Stability of Vanadium Chalcone Complexes

The vanadyl(IV) complexes of substituted chalcones were prepared by reflux-ing vanadyl sulphate with different substituted chalcones in ethanoic medium. The chalcones were prepared with different aromatic aldehydes like benzaldehyde, hydroxy benzaldehyde, nitro benzaldehyde and chloro benzaldehyde. The synthesized Vanadium complexes were characterized by different spectral techniques. The IR spectral studies revealed that the chalcone derivatives are bidentate ligand. Magnetic studies, electron spin resonance and UV studies suggest that the complexes are in square pyramidal geometry. Conductance measurements suggest that all complexes are non-electrolyte in DMF. The thermal study explained the stability of complex and its decomposition. The synthesized ligand and metal complexes were screened for their antibacterial activity against E. coli and Staphylococcus aureus bacterial strains and for antifungal activity against P. notatum . and square pyramidal geometry of the VO(II) complexes which is supported by the electronic spectra. The IR spectra shows bonding of the metal through O-donor atoms of the ligands. In this study VO(II) complexes shows the moderate activity against Escherichia coli , Staph aureus bacteria and against P. notatum fungi.


Introduction
Chemically chalcones consist of open chain flavonoids in which the two aromatic ring joined by a three carbon. Chalcones are α, β-unsaturated ketones and contain a reactive keto-ethylenic group 2-hydroxychalcones and their heterocyclic analogues are also reported to form coordination complexes.
Chalcones are widely distributed in fruits, vegetables, tea, spices and also have applications in biological activities. These are abundant in edible plants and are considered to be the precursors of flavonoids and isoflavonoids [1]. Some chalcones has been employed for the gravimetric estimation of divalent ions of palladium, copper and nickel. Calcium determination by complexometric titration by using chalcone-metanil as indicator also has been reported. Chalcones possesses several biological activities with wide ranging. Chalcones use for the treatment of chronic diseases which involve inflammatory processes, for example, diabetes mellitus [2]. The study of chalcone derivatives has become of much interest in recent years on account of their antibacterial, antiviral activities [3]. Many chalcones based have been recognized in a number of screening assays for modulating important pathways or molecular targets in cancer [4][5].
The coordination chemistry of oxovanadium(IV) (i.e. VO 2+ or vanadyl ion) is more interesting and rather more important because of two main reasons. Firstly, the vanadyl complexes are finding more and more importance in biological systems. Secondly, the coordination number and geometry of this metal is highly ligand dependent. Moreover vanadyl ion is less toxic. In last year's research has been directed towards the synthesis of efficient bioactive compounds with low toxicity, in order to achieve this goal the type and the position of substituent into ligand were varied [6].
A wide variety of oxovanaduim(IV) complexes have been prepared and characterized by various physicochemical methods. Oxovanadium(IV) complexes have a square pyramidal geometry with apical oxygen and the vanadium atom lying above the plane defined by the donor atoms of the equatorial ligands. These square pyramidal complexes generally exhibit strong tendency to remain five coordinate [7,8]. The coordination chemistry of vanadium(V) compounds is dominated by oxo complexes, containing the VO 3+ or the VO 2+ moiety [9]. The absorption band due to V〓O stretching vibration of oxovanadium(IV) complexes is usually observed at a higher wavenumber compared to those of vanadate(V) complexes. However, the V〓O stretching vibration is susceptible to a number of influences including electron donation from basal plane ligand atoms, solid-state effects, and coordination of additional molecules. Therefore, there has been considerable work done to assign the V〓O stretching frequencies in oxovanadium(IV) compounds [10].
An increasing research interest of vanadium in coordination chemistry is not only due to its exhibiting a range of oxidation dates from +5 to −1 but also complexities exhibited by vanadium complexes and their industrial [11,12], biological [13,14] and medicinal [15,16] applications. Vanadium complexes have been reported to have interesting antibacterial activities [17][18][19][20]. It has been reported that V(IV) complexes high effect on anticancer activity [21].

