Oxovanadium(IV) complexes of bioinorganic and medicinal relevance: Synthesis, characterization and 3D molecular modeling of some oxovanadium(IV) complexes involving O, N-donor environment of salicylaldehyde-based sulfa drug Schiff bases

2.1 Materials used

Vanadyl sulfate pentahydrate (Thomas Baker Ltd., Mumbai), salicylaldehyde (E. Merck, Germany), sulfa drugs (sulfadiazine, sulfaguanidine, sulfanilamide, sulfamerazine, (Sigma Chemicals Co., USA), daphsone (Fluka Chemie A.G., Switzerland), ethanol (Bengal Chemicals and Pharmaceuticals Ltd., Kolkata), were used as supplied. All other chemicals used were of analytical reagent grade.

2.2 Synthesis of Schiff bases

The Schiff bases of sulfa drugs were prepared as follows: An ethanolic solution (10 mL) of salicylaldehyde (0.244 g, 0.209 mL, 2 mmol) was added to the ethanolic solution (∼20 mL) of sulfadiazine (0.500 g, 2 mmol), sulfanilamide (0.344 g, 2 mmol), sulfaguanidine (0.428 g, 2 mmol), sulfamerizine (0.528 g, 2 mmol) or 4,4′-diaminodiphenylsulfone (0.248 g, 1 mmol). A colored solid mass separated out when the mixture was heated at 80 °C for ∼20 min. It was filtered, washed several times with ethanol and diethyl ether and subsequently, dried over anhydrous CaCl₂ in a desiccator. The characterization data of Schiff bases are given in Table 1.

2.3 Synthesis of complexes

The following general procedure was used in the synthesis of all the complexes: A suspension of the Schiff base ligand sal-sdzH (0.708 g, 2 mmol), sal-sgnH (0.636 g, 2 mmol), sal-snmH (0.552 g, 2 mmol), sal-smrH (0.736 g, 2 mmol), and sal-dadpsH₂ (0.456 g, 1 mmol) in the minimum quantity of DMF was mixed with ethanol (∼20 mL). The resulting mixture was refluxed with stirring on a magnetic stirrer equipped with heater for 1 h to get a clear solution. The salt of VOSO₄·5H₂O (0.253 g, 1 mmol) was dissolved in ethanol-water (4:1, 5 mL) and the solution so obtained was added to a hot, stirred ethanolic solution of the corresponding Schiff bases. The resulting solution was refluxed for 10–12 h, and then concentrated to half of its volume. The resulting colored precipitate was then filtered and washed several times with ethanol to remove any unreacted ligand and the metal salt. The product was dried in vacuo. The analytical data of the complexes are given in Table 2.

2.4 Analyses

Carbon, hydrogen and nitrogen were determined micro-analytically at SAIF, Indian Institute of Technology, Mumbai. The vanadium content of each of the complexes was determined as follows. A 100 mg of sample of the compound was placed in a silica crucible, decomposed by gentle heating and then adding 1–2 mL of concentrated HNO₃, 2–3 times. An orangish mass (V₂O₅) was obtained after decomposing and complete drying. It was dissolved in the minimum amount of dilute H₂SO₄, and the solution so obtained was diluted with distilled water to 100 mL in a measuring flask. The vanadium content of each of the complexes was determined volumetrically using decinormal KMnO₄ solution as an oxidizing agent in the presence of sulfurous acid. The amount of vanadium in the sample solution was calculated using the standard (Furman, 1962) relationship: 1 mL of 0.1 N KMnO₄ = 5.094 mg vanadium.

2.5 Physical methods

The following physical methods were used in the present investigation. The solid-state infrared spectra were obtained using KBr pellets with a Perkin–Elmer model 1620 FT-IR spectrophotometer at the Central Drug Research Institute, Lucknow. Thermogravimetry of the complexes was performed on a Perkin–Elmer Thermoanalyser at S. A. I. F., Indian Institute of Technology, Mumbai. Electronic spectra were recorded on an ATI Unicam UV-1-100 UV/Visible Spectrophotometer in our laboratory. Conductance measurements were made in DMF solution using a Toshniwal conductivity bridge and dip-type cell with a smooth platinum electrode of cell constant 1.02. Magnetic measurements were done by a vibrating sample magnetometer at the Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Chennai. The decomposition temperatures of the complexes were recorded by an electrically operated melting point apparatus (Kumar Industries, Mumbai) of heating capacity up to 360 °C. The X-Band and EPR spectra of the complexes were measured on a Bruker ESP X-Band EPR spectrometer using powdered samples at the Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Chennai. The FAB mass spectrum of a representative complex was recorded on a JEOL SX 102/DA-6000 Mass Spectrometer/Data system using Argon/Xenon (6KV, 10 mA) as the FAB gas. The accelerating voltage was 10 KV and the spectrum was recorded at room temperature and m-nitrobenzyl alcohol (NBA) was used as the matrix.

