Synthesis of Oxovanadium(IV) Complexes with Tetraaza Coordinating Ligands

Singh, Sanjay; Yadav, Hardeo S.; Yadava, Ashok Kumar; Rao, Devendra Pratap

doi:https://doi.org/10.1155/2013/947325

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Volume 2013 | Article ID 947325 | https://doi.org/10.1155/2013/947325

Synthesis of Oxovanadium(IV) Complexes with Tetraaza Coordinating Ligands

Sanjay Singh,¹Hardeo S. Yadav,²Ashok Kumar Yadava,²and Devendra Pratap Rao³

Academic Editor: Alexander Kornienko

Received18 Jun 2012

Accepted28 Aug 2012

Published08 Oct 2012

Abstract

Oxovanadium(IV) complexes of the type [VO(mac)]SO₄ (where mac = tetraaza macrocyclic ligands derived by condensation of thenil with 1,4-diaminobenzene or 3,4-diaminopyridine and their reaction with β-diketones) have been prepared using vanadyl ion as kinetic template. The prepared macrocyclic complexes were characterized by elemental analyses, molar conductance, magnetic moments, and infrared, electronic, and electron spin resonance data. From the spectroscopic studies, five-coordinate square-pyramidal geometry for the VO²⁺ complexes have been proposed wherein derived ligands act as tetradentate chelating agents.

1. Introduction

The chemistry of vanadium has generated great interest since the discovery of vanadium in organisms such as certain ascidians and Amanita mushrooms and being a constituent of the cofactors in vanadate-dependent haloperoxidases and vanadium nitrogenase [1–3]. The literature contains several reports about oxovanadium(IV) complexes which show modulating activities of various enzymes [4, 5]. Vanadium compounds with vanadyl ion having oxidation state +4 and +5 exist in the environment and in biological systems. These complexes also have biological activities such as antibacterial, antifungal, antiviral, and anticancer drugs [6–8]. Thenil is a versatile chelating agent having two reactive carbonyl groups capable of undergoing Schiff-base condensation with a variety of di- and polyamines. The use of metals as templates in such reaction has led to the synthesis of metal complexes of macrocyclic ligands [9]. Thus, thenil has played an important role in the development of macrocyclic complexes. Such complexes show unusual structure and stability and are known to have relevance to biological system. This provides an opportunity to design and study the model biological systems to understand the chemical changes taking place in such cases [10]. However, in most cases, the template effect of metal ions of the first transition series has been studied and the chemistry of metal complexes with macrocyclic ligands of oxovanadium(IV) incorporating four nitrogen donor atoms has received less attention[11, 12].

With this aspect, some oxovanadium(IV) complexes with new denticity ligands derived from condensation of thenil with 1,4-diaminobenzene or 3,4-diaminopyridine, capable of undergoing cyclization with β-diketones via the metal template effect, have been prepared and characterized, and their tentative structures are ascertained in this communication.

2. Experimental

2.1. Chemicals and Materials

All the chemicals and the solvents used were of the reagent grade. Oxovanadium(IV) sulfate was procured from Aldrich Chemical Co. The β-diketones, namely, acetylacetone, benzoylacetone, thenoyltrifluoroacetone and dibenzoylmethane, were SRL products and the diamines used were reagent-grade products. Thenil used was an Aldrich product.

2.2. Analytical Methods and Physical Measurements

Vanadium was estimated gravimetrically as its vanadate after decomposing the complex with concentrated nitric acid [13]. Sulfur was estimated as barium sulfate in the laboratory [14]. The standard technique of melting point (uncorrected) determination using sulfuric acid bath was employed. Toshniwal conductivity bridge, model number CLO102A, was used for conductance measurements at room temperature. The magnetic susceptibility of the complexes in powder form was carried out at room temperature using Gouy’s balance. Mercury tetrathiocyanatocobaltate(II), Hg[Co(CNS)₄] (χg = 16.44 × 10⁻⁶ c.g.s. unit at 20°C ) was used as calibrant. The electronic spectra of the complexes were recorded on Beckmann DU-2 spectrophotometer in the ranges 2000–185 nm. The room temperature and liquid nitrogen temperature ESR. spectra were recorded at RSIC, IIT, Chennai, India. The infrared spectra of the complexes in the range 4000–200 cm⁻¹ were measured on Perkin-Elmer 621 using KBr medium.

