Vanadium(V) Complexes with Substituted 1,5-bis(2-hydroxybenzaldehyde)carbohydrazones and Their Use As Catalyst Precursors in Oxidation of Cyclohexane

Abstract

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.

Transition metal complexes of a versatile polyalkoxy oxazolidine-based ligand derived fromin situcyclization

“…[5][6][7][8][9][10][11] One of the popular ways to design new polyalkoxy ligands is through Schiff-base condensation between hydroxyaldehydes and aminoalcohols. [12][13][14][15][16][17][18][19][20][21][22][23] In line with our interest in designing new polynuclear coordination complexes for studying magnetic and biomimetic properties, we coupled 8-hydroxyquinoline and tris(hydroxymethyl)aminomethane to design the desired Schiff-base ligand H 4 L 1 (Scheme 1). However, the reaction between 8-hydroxyquinoline-2-carboxaldehyde (HQC) and tris(hydroxymethyl)aminomethane (TRIS) yielded an oxazolidine derivative by in situ cyclization, and produces H 3 L 2 during the synthesis.…”

Liquid‐phase oxidation of olefins with rare hydronium ion salt of dinuclear dioxido‐vanadium(V) complexes and comparative catalytic studies with analogous copper complexes

“…Herein (1) can be found in earlier literature. [13] However, here, we were able to crystallize complex 1 in the form of its hydronium ion salt with 2 eq. of DMF as the solvent of crystallization.…”

Copper(II) Complexes of Arylhydrazone of 1H-Indene-1,3(2H)-dione as Catalysts for the Oxidation of Cyclohexane in Ionic Liquids

“…When using H 2 O 2 as oxidant, a higher selectivity towards the alcohol is observed, while with TBHP, the amount of this product is lower than that of the ketone ( [13][14][15][16].…”