid state), elemental analysis, N2 adsorption, and FT-IR spectroscopy. When used as catalyst precursors for the epoxidation of cis-cyclooctene by tert-butylhydroperoxide at 55 °C (1 mol% catalyst (Mo), no additional co-solvent), 1,2-epoxy-cyclooctane was obtained as the only reaction product in quantitative yield after 5 h for 1 and 36% yield after 24 h for the supported complex. The use of the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate as co-solvent led to lower catalytic activities (epoxide selectivity was always 100%) but allowed the catalyst/IL mixtures (homogeneous mixture for IL+1 and a biphasic solid + IL system for IL+MCM-41-pyim/Mo) to be easily recovered and reused in subsequent runs without loss of catalytic performance.

Graphical abstractA molybdenum tetracarbonyl complex bearing a pyridylimine ligand is a precursor to an active and selective catalyst for cis-cyclooctene epoxidation. A heterogenised version of the complex based on a chemically modified ordered mesoporous silica is described. The use of an ionic liquid as co-solvent allows efficient catalyst recycling.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Microwave-assisted synthesis of free and supported Mo0 tetracarbonyl complexes. ► Precursors to effective MoVI catalysts for the epoxidation of cis-cyclooctene. ► Efficient recycling of the catalyst systems by using an ionic liquid as co-solvent.