Dioxomolybdenum(VI) and dioxotungsten(VI) complexes chelated with the ONO tridentate hydrazone ligand: synthesis, structure and catalytic epoxidation activity

Synthesis of the dioxomolybdenum(vi) complexes [MoO2(L-3OMe)(EtOH)) (1), (MoO2(L-4OMe)(EtOH)] (2) and [MoO2(L-H)(EtOH)] (3) and dioxotungsten(VI) complexes [WO2(L-3OMe)(EtOH)] (4), [WO2(L-4OMe)(EtOH)] (5) and [WO2(L-H)](n) (6a) was carried out using [MO2(C5H7O2)(2)] = Mo or W) and the corresponding aroythydrazone ligand H2LR (3-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H2L3OMe), 4-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H2L4OMe), or salicylaldehyde 4-hydroxybenzhydrazone (H2LH) in ethanol. Compounds obtained upon heating of the mononuclear complexes in acetonitrile or dichtoromethane, [MO2(L-R)](n) (1a-6a) or [MoO2(L-3OMe)](2) (1b), respectively, were also investigated. Crystal and molecular structures of the mononuclear 1, 2 and 3, polynuclear 1a.MeCN and dinuclear 1b complexes were determined by the single crystal X-ray diffraction method. Powder X-ray diffraction showed isostructurality of 1 and 4, and 2 and 5. The complexes were further characterized by elemental analysis, IR spectroscopy, TO and DSC analyses, and one- and two-dimensional NMR spectroscopy. The catalytic performances of 1-5 and 6a were investigated for epoxidation of cyclooctene using aqueous tert-butyl hydroperoxide (TBHP) as the oxidant.