2.3. In-Situ Preparation of Oxovanadium(IV) Complexes with Ligands Derived by Condensation of Thenil with 1,4-Diaminobenzene or 3,4-Diaminopyridine

Vanadyl sulfate (2 mmol) dissolved in methanol (25 mL) was added to a refluxing solution of thenil (2 mmol) and 1,4-diaminobenzene (4 mmol) or 3,4-diaminopyridine (4 mmol) in ethanol (25 mL). The mixture was refluxed for 6 h, when the color of the solution turned green. The solvent was removed under vacuo at room temperature and the dark green color product was isolated. The complex was thoroughly washed with methanol/ethanol mixture. The yield was 70%.

2.4. In-Situ Preparation of Macrocyclic Complexes of Oxovanadium(IV)

Vanadyl sulfate (2 mmol) dissolved in methanol (25 mL) was added to a refluxing solution of thenil (2 mmol) and 1,4-diaminobenzene or 3,4-diaminopyridine (4 mmol) in ethanol (25 mL). The mixture was refluxed for 5 h, when the color of the solution intensified and turned green. To this reaction mixture, an ethanolic solution (10 mL) of acetylacetone (2 mmol) and glacial acetic acid (2 mL) were added. The reaction mixture was refluxed for about 5 h then green precipitate was obtained. The complex was purified by washing with the mixture (10 mL) of methanol/ethanol (1 : 1). The yield was 60%. The same procedure was adopted for the synthesis of other oxovanadium(IV) macrocyclic complexes using benzoylacetone, thenoyltrifluoroacetone, and dibenzoylmethane. The physical and analytical data of the complexes are presented in Table 1.

3. Results and Discussion

The oxovanadium(IV) complexes were synthesized using in situ method by refluxing the reaction mixture of thenil, diamines, and vanadylsulfate in 1 : 2 : 1 molar ratio in aqueous ethanol. The reaction appears to proceed according to Schemes I and II given in Figure 1.

3.1. Infrared Spectra

The important bands of the infrared spectra for the complexes are listed in Table 2. The macrocyclic complexes of oxovanadium(IV) exhibit >C=N absorption around 1625–1618 cm⁻¹, which normally appears at 1660 cm⁻¹ in free ligands [15–18]. The lowering of this band in the complexes (type I) indicates the coordination of nitrogen atoms of the azomethine groups to the vanadium [19, 20]. The presence of a band at around 302 cm⁻¹ may be assigned to ν (V–N) vibration [21]. The appearance of >C=N band and the absence of the >C=O band around 1700 cm⁻¹ are a conclusive evidence for condensation of the diamines with the two keto group of thenil [18]. The bands appearing at 3350 and 3180 cm⁻¹ may be assigned to asymmetrical and symmetrical N–H stretching modes of the coordinated terminal amino group [21]. The oxovanadium(IV) complexes show a band at around 981 cm⁻¹, which is assigned to ν (V=O) vibration [22]. The presence of an ionic sulfate group in the complexes is confirmed by the appearance of three bands at ca. 1131–1135 cm⁻¹ (ν₃), 955–960 cm⁻¹ (ν₁) and 602–606 cm⁻¹ (ν₄). The absence of a ν₂ band and nonsplitting of ν₃ band indicate that Td symmetry is retained [23, 24]. The infrared spectra of macrocyclic complexes of type II show the same pattern of bands but the asymmetrical and symmetrical N–H stretching modes of terminal amino groups disappear due to condensation of these amino groups with carbonyl group of β-diketones in cyclization reactions.

