Synthesis, Spectral, Thermogravimetric, XRD, Molecular Modelling and Potential Antibacterial Studies of Dimeric Complexes with Bis Bidentate ON–NO Donor Azo Dye Ligands

The dimeric complexes of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) with two new symmetrical ON–NO donor bis bidentate (tetradentate) azo dye ligands, LH 2 = 4,4-bis(4-hydroxyquinolinolinylazo)diphenylsulphone, and LH 2 = 4,4bis(acetoacetanilideazo)diphenylsulphone have been synthesized. The metal complexes have been characterised by elemental analytical, conductance, magnetic susceptibility, IR, electronic spectra, ESR, NMR, thermogravimetry, X-ray diffraction (powder pattern) spectra, and molecular modelling studies. The Co(II) and Ni(II) complexes are found to be octahedral, Cu(II) complexes are distorted octahedral, and a tetrahedral stereochemistry has been assigned to Zn(II), Cd(II), and Hg(II) complexes. The thermogravimetric study indicates that compounds are quite stable. The energy optimized structures are proposed using the semiempirical ZINDO/1 quantum mechanical calculations. The potential antibacterial study of the ligands and some metal complexes has been made with one gram positive bacteria Staphylococcus aureus and one gram negative bacteria E. coli which gives encouraging results. Both the Co(II) complexes are found to possess monoclinic crystal system.


Introduction
Azo dyes are found to be pharmacologically active and hence they are used as chemotherapeutic agents in the manufacture of potential drugs [1,2].Azo dyes are also used as indicator in the chemical laboratories and as preservative and dyeing agents in food industries [3].Besides their applications azo dyes can also form stable complexes with transitional and nontransitional metal ions because of the presence of azo (-N=N-) group [4][5][6][7][8][9].The present study reports the synthesis of two new ON-NO donor tetradentate azo dyes and their twelve dimeric complexes and characterization of these complexes are made by using various physicochemical methods.Antibacterial study and thermogravimetric and molecular modelling of the azo dyes and some metal complexes have also been described.

Synthesis of Metal
Complexes.The metal chlorides in ethanol were mixed separately with ethanolic solution of the ligands in 2 : 1 molar ratio and the resulting solutions were heated to 60-70 ∘ C for about one hour on a heating mantle.
The solution was then cooled down to room temperature and the pH was raised to ∼7 by adding conc.ammonia drop by drop with stirring.The solid complexes thus separated were then washed with ethanol followed by ether and dried in vacuum.

Results and Discussion
The physical characteristics and microanalytical data of the ligands and the complexes are given in (Table 1  are amorphous in nature and have high melting points and are insoluble in common organic solvents like methanol, ethanol, and benzene but soluble in dimethylformamide and dimethylsulfoxide.Nonelectrolytic nature of the complexes is indicated from the low conductance values (4.2-5.6 Ω −1 cm 2 mol −1 ) in 10 −3 M solution in DMF [11].

IR Spectra.
In the IR spectra of the azodye ligands (Table 2) a broad band obtained at 3390 cm −1 (LH 2 ) and at 3443 cm −1 (L  H 2 ) be assigned to O-H⋅ ⋅ ⋅ N and O-H⋅ ⋅ ⋅ O intramolecular hydrogen bonding.The absence of this band in the spectra of metal complexes indicates the deprotonation of hydrogen bonded N⋅ ⋅ ⋅ H or O⋅ ⋅ ⋅ H group on complexation and subsequent coordination of the phenolic/enolic oxygen atoms to the metal ions [12].The sharp band of the ligands appear at 1625 cm −1 (LH 2 ) and at 1633 cm −1 (L  H 2 ) can be attributed to ](-N=N-) vibration.There is no shift of this band in the metal complexes indicating noncoordination of the azo group to the metal ions.The band observed at 1148 cm −1 (LH 2 ) is attributed to ](C-O) vibration and the bathochromic shift of ∼15 cm −1 in the metal complexes indicates bonding of oxine oxygen to the metal ions [13].In the spectrum of the ligand (LH 2 ) an intense band is observed at 1407 cm −1 due to C-N vibration of the oxinate group [14].In the metal complexes this band occurs at ∼1324 cm −1 .The shift of this band to lower frequency regions shows considerably lower double bond character of the C-N bond due to involvement of the ring nitrogen on complexation [15,16].In the ligand (L  H 2 ) the band observed at 1668 cm −1 can be assigned to ](C=O) vibration and shifting of this band by 20-25 cm −1 to lower frequency region in the metal chelates indicates the coordination of the amidic oxygen atoms to the metal ions.The band shown at 1263 cm −1 in the ligand (L  H 2 ) can be assigned to enolic (C-O) vibration and decrease of this frequency by 20-30 cm −1 on complexation is indicative of bonding of enolic oxygen atoms to the metal ions.In the metal complexes broad bands appear at ∼3350-3399 cm −1 followed by sharp peaks at ∼833-842 cm −1 and at ∼727-736 cm −1 assignable to -OH starching, rocking, and wagging vibrations, respectively, indicating the presence of coordinated water molecules in the complexes [17].The conclusive evidence of bonding of the azo dye ligands to the metal ions is proved by the appearance of bands at ∼508-   [20].The electronic spectra of the copper(II) complexes exhibit one broad band at 13300-14470 cm −1 with maxima at 13320(13345) cm −1 assignable to 2 Eg → 2 T 2g transition in support of a distorted-octahedral configuration of the copper (II) complex [21,22].The magnetic moment of the metal complexes were recorded at RT.The observed magnetic moment value of the Co II , Ni II , and Cu II complexes are found to be ∼5.0,∼3.1, and ∼1.8 B.M., respectively, indicating octahedral configuration of the complexes, which is further supported by their electronic spectral data [23,24].[26].This type of spectrum may be due to dynamic or pseudorotational type of Jahn-Teller distortion (Figures 6(a) and 6(b)).The spin-orbit coupling constant () can be calculated from the equation

