No abstract

Review: 330 refs.

Six dinuclear vanadium(V) complexes have been synthesized: NH4[(VO2)2((H)LH)] (NH4[1]), NH4[(VO2)2((t-Bu)LH)] (NH4[2]), NH4[(VO2)2((Cl)LH)] (NH4[3]), [(VO2)(VO)((H)LH)(CH3O)] (4), [(VO2)(VO)((t-Bu)LH)(C2H5O)] (5), and [(VO2)(VO)((Cl)LH)(CH3O)(CH3OH/H2O)] (6) (where (H)LH4 = 1,5-bis(2-hydroxybenzaldehyde)carbohydrazone, (t-Bu)LH4 = 1,5-bis(3,5-di-tert-butyl-2-hydroxybenzaldehyde)carbohydrazone, and (Cl)LH4 = 1,5-bis(3,5-dichloro-2-hydroxybenzaldehyde)carbohydrazone). The structures of NH4[1] and 4-6 have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit -N-N-. In two pockets of NH4[1], two identical VO2(+) entities are present, whereas, in those of 4-6, two different VO2(+) and VO(3+) are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO(3+) in 4-6, charge density measurements on 4, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH4[1], NH4[2], and 4-6 were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent-free and additive-free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst.

Abstract:A series of first-row-transition-metal ferrite magnetic nanoparticles (NPs) MFe 2 O 4 [M = Mn 2+ (1), Fe 2+ (2), Co 2+ (3), Ni 2+ (4), Cu 2+ (5) or Zn 2+ (6)] were prepared by the co-precipitation method and characterized by Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscope -energy dispersive X-ray spectrometry (SEM-EDS), vibrating sample magnetometer (VSM) and X-ray photoelectron spectroscopy (XPS). Those NPs were used as catalysts for the microwave-assisted oxidation of various alcohols in solvent-free medium. MnFe 2 O 4 (1), CoFe 2 O 4 (3) and CuFe 2 O 4 (5) act as catalysts for the conversion of alcohols to the corresponding ketones or aldehydes with a yield range of 81 to 94% in 2 h at 120 • C using t-BuOOH as an oxidant. These catalysts can be readily isolated by using an external magnet and no significant loss of activity is observed when reused up to 10 consecutive runs. The effects of some parameters, such as temperature, time, type of oxidant and presence of organic radicals, on the oxidation reactions were also investigated. The presented literature overview highlights the advantages of our new 1-6 NPs catalytic systems in terms of efficiency and economy, mainly due the used microwave (MW) heating mode.

Cooperative coordination and ionic interactions assisted E/Z isomerization of arylhydrazone ligands lead to a variety of CuII complexes, which effectively catalyse the homogeneous oxidation of alcohols to carbonyl compounds.