Pyridoxal hydrazonato molybdenum(VI) complexes: assembly, structure and epoxidation (pre)catalyst testing under solvent-free conditions
Résumé
Pyridoxal hydrazonato molybdenum(VI) complexes were prepared by the reaction of the corresponding hydrazone (H2L1 = pyridoxal isonicotinic acid hydrazone, H2L2 = pyridoxal benzhydrazone, H2L3 = pyridoxal 4-hydroxy benzhydrazone) and [MoO2(acac)(2)] under appropriate conditions. The complexes can be classified into three categories: mononuclear [MoO2(L1-3)(MeOH)], polynuclear [MoO2(L1-3)](n) and hybrid organic-inorganic compounds with the Lindqvist polyoxomolybdate [MoO2(HL1-3)](2)Mo6O19. A unique example of a cationic polymer assembly with Lindqvist anions is reported herein for the first time. The compounds were characterised by elemental, TG and DSC analyses and by spectroscopic (IR, UV-Vis, H-1, C-13 NMR) techniques. The crystal and molecular structure of the pyridoxal benzhydrazone H2L2, three mononuclear complexes [MoO2(L1-3)(MeOH)], and the Lindqvist-containing compounds [MoO2(HL2)](2)Mo6O19 center dot 2MeCN and (H4L1)Mo6O19 were determined by single crystal X-ray diffraction. All complexes were tested as (pre)catalysts for the epoxidation of cyclooctene under solvent-free conditions with the use of aqueous TBHP (TBHP tert-butylhydroperoxyde) as an oxidant. Optimal results in terms of conversion, selectivity, TOF and TON were obtained at very low (pre)catalyst loadings (0.05% [Mo] vs. substrate). The influence of the Linqvist anion on catalytic performance is discussed.