2.6 3D Molecular modeling studies

The 3D molecular modeling of one of the synthesized compound was carried out on CS Chem 3D Ultra Molecular Modeling and Analysis Program (http://www.cambridgesoft.com). It is an interactive graphics program that allows rapid structure building, geometry optimization and molecular display. It has the ability to handle transition metal compounds.

3.1 Infrared spectral studies

All the ligands except (sal-dadpsH₂ and sal-snmH) contain five donor sites: (i) the phenolic oxygen, (ii) the azomethine nitrogen, (iii) sulfonamide oxygens, (iv) sulfonamide nitrogen, (v) ring nitrogens in the case of sal-sdzH and sal-smrH or amino nitrogen in the case of sal-sgnH. The ligand, sal-snmH contains only four potential donor sites, (i)–(iv) given above. The remaining ligand, sal-dadpsH₂ possesses five potential donor sites (i) two azomethine nitrogens, (ii) two phenolic oxygens, (iii) sulfonamide oxygens. All the Schiff base ligands show a sharp and strong band due to ν(C⚌N) of the azomethine group at 1615–1620 cm⁻¹. The observed low energy shift (Maurya and Rajput, 2006) of this band in the complexes appearing at 1580–1600 cm⁻¹, suggests the coordination of azomethine nitrogen to the metal center. This is further supported by the appearance of a new band at 450–470 cm⁻¹ due to the ν(V–N) band (Maurya et al., 2015a,b,c).

The ν(NH) mode of the sulfonamide group/amino group in the uncoordinated Schiff bases remains unchanged in the spectra of their complexes (see Tables 3 and 4). This suggests that the sulfonamide nitrogen or amino group is not taking part in coordination. The bands, in these ligands, due to ν_as(SO₂) and ν_s(SO₂) appear at 1300–1340 and 1150–1170 cm⁻¹, respectively. These remain almost unchanged in the spectra of complexes, indicating that sulfonamide oxygens are not participating in coordination. This is consistent with our previous observation (Maurya et al., 2003a) in sulfa drug based Schiff bases.

The characteristic phenolic ν(OH) mode in all the ligands due to the presence of a hydroxyl group at the ortho position, was observed at 3340–3480 cm⁻¹. A medium band at 1480–1490 cm⁻¹ due to ν(C–O) of the phenolic group was also observed in these ligands. The band due to the phenolic (OH) group of these ligands should be absent in all of the complexes under study to indicate the coordination of the phenolic oxygen, after deprotonation, to the metal ion. But due to the presence of a broad band (vide infra) for lattice and coordinated water, it is difficult to confirm with certainty the coordination of the phenolic oxygen after deprotonation. However, such coordination is supported by the observed shift of the phenolic ν(C–O) band to a lower wave number (Maurya et al., 2008a,b) at 1440–1450 cm⁻¹ in the complexes. The coordination of phenolic oxygen is further supported by the appearance of a non ligand band at 550–580 cm⁻¹, due to ν(V–O) (Maurya et al., 2003b) in the complexes. The band due to ν(C⚌N) ring appearing at 1580 and 1588 cm⁻¹ in the ligands, sal-sdzH and sal-smrH remains unchanged in the respective complexes. This suggests that ring nitrogens of the ligand are not taking part in coordination. This is logical in terms of six-member chelate ring formation including metal on account of the bidentate coordination of azomethine nitrogen and phenyl oxygen from the ortho position.

The presence of lattice/coordinated water (Maurya et al., 2003a,b,c,d) in the complexes is revealed by the presence of either a broad band at 3400 cm⁻¹ or two weak bands at 3400–3500 and 3350–3380 cm⁻¹.