Preparation of substituted chalcone
The substituted chalcones prepared by stirring the equimolar concentration mixture of 2-hydroxy-4,5-dimethyl acetophenone (0.01 mol) and substituted aromatic benzaldehyde (0.01 mol) in 20 ml ethanol for 1 h in presence of 50% NaOH. The mixture stirred till completion of the reaction (progress of reaction checked by TLC). The crude mixture poured into ice water then acidified the product with 10% hydrochloric acid. The coloured compound formed was filtered, washed with water and dried. The compounds recrystallized from absolute ethanol (Figure 1).

Instrumental methods
The IR spectra of complexes were recorded on a Perkin-Elmer instrument in KBr pallets in the range of 4000-400 cm −1 . TGA analysis of metal complexes were carried out in nitrogen atmosphere in the range 25-900°C on Rigaku Thermo Plus-8120 TG-DTA instrument with a heating rate 10°C min −1 using Alumina as a standard. UV-Visible spectra were recorded using DMF as solvent on Shimadzu UV-VIS spectrophotometer in the range 250-950 nm. Electron spin resonance spectra complexes were recorded on E-112 ESR Spectrometer as 'g' marker ( g = 2.00277) at room temperature. The conductance was measured in DMF solvent on Equiptronics Conductivity meter (EQ-664A).

Results and discussion
All the vanadium chalcone complexes are insoluble in water and most common organic solvents but sparingly soluble in DMF. The complexes are stable at room temperature. The elemental analysis shown in Table 1 indicates that, all metal complexes have 1:2 (metal: ligand) stoichiometry for all the complexes (Figure 2).
All synthesized complexes are green in colour. All complexes are decomposed above 300°C. The molar conductance values obtained for these complexes at the concentration are in the range of 15-27 mhos.cm 2 mol −1 suggesting their nonelectrolytic nature [22].

IR spectral studies
Some structurally significant IR bands for uncoordinated chalcones and complexes have been scanned in the 4000-400 cm −1 region and presented in Table 2.
The complexes shows a band in the region 1162-1205 cm −1 which corresponds to strong band in the region 1185-1221 in free ligand of chalcones due to υ (C-O). The shifts of this band were observed to move to lower wave numbers indicates bonding of phenolic oxygen atom to metal center via deprotonation. The band in the region 552-573 cm −1 can be assigned to the stretching modes of the metal to ligand bonds, ν (V-O) [23]. In addition the compound exhibit a strong band in the 952-983 cm −1 range due to the terminal V〓O stretching and this value is close to the majority of oxovanadium(IV) complexes [24]. In the IR spectra of all the ligands an intense band appearing around 1640 cm −1 is attributed [25] to (C〓O) group. In the complexes this band is observed in the 1618-1626 cm −1 range. The shifting of band to lower wave number indicates coordination through oxygen of (C〓O) group. The band appears around 740 cm −1 in chloro complex indicating presence of C-Cl bond. The occurrence of band around 1500 cm −1 in VL 3 complex is due to ν (C-NO 2 ) mode.

Electronic spectra
Electronic absorption spectral study is an important tool used [26] to determine the geometry around a central metal atom. The electronic spectra (Figure 3) of the metal complexes in DMF (10 −3 M) were recorded on Shimadzu UV-VIS spectrophotometer in the range 200-950 nm. The Vanadium chalcone complexes show absorption band in 380-770 nm region ( 2 ) respectively. From these electronic absorption bands and paramagnetic nature, square pyramidal geometry suggested for VO(II) chalcone complexes. As these oxovanadium(IV) belong to the 3d 1 system, there is no interelectronic repulsion in the metal ion so B 0 value of V 4+ is not available. Therefore calculation of Racah parameter is not possible for these vanadium complexes ( Table 3).

Magnetic susceptibility measurement
The magnetic susceptibility for all the Vanadium complexes at room temperature were recorded by the Gouy's method using Hg[Co(SCN) 4 Table 4. The effective magnetic moments were calculated after applying diamagnetic corrections for the ligand components using Pascal's constants. The room temperature μ eff values for the vanadium complexes were found in the range 1.71-1.76 B.M. The magnetic susceptibilities of the complexes are consistent with square pyramidal geometry around the central metal ion [27,28]. The magnetic moments of the compounds investigated are in agreement with the findings of electronic absorption spectra.

