Reaction of VCl₃ with bis[N-(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) in air afforded [VOCl₂(dpp-bian)] (1). The complex was characterized by IR and UV-vis spectroscopies and elemental analysis. The crystal structure of 1 was determined by X-ray diffraction (XRD) analysis. The vanadium atom is in a square-pyramidal OCl₂N₄ coordination environment. The cyclic voltammogram (CV) in dichloromethane reveals an irreversible oxidation process at +1.40 V (vs. Ag/AgCl) assigned to the V(IV)/V(V) couple, and two consecutive quasi-reversible one-electron reduction processes at −0.32 V and −1.05 V (vs. Ag/AgCl), respectively, assigned to the bian/bian^−/˙ and bian^−/˙/bian²⁻ couples, followed by irreversible reduction at −1.6 V (vs. Ag/AgCl). The EPR spectrum of 1 in toluene shows a single 8-line signal typical for oxidovanadium(IV) complexes with d¹ configuration. The magnetic behavior of 1 confirms the presence of one unpaired electron (μ_eff (330 K) = 1.67 μ_B), and the isolation of the paramagnetic centers. Application of 1 to oxidation of alkanes documented high catalytic activity under mild conditions. The kinetics and selectivity of alkane oxygenation by the 1/H₂O₂ and 1/PCA/H₂O₂ systems (PCA is pyrazine-2-carboxylic acid) were studied. The reaction is more efficient in the presence of PCA.

A new monoiminoacenaphthenone 3,5-(CF₃)₂C₆H₃-mian (complex 2) was synthesized and further exploited, along with the already known monoiminoacenaphthenone dpp-mian, to obtain oxidovanadium(IV) complexes [VOCl₂(dpp-mian)(CH₃CN)] (3) and [VOCl(3,5-(CF₃)₂C₆H₃-bian)(H₂O)][VOCl₃(3,5-(CF₃)₂C₆H₃-bian)]·2.85DME (4) from [VOCl₂(CH₃CN)₂(H₂O)] (1) or [VCl₃(THF)₃]. The structure of all compounds was determined using X-ray structural analysis. The vanadium atom in these structures has an octahedral coordination environment. Complex 4 has an unexpected structure. Firstly, it contains 3,5-(CF₃)₂C₆H₃-bian instead of 3,5-(CF₃)₂C₆H₃-mian. Secondly, it has a binuclear structure, in contrast to 3, in which two oxovanadium parts are linked to each other through V=O···V interaction. This interaction is non-covalent in origin, according to DFT calculations. In structures 2 and 3, non-covalent π-π staking interactions between acenaphthene moieties of the neighboring molecules (distances are 3.36–3.40 Å) with an estimated energy of 3 kcal/mol were also found. The redox properties of the obtained compounds were studied using cyclic voltammetry in solution. In all cases, the reduction processes initiated by the redox-active nature of the mian or bian ligand were identified. The paramagnetic nature of complexes 3 and 4 has been proven by EPR spectroscopy. Complexes 3 and 4 exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The yields of products of cyclohexane oxidation were 43% (complex 3) and 27% (complex 4). Based on the data regarding the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play the most crucial role in the reaction. The initial products in the reactions with alkanes are alkyl hydroperoxides, which are easily reduced to their corresponding alcohols by the action of triphenylphosphine (PPh₃). According to the DFT calculations, the difference in the catalytic activity of 3 and 4 is most likely associated with a different mechanism for the generation of ^●OH radicals. For complex 4 with electron-withdrawing CF₃ substituents at the diimine ligand, an alternative mechanism, different from Fenton’s and involving a redox-active ligand, is assumed.

Published data on the coordination compounds of group 5 metals with diimine type ligands are surveyed. Methods of synthesis, structural types, reactivity and properties of these compounds are summarized and systematized. Particular attention is paid to the redox, magnetic and catalytic properties and to biological activities of these complexes, which is important for understanding the areas of their potential application. The bibliography includes 177 references.

Reactions of [VCl₃(thf)₃] or VBr₃ with 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) in a 1:1 molar ratio in air under solventothermal conditions has afforded polymeric oxidovanadium(IV) four complexes 1–4 of a general formula [VO(L)X₂]_n (L = bpy, phen and X = Cl, Br). Monomeric complex [VO(DMF)(phen)Br₂] (4a) has been obtained by the treatment of compound 4 with DMF. The complexes were characterized by IR spectroscopy and elemental analysis. The crystal structures of 3 and 4a were determined by an X-ray diffraction (XRD) analysis. The {VOBr₂(bpy)} fragments in 3 form infinite chains due to the V = O…V interactions. The vanadium atom has a distorted octahedral coordination environment. Complexes 1–4 have been tested as catalysts in the homogeneous oxidation of alkanes (to produce corresponding alkyl hydroperoxides which can be easily reduced to alcohols by PPh₃) and alcohols (to corresponding ketones) with H₂O₂ or tert-butyl hydroperoxide in MeCN. Compound 1 exhibited the highest activity. The mechanism of alkane oxidation was established using experimental selectivity and kinetic data and theoretical DFT calculations. The mechanism is of the Fenton type involving the generation of HO^• radicals.

Two new isomeric complexes [CuBr₂(R-bian)] (R = 4-Me-Ph (1), 2-Me-Ph (2)) were obtained by reacting copper(II) bromide with 1,2-bis[(2-methylphenyl)imino]acenaphthene ligands and characterized. The crystal structure of 2 was determined by X-ray diffraction analysis. The copper atom has a distorted square-planar environment; the ω angle between the CuN₂ and CuBr₂ planes is 37.004°. The calculated ω parameters for optimized structures 1 and 2 were 76.002° and 43.949°, indicating significant deviations from the ideal tetrahedral and square-plane geometries, respectively. Molecules 2 form dimers due to non-covalent Cu···Br contacts, which were analyzed by DFT calculations. The complexes were also characterized by cyclic voltammetry and UV-Vis spectroscopy. A quasi-reversible Cu(II)/Cu(I) redox event with E_1/2 potentials of 0.81 and 0.66 V (vs. SHE) was found for 1 and 2, respectively. The electronic absorption spectra showed the presence of Cu(I) species as a result of the partial reduction of the complexes in the acetonitrile solution. Both complexes were tested as homogenous catalysts for the oxidation of isopropylbenzene (IPB) in acetonitrile at low temperatures. Differences in the mechanism of the catalytic reaction and the composition of the reaction products depending on the oxidizing ability of the catalyst were revealed.