Dichlorodioxomolybdenum(vi) complexes bearing oxygen-donor ligands as olefin epoxidation catalysts

Abstract

Treatment of the solvent adduct [MoO₂Cl₂(THF)₂] with either 2 equivalents of N,N-dimethylbenzamide (DMB) or 1 equivalent of N,N′-diethyloxamide (DEO) gave the dioxomolybdenum(VI) complexes [MoO₂Cl₂(DMB)₂] (1) and [MoO₂Cl₂(DEO)] (2). The molecular structures of 1 and 2 were determined by single-crystal X-ray diffraction. Both complexes present a distorted octahedral geometry and adopt the cis-oxo, trans-Cl, cis-L configuration typical of complexes of the type [MoO₂X₂(L)_n], with either the monodentate DMB or bidentate DEO oxygen-donor ligands occupying the equatorial positions trans to the oxo groups. The complexes were applied as homogeneous catalysts for the epoxidation of olefins, namely cis-cyclooctene (Cy), 1-octene, trans-2-octene, α-pinene and (R)-(+)-limonene, using tert-butylhydroperoxide (TBHP) as oxidant. In the epoxidation of Cy at 55 °C, the desired epoxide was the only product and turnover frequencies in the range of ca. 3150–3200 mol mol_Mo⁻¹ h⁻¹ could be reached. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the cosolvent. Complexes 1 and 2 were also applied in liquid–liquid biphasic catalytic epoxidation reactions by using an ionic liquid of the type [C₄mim][X] (C₄mim = 1-n-butyl-3-methylimidazolium; X = NTf₂, BF₄ or PF₆] as a solvent to immobilise the metal catalysts. Recycling for multiple catalytic runs was achieved without loss of activity.