STRUCTURAL AND THEORETICAL STUDY OF SIX DINUCLEAR Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) COMPLEXES CONTAINING ON-NO DONOR AZODYE LIGAND, 4,4’-(1E,1’E)-(5,5’-(PROPANE-2,2-DIYL)BIS(2-HYDROXY-5,1- PHENYLENE)BIS(DIAZENE-2,1-DIYL)DIBENZENESULFONIC ACID

Six dimeric transitional metal complexes with ON-NO donor bis-bidentate azodye ligand 4,4’(1E,1’E)-(5,5’-(propane-2,2-diyl)bis(2-hydroxy-5,1-phenylene)bis(diazene-2,1-diyl)dibenzenesulfonic acid (PDHPDDDBSA) have been synthesised and characterized by elemental, analytical, conductivity measurement, magnetic moments, spectral, thermal, X-ray diffraction (powder pattern) and theoretical methods. The Co(II) and Ni(II) complexes are found to be octahedral, Cu(II) complex is distorted-octahedral and a tetrahedral stereochemistry has been suggested to Zn(II), Cd(II) and Hg(II) complexes. The thermal analysis data provided the kinetic parameters as order of decomposition reaction, activation energy and frequency factor. All theoretical calculations of the ligand and the Cu(II) and Zn(II) complexes were made using Gaussian 03 rev. A.01 programme package hybrid DFT/B3LYP hybrid functional level of theory with LANL2DZ basic set for copper and zinc atoms in gas phase and 6-31 G(d,p) basic set for other atoms. The DFT study of the ligand and the complexes is used to calculate the bond length, bond angle, HOMO, LUMO and dipole moment values. The calculated HOMO-LUMO energy gap shows that a charge transfer occurs within the molecule. The Cd(II) complex is found to have an orthorhombic crystal system.


INTRODUCTION
A large number of publications have been made in recent years with azodye ligands containing nitrogen, oxygen and sulfur donor atoms and their metal complexes ranging from pure synthetic works to physiochemical and biological studies [1].The azodyes have many applications in various fields as pharmaceutical industry as chemotheraptics [2], chemical laboratory as indicators, chemical reactions as catalysts, textile industry as dyeing agents and food industry as preservatives [3].Bisphenol A [2,2'-di(4-hydroxyphenyl) propane] is used to make plastic based commercial materials [4], colour developer in carbonless copy paper and thermal paper receipts [5,6].Sulphanilic acid is used to make sulpha drugs and dyes [7].Keeping these wide applications in view, these two compounds are coupled to form a novel new ON-NO donor tetradentate azodye.In continuation to our previous works [8], we report here the synthesis of one bis-bidentate (tetradentate) azodye (Figure 1) and its six dimeric metal complexes.The characterization of these metal complexes was made by spectral, thermal, X-ray and theoretical studies.

Materials
Sulphanilic acid and 2,2'-di(4-hydroxyphenyl) propane were E. Merck grade.The chlorides of Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) were of S.R.L. grade.All other reagents and solvents were purchased from commercial sources and were of analytical grade.

Synthesis of the metal complexes
The ethanolic solution of metal chlorides was mixed with ethanolic solution of the ligand in 2:1 molar ratio.

RESULTS AND DISCUSSION
The metal complexes were prepared according to the following reaction scheme: The elemental analysis data agree well with the following composition of the metal complexes (Table 1), [M 2 LCl 2 (H 2 O) 6 ] and [M' 2 LCl 2 (H 2 O) 2 ] where M = Co II , Ni II , Cu II ; M ' = Zn II , Cd II , Hg II .All the complexes are semi-crystalline in nature and have high melting points.The complexes are insoluble in common organic solvents as methanol, ethanol and benzene but soluble in dimethylformamide and dimethyl sulphoxide.The complexes are non-electrolytic nature as evident from their low conductance values (3.4-4.9  -1 cm 2 mol -1 ) in 10 -3 M DMF solution.

