Dioxotungsten(VI) complexes with isoniazid-related hydrazones as (pre)catalysts for olefin epoxidation: solvent and ligand substituent effects
Résumé
The mononuclear dioxotungsten(VI) complexes [WO2(L-3OMe)(D)] (1a and 1b), [WO2(L-4OMe)(D)] (2a and 2b) and [WO2(L-H)(D)] (3a and 3b) (D = EtOH (1a-3a) or MeOH (1b-3b); L-3OMe = 3-methoxy-2-oxybenzaldehyde isonicotinoyl hydrazonato, L-4OMe = 4-methoxy-2-oxybenzaldehyde isonicotinoyl hydrazonato, L-H = 2-oxybenzaldehyde isonicotinoyl hydrazonato) were synthesized by the reaction of [WO2(acac)(2)]center dot 0.5C(6)H(5)Me with the respective isoniazid-related hydrazone. The compounds were characterized by microanalysis, FT-IR and NMR spectroscopy, thermogravimetric analysis, and powder X-ray diffraction method. The crystal and molecular structures of 1a, 1b, 3a and [WO2(acac)(2)]center dot 0.5C(6)H(5)Me were determined by single crystal X-ray diffraction. The structures of 1a, 1b, 3a are mononuclear and form hydrogen bonded centrosymmetric dimers. In all three complexes, the dimers are also held together by pi center dot center dot center dot pi interactions between aromatic rings. The catalytic performances (activity and selectivity) of 1a-3a and 1b-3b towards alkene epoxidation by tert-butyl hydroperoxide (TBHP) were investigated under different conditions.