Pyridoxal hydrazonato molybdenum(vi) complexes: assembly, structure and epoxidation (pre)catalyst testing under solvent-free conditions

Abstract

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(L^1–3)(MeOH)], polynuclear [MoO₂(L^1–3)]_n and hybrid organic–inorganic compounds with the Lindqvist polyoxomolybdate [MoO₂(HL^1–3)]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(L^1–3)(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.