The Effect of Charged Axial Ligands on the EPR Parameters in Oxovanadium(IV) Compounds: An Unusual Reduction of the Az(51V) Values

Abstract

Two series of octahedral oxovanadium(IV) compounds, containing charged or neutral axial ligands, with the tetradentate amidate molecules Hcapca and H2capcah of the general formulae trans-[V(IV)OX(capca)]0/+ (where X = Cl- (1.CH2Cl2), SCN- (2), N3 (3), CH3COO- (4), PhCOO- (5), imidazole (6. CH3NO2), and eta-nBuNH2 (7)) and cis-[V(VI)OX(Hcapcah)]0/+ (where X = Cl- (8.0.5CH2Cl2), SCN (9), N3 (10.2CH3OH), and imidazole (11)), were synthesized and characterized by X-ray crystallography (1.CH3OH,8.CHCl3, 9.2CH3CN, 10.CH3CN and cis-[VO(imidazole)(Hcapcah)+) and continuous-wave electron paramagnetic resonance (cw EPR) spectroscopy. In addition to the synthesis, crystallographic and EPR studies, the optical, infrared and magnetic properties (room temperature) of these compounds are reported. Ab initio calculations were also carried out on compound 8 CHCl3 and revealed that this isomer is more stable than the trans isomer, in good agreement with the experimental data. The cw EPR studies of compounds 1-5, that is, the V(IV)O2+ species containing monoanionic axial ligands, revealed a novel phenomenon of the reduction of their A, components by about 10% relative to the N4 reference compounds ([V(IV)O-(imidazole)4]2+ and [V(IV)O(2,2-bipyridine)2]2+). In marked contrast, such a reduction is not observed in compounds 6. CH3NO2-11, which contain neutral axial ligands. Based on the spin-Hamiltonian formalism a theoretical explanation is put forward according to which the observed reduction of Az is due to a reduction of the electron - nuclear dipolar coupling (P). The present findings bear strong relevance to cw EPR studies of oxovanadium(IV) in vanadoproteins, V(IV)O2+-substituted proteins, and in V(IV)O2+ model compounds, since the hyperfine coupling constant, Az, has been extensively used as a benchmark for identification of equatorial-donor-atom sets in oxovanadium(IV) complexes.

Synthesis, characterization, antibacterial activity, and interaction with DNA of the vanadyl-enrofloxacin complex

“…In all these models the water molecule (O w ) occupies a cis-position with respect to the V@O (O v ), in accordance with the experimental results and just as in similar vanadyl complexes. [19][20][21][22][23][24] The resulted models of all VO(erx) 2 (H 2 O) isomers exhibit comparable average total energy with the two transOc enantiomers (as are arbitrarily designated in Scheme S1) being more stable by $1 kcal mol À1 (Table S1). After application of the distance restraints to M-L bonds (Table S2) and their geometry correction by minimizing the energy of the complex, the structure of the predicted most stable isomer is obtained (Fig.…”

“…21 These bands are typical for distorted octahedral VO 2+ complexes. 19,22 Although diverse crystallization techniques were employed, we did not manage to obtain a crystal of the complex suitable for the structure determination with X-ray crystallography. In order to present a model structure for the complex, eight diastereoisomers of VO(erx) 2 (H 2 O) complex have been constructed (Scheme S1).…”

“…18 The existence and the number of the m(V@O) absorptions is a useful diagnostic tool for the characterization of the complex. The appearance of the V@O stretching frequency in complex 1 at 927 cm À1 implies that a monoanionic ligand lies in 19 This acceptance is indicative of the arrangement of one O carb or O pyr atom in the axial position of the octahedron around V atom and excludes the O w (O water ) atom, which consequently lies in cis-position to the O v .…”

Mononuclear metal complexes with Piroxicam: Synthesis, structure and biological activity

“…Nevertheless, due to the lack of further experimental data regarding the appropriate contribution of O amide and the estimated accuracy of ±3.0 · 10 À4 cm À1 of the ''additivity rule'' some doubts remain as to the reliability of this approach in the present case. Alternatively, one might consider the correlations between g i and A i in order to gain some insights regarding the nature of the equatorial ligands [114,120,123]. By making use of such correlation plots we observe that in the present case the experimentally deduced pair of values (g i $ 1.94, and A i $ 172 · 10 À4 cm À1 ) falls better to the region where an oxygen ''rich'' equatorial plane is favored.…”

“…d z 2 transition although it usually lies under the much stronger ligand-to-metal charge-transfer transition at k = 420 nm (e = 180 M À1 cm À1 ) and can be difficulty distinguished [112,113]. These bands are typical for distorted octahedral VO 2+ complexes [93,114,115].…”