Most of the oxovanadium(IV) complexes display a strong band near 1000 cm⁻¹ assignable to ν(V⚌O) (Selbin, 1966). Contrary to this, several oxovanadium(IV) complexes have been reported in which this stretching mode appears at quite lower (Maurya and Rajput, 2006; Maurya et al., 2001) wave numbers around 900 cm⁻¹ due to the presence of a ⋯V⚌O⋯V⚌O⋯ chain structure, which is formed by the interaction of vanadyl oxygen of one molecule with a vanadium metal in another molecule (Boas and Pessoa, 1987). In the complexes presented here, ν(V⚌O) is found at 930–980 cm⁻¹. This suggests the absence of a ⋯V⚌O⋯V⚌O⋯ chain. The presence of a coordinated water molecule in this complex (discussed above) supports this fact. The IR spectra of the Schiff base ligand (sal-snmH, III) and its complex, [VO(sal-snm)(H₂O)]·H₂O (3) are given in Figs. 1 and 2, respectively.

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Figure 1

IR spectrum of sal-snmH.

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Figure 2

IR spectrum of [VO(sal-snm)(H₂O)]·H₂O (3).

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3.2 Thermogravimetric studies

Thermogravimetric analysis of two representative compounds, namely [VO(sal-sdz)₂(H₂O)]·H₂O (1) and [VO(sal-dadps)(H₂O)]₂·2H₂O (5) were recorded in the temperature range from ambient to 1000 °C at the heating rate of 15 °C/min. The compound (1) exhibits a weight loss of 4.66% at approximately 277.00 °C (calcd. wt. loss for two moles of H₂O, 4.45%) corresponding to the removal of two molecules of water. As the compound starts loosing weight right from about 50.00 °C, one water molecule may be taken as lattice water and the other as the coordinated water. The second weight loss observed at 490.00 °C is found to be 51.48% against a calculated weight loss of 48.08% corresponding to the elimination of one ligand moiety. The final weight loss at approximately 573.13 °C has been found to be 90.54% against a calculated weight loss of 91.72% involving the removal of another ligand moiety. The final residue attaining a constant weight over 600.00 °C (obsd. = 9.46%) corresponds to V₂O₅ (calcd. = 10.25%) (Maurya et al., 2015a,b,c). This weight loss data, thus, agree well with the IR results for this complex. The thermogram of compound, [VO(sal-sdz)₂(H₂O)]·H₂O (1) is given in Fig. 3.

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Figure 3

TG curve of [VO(sal-sdz)(H₂O)]·H₂O (1).

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The compound (5) shows a weight loss of 6.43% at 337.00 °C (calcd. wt. loss for four moles of water, 6.46%) corresponding to the removal of four water molecules similar to compound (1). It also starts loosing weight right from 50.00 °C. This suggests that two water molecules are lattice water and other two are coordinated. Taking the dimeric structure of the compound into account (vide infra), this seems to be logical. Another weight loss of 45.42% by this compound was noticed around 550.00 °C (calculated wt. loss of one ligand moiety, 47.22%), suggesting the removal of one ligand moiety from the complex. The final weight loss of 85.97% observed at ∼700.00 °C (calcd. wt. loss for another ligand moiety, 87.98%), indicates the elimination of second ligand moiety from the complex. The final residue attaining a constant weight over 750 °C corresponds to (obsd. = 14.03%) V₂O₅ (calculated = 14.89%) (Maurya et al., 2015a,b,c). The thermogram of compound, [VO(sal-dadps)(H₂O)]₂·2H₂O (5) is given in Fig. 4.

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Figure 4

TG curve of [VO(sal-dadps)(H₂O)]₂·2H₂O (5).

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3.3 ESR spectral studies

The liquid nitrogen temperature (LNT) X-Band ESR spectra of two representative compounds namely [VO(sal-sdz)₂(H₂O)]·H₂O (1) and [VO(sal-dadps)(H₂O)]₂·2H₂O (5) were recorded in powder form using TCNE (tetracynoethylene) as a marker. Both g‖ and g⊥ components were resolved in compound (1). The observed spectral parameters for this compound are: g⊥ = 1.976, g‖ = 1.966, g_av = 1.969, A⊥ = 80 G, A‖ = 70 G and A_av = 76.66. The deviation of g‖, g⊥ and g_av values in the complex from the free ion value of 2.0027 suggests that the resulting complex is covalent (Dutta and Syamal, 1993) in nature. Furthermore, the feature of the spectrum of the complex is indicative of its monomeric structure (Maurya et al., 2002a,b).