Electronic Spectra and Magnetic
The  value of the former complex is found to be −320.445cm −1 and that of latter complex is −293.823cm −1 .The decrease of the  values of the complexes from the free ion value (−830 cm −1 ) indicates the overlapping of metalligand orbitals in the metal complexes.) that have successfully indexed as figure of merit () is found to be 6.9 and 8.8, respectively, as suggested by de Woulff [28].The density () of the complex was determined by the floatation method in a saturated solution of KBr, NaCl, and benzene separately.The number of formula units per unit cell () is calculated from the relation

Powder XRD
where  = density of the compound,  = Avogadro's number,  = volume of the unit cell, and  = molecular weight of the complex.The value of "" is found to be 2 in both cases which agrees well with the suggested structure of the complexes.The crystal system of both the complexes was found to be monoclinic.The Debye-Scherrer equation in Xray diffraction and crystallography is a formula which relates the size of the crystallites in a solid to the broadening of a peak in a diffraction pattern.The Debye-Scherrer equation is where  = crystallite size,  = wavelength of X-ray radiation (CuK = 0.154060 nm),  = constant taken as 0.   and  = full width at half maximum height (FWHM) (2.77 nm).So crystallite size of this complex is found to be 2.61 nm [29].[31] method.In this method, the equation used is where   = rate of heating,  = weight fraction of reacting materials,   = activation energy,  = order of reaction, and  = frequency.This equation in the difference form will be Δ log  = Δ log  − (/2.303)⋅ Δ1/; when Δ(1/) is kept constant, a plot at Δlog  versus Δlog  will give a linear relationship whose slope and intercept provide the value of  and , respectively.The order of the decomposition reaction, the activation energy, and correlation coefficient are given in (Table 5).The calculated values of the activation energy is found to be low due to the autocatalytic [32,33] effect of the metal ion on the thermal decomposition of the complex.

Optimized Geometry Studies of the Ligands & Complexes
by Molecular Modelling Method.Molecular modelling of the ligands (LH 2 ), (L  H 2 ) and metal complexes of Co(II) have been carried out using molecular mechanics and Hartree-Fock (HF) Quantum methods.The standard 6-31 g basic set was used in conjugation with the HF method.All calculations are made using Gaussian 98 programme package [34][35][36][37].The metal complexes were built and the optimization of their geometries was done at mm/H-F/6-31 g level of theory Figures 1, 2, 3, and 4. The findings of these computed works are in good agreement with the experimental results.The selected bond lengths, bond angles of the ligand, bond angles of the complexes, and their bond energies are given in Tables 3(a), 3(b), 3(c), 3(d), 3(e), 3(f), 3(g), and 3(h), respectively.The total energies of both the complexes have been found to be 287.403kcal/mole and 247.322 kcal/mole, respectively.3.8.Antibacterial Activity.The ligands and metal complexes have been screened for antibacterial activities and results have been shown in (Table 6).The antibacterial activity of the compounds is examined against two strains of bacteria, one gram positive Staphylococcus aureus and one gram negative E. coli.The effectiveness of the compounds is classified into three categories, Sensitive, intermediate, and resistant.If a compound is sensitive to a bacteria then it can be applied to cure the disease caused by the bacteria, while it fails to do so if it is resistant to the bacteria.Accordingly the effectiveness of the compound can be predicted by knowing the zone of inhibition value in mm.The results (Figure 9) show that the ligand was found to posses more antibacterial activity than the complexes against different bacteria.The increase in biological activity of the metal complexes than the ligands may be due to complexation and it can be explained on the basis of chelation theory [38].

Conclusion
The Co II and Ni II complexes are found to be octahedral and Cu II complexes distorted-octahedral; Zn II , Cd II , and Hg II complexes are assigned to have tetrahedral geometry.Both the azo dyes behave as dibasic tetradentate ligands coordinating through oxine nitrogen, phenolic oxygen, enolic oxygen, and amidic oxygen atoms.All the complexes are dimeric in nature.The complexes are found to be thermally stable.From the thermal study of the complexes the order of decomposition reaction, activation energy and correlation coefficients has been calculated.The XRD study indicates a monoclinic crystal system for both the Co II complexes.All calculations based on molecular mechanics on the optimized geometries fit well with the experimental findings.The crystallite sizes of the complex compounds have been determined.
The potential antibacterial study of the ligands as well as Co II , Ni II , Cu II , and Zn II complexes has been made against gram positive and gram negative bacteria which gives encouraging results.

