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 (H sub(2)L super(1) = pyridoxal isonicotinic acid hydrazone, H sub(2)L super(2) = pyridoxal benzhydrazone, H sub(2)L super(3) = pyridoxal 4-hydroxy benzhydrazone) and [MoO sub(2)(acac) sub(2)] under appropriate conditions. The complexes can be classified into three categories: mononuclear [MoO sub(2)(L super(1-3) )(MeOH)], polynuclear [MoO sub(2)(L super(1-3) )] sub()nand hybrid organic-inorganic compounds with the Lindqvist polyoxomolybdate [MoO sub(2)(HL super(1-3))] sub(2)Mo sub(6)O sub(19). 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, super(1)H, super(13)C NMR) techniques. The crystal and molecular structure of the pyridoxal benzhydrazone H sub(2)L super(2) , three mononuclear complexes [MoO sub(2)(L super(1-3) )(MeOH)], and the Lindqvist-containing compounds [MoO sub(2)(HL super(2))] sub(2)Mo sub(6)O sub(19).2MeCN and (H sub(4)L super(1))Mo sub(6)O sub(19) 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% vs.substrate). The influence of the Linqvist anion on catalytic performance is discussed.