Pyridoxal hydrazonato molybdenum(vi) complexes: assembly, structure and epoxidation (pre)catalyst testing under solvent-free conditions†
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)]2Mo6O19. 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, 1H, 13C 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)]2Mo6O19·2MeCN 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.