Electron spin resonance study
ESR spectrum of VO(II) complexes was recorded at room temperature as polycrystalline sample, on x-band at 9.1 GHz, under the magnetic field strength 3000 G using TCNE as marker. The value of g || and g ┴ is shown in Table 5. Paramagnetism of vanadyl ion arises from the single unpaired electron, its paramagnetism is due to spin angular momentum and VO 2+ is most stable diatomic cation. In V(IV) complexes value of g is below value of free electron. The trend 2.0023 > g ┴ > g ||, observed for the complexes. The EPR signal are split into eight lines suggested that V(IV) complexes are mononuclear [29]. The oxovanadium(IV) ion belong to the 3d 1 system. Because of strong multiple covalent vanadium oxo-bond, a tetragonal compression occurs and therefore square pyramidal geometry proposed for VO(II) complex. The d orbital split into different level, dx 2 -y 2 orbital is least stable as it is face to face with all four equatorial ligands. d xy being most stabilized orbital therefore d 1 electron lies in d xy orbital. As G values of present complexes are lower than four indicates, the ligands involve in the complex formation are the strong field ligands.

Thermal measurements
A plot of change in weight vs. temperature or time is known as Thermo gravimetric curve. It records the loss in weight as a function of temperature for transition that involves dehydration or decomposition. The TGA studies of the complexes was recorded in nitrogen atmosphere on Rigaku Thermo Plus-8120 instrument by increasing the temperature from room temperature upto 900°C at the heating rate of 10°C min −1 . The decomposition pattern obtained for VO(II) complexes of chalcone ligands follow two major stages (Figure 4). The thermal dehydration of this complex in the first stage was attributed to the loss of water leaving behind the nonhydrated complex takes place between 80 and 120°C. In the second stage the complex starts decomposing gradually which occurs in the temperature range corresponds to 220-310°C, this range correspond to the decomposition of part of chalcone ligand and the observed mass loss was recorded as 29.81% for VL 1

Antibacterial activity
It is known that killing of bacteria by chelated ligand is more powerful than non-chelated ligand, therefore chelated ligand acts as potent bactericidal agents. It was observed that due to delocalization of the π electrons over whole ligand and partial sharing of its positive charge with donor groups reduce the polarity of a metal ion in a complex. Thus, chelation increases the lipophilic character in complexes and results in an enhancement of activity [30,31]. The antibacterial results, Table 7, showed that the complexes shows moderate activity against the bacteria E. coli and S. aureus. Table 7 showed that metal complexes exhibited significant antifungal activity than chalcones at the same concentration against the fungi P. notatum. The order of inhibition with respect to metal complexes was VL 2 > VL 4 > VL 1 > VL 3 .

Antifungal activity
It was concluded from above data that electron withdrawing substituents like nitro, cyno at para position in ring A decreases the antibacterial and antifungal activity. Electron donating group like hydroxyl, methoxy at para position increases the activity. Presence of halogen group at meta position in ring A shows good activity than the unsubstituted ring. The increased activity of the chelates can be explained based on the overtone concept and the Tweedy chelation theory [32]. According to the overtone concept of cell permeability, the lipid membrane that surrounds the cell favors the passage of only lipid-soluble material.

Conclusion
Based on the results the following conclusion maybe drawn. The higher decomposition temperature and electrical conductance studies show the presence of strong metal-ligand bonding and non-electrolytic nature of the complexes, respectively. Room temperature magnetic studies are indicative of paramagnetic nature  and square pyramidal geometry of the VO(II) complexes which is supported by the electronic spectra. The IR spectra shows bonding of the metal through O-donor atoms of the ligands. In this study VO(II) complexes shows the moderate activity against Escherichia coli, Staph aureus bacteria and against P. notatum fungi.
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