SYNTHESIS, CHARACTERIZATION AND CRYSTAL STRUCTURES OF OXOVANADIUM(V) COMPLEXES DERIVED FROM SIMILAR AROYLHYDRAZONE LIGANDS

Reaction of [VO(acac)2] (acac = acetylacetonate) with N’-(5-chloro-2-hydroxybenzylidene)-3methoxybenzohydrazide (H2L) and N’-(2-hydroxy-4-methoxybenzylidene)-4-nitrobenzohydrazide (H2L) in methanol affords methanol-coordinated mononuclear oxovanadium(V) complexes, [VOL(OMe)(MeOH)] (1) and [VOL(OMe)(MeOH)] (2), respectively. The complexes were characterized by elemental analysis, FT-IR, H NMR and C NMR spectra. Crystal and molecular structures of the complexes were determined by single crystal X-ray diffraction method. Single crystal X-ray structural studies indicate that the hydrazone ligands coordinate to the VO core through enolate oxygen, phenolate oxygen and azomethine nitrogen. The V atoms in the complexes are in octahedral coordination. Thermal stabilities of the complexes have also been studied.


Materials and measurements
Commercially available 5-chlorosalicylaldehyde, 4-methoxysalicylaldehyde, 3-methoxybenzohydrazide and 4-nitrobenzohydrazide were purchased from Aldrich and used without further purification.[VO(acac) 2 ] was purchased from Shenyang Haizhongtian Chemical Company (China).Other solvents and reagents were made in China and used as obtained.C, H and N elemental analyses were performed with a Perkin-Elmer elemental analyser.Infrared spectra were recorded on a Nicolet AVATAR 360 spectrometer as KBr pellets in the 4000-400 cm -1 region.Thermal stability analysis was performed on a Perkin-Elmer Pyris Diamond TG-DTA thermal analyses system. 1 H NMR and 13 C NMR spectra were recorded on a Bruker DPX 300 model spectrometer at 25 °C with TMS and solvent signals allotted as internal standards.Chemical shifts were reported in ppm (δ).

X-Ray diffraction
Diffraction intensities for the complexes were collected at 298(2) K using a Bruker D8 Venture Photon diffractometer with MoK radiation ( = 0.71073 Å).The collected data were reduced using SAINT [23], and multi-scan absorption corrections were performed using SADABS [24].Structures of the complexes were solved by direct method and refined against F 2 by full-matrix least-squares methods using SHELXTL [25].All the non-hydrogen atoms were refined anisotropically.The methanol H atoms in both complexes were located from difference Fourier maps and refined isotropically, with O-H distance restrained to 0.85(1) Å.The remaining H atoms were placed in idealized positions and constrained to ride on their parent atoms.The crystallographic data for the complexes are summarized in Table 1.Selected bond lengths and angles are given in Table 2.

RESULTS AND DISCUSSION General
Replacement of two acetylacetonate ligands in [VO(acac) 2 ] by aroylhydrazone, methanol and methanolate ligands resulted in the mononuclear oxovanadium(V) complexes.The dinegative ligands are coordinated to the VO cores via the phenolate-oxygen, imino-nitrogen and enolateoxygen.The complexes are soluble in methanol, ethanol and acetonitrile.Molar conductance of the complexes at concentration of 10 -4 M is about 30 Ω -1 cm 2 mol -1 , indicating they are nonelectrolyte [26].

IR spectra
The aroylhydrazone ligands showed stretching bands attributed to C=O, C=N, C-OH and NH at about 1650, 1605, 1160 and 1230, and 3220 cm -1 , respectively.The complexes exhibit two bands at about 960 cm -1 , assigned to stretching vibrations of the V=O groups [27].The bands due to ν C=O are absent in the complexes, but new C-O stretch appeared at about 1245 cm -1 .This suggests occurrence of keto-imine tautomerization of the aroylhydrazone ligands during complexation.The ν C=N absorption observed at 1605 cm -1 in the free aroylhydrazone ligands shifted to 1600 cm -1 upon coordination to the V atoms [28].

Thermal property
Differential thermal (DT) and thermal gravimetric analyses (TGA) were conducted to examine the stability of the complexes (Figures 1 and 2).For the decomposition of complex (1), the first step started from 90 °C and ended at 110 °C, corresponding to the loss of the ligand (CH 3 OH).The observed weight loss of 7.2% is close to the calculated value of 7.4%.The second step, from 150 °C to 210 °C, corresponds to the loss of the methanolate ligand (CH 3 O -).The observed weight loss of 7.1% is close to the calculated value of 7.2%.The last step, from 250 °C to 510 °C, corresponds to the loss of the aroylhydrazone ligand (C 15 H 11 ClN 2 O 3 2-), and the formation of the final product (V 2 O 5 ).The total weight loss of 79.3% is close to the calculated value of 79.0%.For the decomposition of complex (2), there are several stages of decomposition, and it is very difficult to ascribe them.The complex decomposed from 100 °C and completed at 500 °C.The total weight loss of 79.5% is the same as the calculated value of 79.5%.

Structure description of the complexes
Molecular structures and atom numbering schemes of the complexes are shown in Figures 3-6

CONCLUSION
In summary, two new methanol and methanolate-coordinated mononuclear oxovanadium(V) complexes derived from similar aroylhydrazone ligands have been prepared and structurally characterized by single crystal X-ray diffraction method, as well as elemental analysis and FT-IR spectroscopic technique.The aroylhydrazone ligands coordinate to the VO core through the enolate oxygen, phenolate oxygen and azomethine nitrogen.Thermal stability of the complexes have also been studied.

H 2 L
1 and H 2 L 2

Figure 4 .
Figure 4. Molecular packing of complex (1).Hydrogen bonds are drawn as dashed lines.Hydrogen atoms not related to hydrogen bonds are omitted for clarity.

Figure 6 .
Figure 6.Molecular packing of complex (2).Hydrogen bonds are drawn as dashed lines.Hydrogen atoms not related to hydrogen bonds are omitted for clarity.
min at room temperature to give colorless solution.The solution was left still in air to slow evaporate of most of the solvent, to give crystalline product of H 2 L 2 .
a R1 = Fo-Fc/Fo, wR2 = [w( . Both structures are very similar except for the aroylhydrazone ligands.The V atoms in the complexes are in octahedral coordination, with the phenolate O, azomethine N and enolate O atoms of the aroylhydrazone ligands as well as the methanolate O atom defining the equatorial plane, and with the methanol O atom and an oxo O atom locating at the axial positions.