Synthesis and catalytic properties in olefin epoxidation of dioxomolybdenum(vi) complexes bearing a bidentate or tetradentate salen-type ligand

The monomeric cis-dioxomolybdenum(VI) complexes [MoO2(oep-saldpen)] and [MoO2Cl2(oep-H2saldpen)], with a tetradentate [N2(imine)O2] and bidentate [N2(imine)] salen-type ligand functionalised with two pyrrole derivative pendant arms [oep-H2saldpen = 1,2-diphenylethylenebis(3-oxyethylpyrrole)salicylideneimine], were synthesised and characterised by 1H NMR, IR and Raman spectroscopy. The solid-state structure of the free ligand oep-H2saldpen was determined by single crystal X-ray diffraction. Assignment of the vibrational spectra of the molybdenum complexes was supported by carrying out ab initio calculations for the possible isomers using [MoO2(salen)] and [MoO2Cl2(H2salen)] as model compounds [H2salen = N,N′-ethylenebis(salicylideneimine)]. The oep-saldpen complexes were examined as catalysts for the epoxidation of cyclooctene, (R)-(+)-limonene, styrene, α-pinene, and cis and trans-β-methylstyrene, with tert-butyl hydroperoxide as the oxidant. Both complexes exhibited high selectivity for the epoxidation reaction, with the bis(chloro) complex being always the more active of the two.

Graphical abstractThe molybdenum complexes [MoO2(oep-saldpen)] and [MoO2Cl2(oep-H2saldpen)], with a tetradentate [N2(imine)O2] and bidentate [N2(imine)] salen-type ligand, respectively, were prepared and characterised by 1H NMR, IR and Raman spectroscopy. The complexes were examined as catalysts for the epoxidation of cyclooctene, (R)-(+)-limonene, styrene, α-pinene, and cis and trans-β-methylstyrene, with tert-butyl hydroperoxide. Both complexes exhibit high selectivity for the epoxidation reaction, with the bis(chloro) complex being always the more active catalyst.Figure optionsDownload full-size imageDownload as PowerPoint slide