Synthesis of Oxovanadium ( IV ) Complexes with Tetraaza Coordinating Ligands

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 reactionwithββ-diketones) have been prepared using vanadyl ion as kinetic template. e prepared macrocyclic complexes were characterized by elemental analyses, molar conductance, magnetic moments, and infrared, electronic, and electron spin resonance data. �rom the spectroscopic studies, �ve-coordinate s�uarepyramidal geometry for the VO complexes have been proposed wherein derived ligands act as tetradentate chelating agents.


Introduction
e 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][2][3].e 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.ese complexes also have biological activities such as antibacterial, antifungal, antiviral, and anticancer drugs [6][7][8].enil is a versatile chelating agent having two reactive carbonyl groups capable of undergoing Schiff-base condensation with a variety of di-and polyamines.e use of metals as templates in such reaction has led to the synthesis of metal complexes of macrocyclic ligands [9].us, 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.is 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 �rst 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 -diketones via the metal template effect, have been prepared and characterized, and their tentative structures are ascertained in this communication.

Chemicals and Materials.
All the chemicals and the solvents used were of the reagent grade.Oxovanadium(IV) sulfate was procured from Aldrich Chemical Co.e diketones, namely, acetylacetone, benzoylacetone, thenoyl-tri�uoroacetone and dibenzoylmethane, were SRL products and the diamines used were reagent-grade products.enil used was an Aldrich product.  1) of complexes show 1 : 1 metal to ligand stoichiometry.

Analytical Methods and Physical
Measurements.Vanadium was estimated gravimetrically as its vanadate aer decomposing the complex with concentrated nitric acid [13].
Sulfur was estimated as barium sulfate in the laboratory [14].e 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.e 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 ] (g = 16.44 × 10 −6 c.g.s. unit at 20 ∘ C) was used as calibrant.e electronic spectra of the complexes were recorded on Beckmann DU-2 spectrophotometer in the ranges 2000-185 nm.e room temperature and liquid nitrogen temperature ESR.spectra were recorded at RSIC, IIT, Chennai, India.e infrared spectra of the complexes in the range 4000-200 cm −1 were measured on Perkin-Elmer 621 using KBr medium.

In-Situ Preparation of Oxovanadium(IV) Complexes with Ligands Derived by Condensation of enil with 1,4-
Diaminobenzene or 3,4-Diaminopyridine.Vanadyl sulfate (2 mmol) dissolved in methanol (25 mL) was added to a re�uxing solution of thenil (2 mmol) and 1,4-diaminobenzene (4 mmol) or 3,4-diaminopyridine (4 mmol) in ethanol (25 mL).e mixture was re�uxed for 6 h, when the color of the solution turned green.e solvent was removed under vacuo at room temperature and the dark green color product was isolated.e complex was thoroughly washed with methanol/ethanol mixture.e yield was 70%.
To this reaction mixture, an ethanolic solution (10 mL) of acetylacetone (2 mmol) and glacial acetic acid (2 mL) were added.e reaction mixture was re�uxed for about 5 h then green precipitate was obtained.e complex was puri�ed by washing with the mixture (10 mL) of methanol/ethanol (1 : 1).e yield was 60%.e same procedure was adopted for the synthesis of other oxovanadium(IV) macrocyclic complexes using benzoylacetone, thenoyltri�uoroacetone, and dibenzoylmethane.e physical and analytical data of the complexes are presented in Table 1.

Results and Discussion
e oxovanadium(IV) complexes were synthesized using in situ method by re�uxing the reaction mixture of thenil, diamines, and vanadylsulfate in 1 : 2 : 1 molar ratio in aqueous ethanol.e reaction appears to proceed according to Schemes I and II given in Figure 1.
3.1.Infrared Spectra.e important bands of the infrared spectra for the complexes are listed in Table 2. e 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][16][17][18].e lowering of this band in the complexes (type I) indicates the coordination of nitrogen atoms of the azomethine groups to the vanadium [19,20].e presence of a band at around 302 cm −1 may be assigned to  (V-N) vibration [21].e 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].e 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].e oxovanadium(IV) complexes show a band at around 981 cm −1 , which is assigned to  (V=O) vibration [22].e presence of an ionic sulfate group in the complexes is con�rmed by the appearance of three bands at ca. 1131-1135 cm −1 ( 3 ), 955-960 cm −1 ( 1 ) and 602-606 cm −1 ( 4 ).e absence of a  2 band and nonsplitting of  3 band indicate that Td symmetry is retained [23,24].e 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 -diketones in cyclization reactions.

Molar Conductance
Measurements.e molar conductivity (Λ M ) values of all the oxovanadium(IV) complexes were measured in dimethylformamide and the obtained values were between 100 to 106 ohm −1 cm 2 mol −1 indicating their 1 : 1 electrolytic nature.

Magnetic Moment Measurements.
Effective magnetic moments (μ eff ) of oxovanadium(IV) complexes were measured at room temperature and the values are given in Table 1.e effective magnetic moments of complexes were found in the range 1.71-1.76BM which correspond to a single electron of the d 1 system of square-pyramidal oxovanadium(IV) [26].
3.5.ESR Spectra.e 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] e room temperature ESR spectra show eight lines, which are due to hyper�ne splitting arising from the interaction of the unpaired electron with a 51 V nucleus having the nuclear spin   2.is con�rms the presence of a single oxovanadium(IV) cation as the metallic centre in the complex.e anisotropy is not observed due to rapid tumbling of molecules in solution at room temperature and only g-average values are obtained.e anisotropy is clearly visible in the spectra at liquid nitrogen temperature and eight bands each due to  || and  ⟂ are observed separately.e  || ,  ⟂ ,  || , and  ⟂ values (RT/LNT) are measured from the spectra, which are in good agreement for a square-pyramidal structure [33][34][35].e  iso value from mobile solution at room temperature and  av 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.

Conclusions
e 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 1,4diaminobenzene or 3,4-diaminopyridine and their cyclisation reaction with -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.e analytical data show the presence of one metal ion per ligand molecule and suggest a mononuclear structure for complexes.e magnetic moment values and electronic data are in the favour of square pyramidal structure for oxovanadium(IV) complexes.e 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.