3.2. Electronic Spectra

The electronic spectra show bands in the regions 11,040–11,980 cm⁻¹, 15,038–15,916 cm⁻¹, and 21,080–22,380 cm⁻¹. These spectra are similar to other five-coordinate oxovanadium(IV) complexes involving nitrogen donor atoms. These spectral bands are interpreted according to an energy level scheme reported by Tsuchimoto et al. [25] for distorted, five-coordinate square pyramidal oxovanadium(IV) complexes [26, 27]. Accordingly, the observed bands can be assigned to ²B₂ → ²E, ²B₂ → ²B₁ and ²B₂ → ²A₁ transitions, respectively. One more band is observed in the region 35,260–35,760 cm⁻¹, which may be due to transition of the azomethine linkages [28, 29].

3.3. Molar Conductance Measurements

The molar conductivity () values of all the oxovanadium(IV) complexes were measured in dimethylformamide and the obtained values were between 100 to 106 ohm⁻¹ cm² mol⁻¹ indicating their 1 : 1 electrolytic nature.

3.4. Magnetic Moment Measurements

Effective magnetic moments () of oxovanadium(IV) complexes were measured at room temperature and the values are given in Table 1. The effective magnetic moments of complexes were found in the range 1.71–1.76 BM which correspond to a single electron of the d¹ system of square-pyramidal oxovanadium(IV) [26].

3.5. ESR Spectra

The X-band ESR spectra of an oxovanadium(IV) complex were recorded in DMSO at room temperature and at liquid nitrogen temperature (177 K). ESR spectra of the complexes were analyzed by the method of Mishra and Pandey [30], Sands [31], and Ando et al. [32] The room temperature ESR spectra show eight lines, which are due to hyperfine splitting arising from the interaction of the unpaired electron with a ⁵¹V nucleus having the nuclear spin . This confirms the presence of a single oxovanadium(IV) cation as the metallic centre in the complex. The anisotropy is not observed due to rapid tumbling of molecules in solution at room temperature and only g-average values are obtained. The anisotropy is clearly visible in the spectra at liquid nitrogen temperature and eight bands each due to and are observed separately. The , , , and values (RT/LNT) are measured from the spectra, which are in good agreement for a square-pyramidal structure [33–35]. The value from mobile solution at room temperature and from frozen solution at liquid nitrogen temperature do not agree very closely since the and tensors are corrected for second order. Further, values are all very close to the spin-only value (free electron value) of 2.0023, suggesting little spin-orbit coupling. On the basis of the above studies, the tentative structures may be proposed for these oxovanadium(IV) complexes of the type (I) and macrocyclic complexes of the type (II) are shown in Figure 1.

4. Conclusions

The spectral data show that the Schiff base condensation has been possible due to thenil being a versatile chelating agent having two reactive carbonyl groups with 1,4-diaminobenzene or 3,4-diaminopyridine and their cyclisation reaction with β-diketones is achieved by virtue of kinetic template effect of oxovanadium(IV) cation in aqueous ethanol medium. Schiff bases behave as tetradentate ligands by bonding to the metal ion through the azomethine nitrogen and amine nitrogen providing controlled geometry. The analytical data show the presence of one metal ion per ligand molecule and suggest a mononuclear structure for complexes. The magnetic moment values and electronic data are in the favour of square pyramidal structure for oxovanadium(IV) complexes. The purpose of synthesizing oxovanadium(IV) complexes is to provide new vanadyl centres in controlled geometry to explore biological importance as few vanadyl complexes are reported showing DNA cleavage activity in near-IR light.

Acknowledgment

The authors are thankful to the Director of NERIST, Nirjuli, Itanagar, Arunachal Pradesh, India, for providing laboratory facilities for synthetic work and central research facility for microanalysis of carbon, hydrogen, and nitrogen.

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Copyright © 2013 Sanjay Singh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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