The ESR spectrum of compound (5) does not exhibit usual shape of a paramagnetic sample. Instead, it shows a zigzag straight-line curve. This is most probably due to the partial pairing of two unpaired electron spins on two oxovanadium centers because of dimer formation involving ligand bridging. The ESR spectra of compound (1) and (5) are given in Figs. 5 and 6, respectively.

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Figure 5

ESR spectrum of [VO(sal-sdz)(H₂O)]·H₂O (1).

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Figure 6

ESR spectrum of [VO(sal-dadps)(H₂O)]₂·2H₂O (5).

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3.4 Mass spectral studies

In the FAB mass spectrum (Fig. 7) of a representative complex [VO(sal-sdz)₂(H₂O)]·H₂O the matrix peaks were supposed to appear at m/z 136, 137, 154, 289, 307 in the absence of any metal ion. If metal ions are present, these peaks may be shifted accordingly. Besides matrix peaks at 136, 137, 154, 289 and 307 m/z, and other spectral peaks were observed at 381, 419, 460, 613, 773 m/z in the compound in question which are most probably due to following types of ion associations:

1. [Molecular ion]⁺ (772.94) − ^*[C₄N₃H₄SO₂]⁺ (157) − ^**[C₁₀N₃H₉SO₂]⁺ (235) = 380.94 (∼381).

2. [sal-sdz]⁺ (353) + [V⚌O]⁺ (66.94) − H⁺ = 418.94 (∼419).

3. [Molecular ion]⁺ (772.94) − ^*[C₄N₃H₄SO₂]⁺ (157) −^**[C₄N₃H₄SO₂]⁺ (157) = 458.94 ∼ 460.

4. [Molecular ion]⁺ (772.94) − ^*[C₄N₃H₄SO₂]⁺ (157) − 3H⁺ = 612.94 ∼ 613.

5. [Molecular ion]⁺ (772.94) = 772.94 ∼ 773.

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Figure 7

Mass spectrum of [VO(sal-sdz)(H₂O)]·H₂O (1).

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^∗ = From one diazine moiety; ^∗∗ = from another diazine moiety.

From the above fragmentation patterns it appears that the mass spectral data are consistent with proposed formulation of the complex in question.

3.5 Electronic spectral studies

The electronic spectra of the complexes were recorded in 10⁻³ M dimethylformamide solutions in the range of 280–800 nm solutions, and the spectral data are given in Table 5. Besides high intensity charge transfer transitions, compound (1), (3), (4) and (5) displayed one low intensity d–d transition assignable to $b_2\to {}^1{a}_1^{\ast }$ while compound (2) exhibited two low intensity d–d transitions at 339 and 426 nm assignable to $b_2\to {}^1{a}_1^{\ast }$ and $b^2\to b_1^{\ast }$ , respectively (Maurya et al., 1997). The assignment of $b_2\to {}^1{a}_1^{\ast }$ transition in each case assumes idealized C₂v symmetry. These spectra are typical of oxovanadium(IV) complexes (Maurya and Rajput, 2004; Dutta and Syamal, 1993). The electronic spectrum of compound, [VO(sal-sdz)₂(H₂O)]·H₂O (1) is shown in Fig. 8.

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Figure 8

Electronic spectrum of [VO(sal-sdz)(H₂O)]·H₂O (1).

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3.6 Magnetic studies

The observed magnetic moments of the compounds (1)–(4) at room temperature are in the range 1.68–1.74 B.M., expected for monomeric oxovanadium(IV) complexes. A low magnetic moment value of 1.61 B.M. of compound (5) suggests spin quenching most probably due to ligand bridged dimeric structure (Maurya et al., 2008b) for this complex. This is consistent with the zigzag straight line ESR spectrum of this compound (vide supra).

3.7 Conductance measurements

The observed molar conductance (14.1–24.3 ohm⁻¹ cm² mol⁻¹) in 10⁻³ molar DMF solutions of these complexes are given in Table 2, and are consistent with the non-electrolytic (Geary, 1971) nature of these complexes. Such a non-zero molar conductance value for each complex in the present study is most probably due to the strong donor capacity of DMF, which may lead to the displacement of anionic ligand and change of electrolyte type.