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

The monomeric cis-dioxomolybdenum(VI)complexes [MoO2(oep-saldpen)] and [MoO2Cl2(oep-H(2)saldpen)], with a tetradentate [N-2(imine)O-2] and bidentate [N-2(imine)] salen-type ligand functionalised with two pyrrole derivative pendant arms [oep-H(2)saldpen = 1,2-diphenylethylenebis(3-oxyethylpyrrole)salicylideneimine], were synthesised and characterised by H-1 NMR, IR and Raman spectroscopy. The solid-state structure of the free ligand oep-H(2)saldpen 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(H(2)salen)] as model compounds [H(2)salen = N,N '-ethylenebis(salicylideneimine)]. The oep-saldpen complexes were examined as catalysts for the epoxidation of cyclooctene, (R)(+)-limonene, styrene, alpha-pinene, and cis and trans-beta-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. (C) 2007 Elsevier B.V. All rights reserved.