3. 3 .
1 H-NMR Studies.The 1 H NMR spectra of the ligands LH 2 and L  H 2 were recorded in DMSO.The complex pattern observed at  6.746-9.344ppm and at  7.039-7.956ppm corresponds to eighteen phenyl protons in each ligand.The sharp peak obtained at  13.629 ppm LH 2 corresponds to two phenolic protons.The sharp peaks obtained at  3.570 ppm, at  2.507 ppm,  10.913 ppm, and at  13.026 ppm in the ligand L  H 2 correspond to six methyl (-CH 3 ) protons, two methylene (>CH) protons, two amino (>NH) protons, and two enolic (>C-OH) protons, respectively[25].(Figures5(a) and 5(b)).

3. 6 .
Thermogravimetric Study.The complex [Ni 2 L  Cl 2 (H 2 O) 6 ] suffers a mass loss of 3.4% at 100 ∘ C which corresponds to the removal of two lattice held H 2 O molecules supported by an endothermic peak on the DTA curve at 95 ∘ C[30].Again, the complex moiety loses a mass of 23.52% at 250 ∘ C indicating removal of all coordinated H 2 O molecules and 1/6th of the ligand mass supported by an endothermic peak at about 240 ∘ C on the DTA curve.Thereafter at 450 ∘ C compound loses a mass of 23.07%which corresponds to the removal of 1/3rd of the ligand moiety supported by an exothermic peak at 420 ∘ C. Again the compound loses a mass of 37.5% indicating removal of 2/3rd of the ligand moiety.Again the compound loses 55% mass which corresponds to the removal of rest of the ligand moiety and two chlorine atoms and formation of NiO as residue (Figure8(a)).The complex [Co 2 L  Cl 2 (H 2 O) 6 ] loses a mass of 11.6% at 150 ∘ C with the removal of all coordinated H 2 O molecules supported by an endothermic peak at 140 ∘ C on the DTA curve.Then, the compound loses a mass of 13.5% indicating removal of 1/6th of the ligand moiety supported by an endothermic peak at 240 ∘ C. Thereafter, the complex moiety suffers a mass loss of 15.15% at 400 ∘ C which corresponds to the removal of 1/5th of the ligand moiety supported by an endothermic peak at 380 ∘ C. Finally the compound loses a mass of 64% at 700 ∘ C indicating removal of rest of the ligand moiety and two chlorine atoms with the formation of CoO as the residue (Figure 8(b)).The complex [Ni 2 LCl 2 (H 2 O) 6 ] suffers a mass loss of 23.52% at 150 ∘ C indicating removal of all the coordinated H 2 O molecules along with 1/6th of the ligand supported by an endothermic peak at 140 ∘ C on the DTA curve.Then the compound loses a mass of 24.24% at 400 ∘ C which corresponds to the removal of 1/3rd of the ligand moiety supported by an exothermic peak at 325 ∘ C on the DTA curve.Finally, the compound loses 64% of mass indicating removal of rest of the ligand moiety and two chlorine atoms which is supported by an endothermic peak

Figure 9 :
Figure 9: Effect of the complexes on the growth of selected E. coli and S. aureus.

Table 1 :
Analytical and physical data of the ligands and its complexes.

Table 2 :
Infrared spectra of the ligand and the complexes in cm −1 .
The ESR spectra of the Cu II Complexes [Cu 2 LCl 2 (H 2 O) 6 ] and [Cu 2 L  Cl 2 (H 2 O) 6 ] have been recorded at X-band at RT.The "g av " values of the complexes are found to be 2.09623 and 2.08807, respectively, by applying Kneubuhl's method 3.4.ESR Studies.

Table 3 :
(a) Selected bond lengths and bond energies of the ligand (LH 2 ).(b) Selected bond angles and bond energies of the ligand (LH 2 ).(c) Selected bond lengths and bond energies of the ligand (L  H 2 ).(d) Selected bond angles and bond energies of the ligand (L  H 2 ).(e) Selected bond lengths and bond energies of the [Co 2 LCl 2 (H 2 O) 6 ] complex.(f) Selected bond angles and bond energies of the [Co 2 LCl 2 (H 2 O) 6 ] complex.(g) Selected bond lengths and bond energies of the [Co 2 L  Cl 2 (H 2 O) 6 ] complex.(h) Selected bond angles and bond energies of the [Co 2 L  Cl 2 (H 2 O) 6 ] complex.

Table 5 :
Kinetic parameters of the complexes.

Table 6 :
Antibacterial activities of the ligands and the complexes (data presented as diameter of zone of inhibition, mm).C on the DTA curve with the formation of NiO as the residue (Figure8(c)).The kinetic parameters such as order of reaction and activation energy for the thermal decomposition of[Cu 2 L  Cl 2 (H 2 O) 6 ], [Ni 2 LCl 2 (H 2 O) 6 ], and [Ni 2 L  Cl 2 (H 2 O) 6 ]complexes have been determined byFreeman-caroll