IR spectra
In the IR spectrum of the ligand, one broad band is observed at 3344 cm -1 that can be attributed to intramolecular O-H…N hydrogen bonding.The absence of this band in the spectra of the metal complexes indicates deprotonation of the phenolic -OH groups and bonding of phenolic oxygen atom to the metal ion [17].This is further supported by the shift of the band at 1509 cm -1 (LH 2 ) to ~1485 cm -1 in the metal complexes.The sharp band of the ligand at 1597.9 cm -1 can be attributed to ν (-N=N-) vibration and in the metal complexes these bands shown at ~1562 cm -1 , that indicates the coordination of one of the azo nitrogen atoms to the metal ions [18].In the metal complexes broad band appears ~3369 cm -1 followed by sharp peaks at ~829.5 cm -1 and at ~681.4 cm -1 assignable to -OH stretching, rocking and wagging vibrations, respectively, indicating the presence of co-ordinated water molecules in the complexes [19].The conclusive evidence of bonding of the ligand to the metal ions is proved by the appearance of bands at ~533 cm -1 (M-O) and ~456 cm -1 (M-N) [20].
This confirms an octahedral symmetry for the complex.The electronic spectrum of the copper(II) complex exhibit one broad band at ~13360-14490 cm -1 with maxima at ~14370 cm -1 assignable to 2 Eg 2 T 2g transition in support of a distorted-octahedral configuration of the complex [25,26].The magnetic moment of high-spin of Co II complexes are found in the range 4.7-5.2B.M. [27,28].The observed magnetic moment of the Co II complex is found to be 5.1 B.M. indicating octahedral configuration of the complex that is further supported by the electronic spectral data [29].The magnetic moment values of Ni II and Cu II complexes are found to be 3.2 and 1.8 B.M. respectively indicating octahedral environment around the metal ions [30] (Table 1).These three complexes are no doubt binuclear in nature, they exhibit normal magnetic moments of mononuclear complexes attributable to absence of M-M interaction between the metal ions.

H-NMR studies
The 1 H NMR spectrum of the ligand LH 2 was recorded in DMSOd 6 .The complex pattern observed at  6.7310-7.2598ppm corresponds to fourteen phenyl protons [31].The sharp peak obtained at  1.6183 ppm corresponds to six methyl protons.The peak for phenolic proton (OH) was not observed as it is beyond the range of the instrument used (Figure 1).

ESR studies
The ESR spectrum of the copper(II) complex has been recorded at X-band at RT.The 'g av ' value of the complex is found to be 2.036 by applying Kneubuhl's method [32].This kind of spectrum may be attributable to dynamic or pseudo rotational type of Jahn-Teller distortion.The spinorbit coupling constant (ƛ) can be calculated from the relation [33]: The ƛ value of the complex is found to be -372.18cm -1 .The decrease of the ƛ values from the free ion value (-830 cm -1 ) indicates the overlapping of metal ligand orbitals in the metal complexes (Figure 2).

Thermogravimetric studies
The thermal decomposition behaviour of the complex [Cd 2 LCl 2 (H 2 O) 2 ] was studied by using TG and DTA techniques in static atmosphere of nitrogen at a heating rate of 10 °C/min (Figure 3).The mass loss in complex compound was recorded from in ambient temperature to 1200 °C.
The complex remains stable up to 200 °C and suffered a mass loss of 11.95% at about 200-300 °C that correspond to loss of two water molecules and 8% of the ligand supported by an endothermic peak at 300 °C.Then the compound remained stable up to 600 °C.Thereafter, the compound losses almost all its masses gradually between 600-700 °C supported by endothermic peak at 600 °C.Finally, the thermogram becomes a straight line after 700 °C.The kinetic parameters as order of the reaction (n) and activation energy (E a ) can be calculated by Freeman-Carrol method [37].The given equation used for this purpose is Where R H = rate of heating, W = weight fraction of the reacting material, E a = activation energy and Z = frequency factor.This equation in the difference form can be written as ∆log R T = n∆logW -E a /2.303R∆(1/T)When Δ(1/T) is kept constant a plot of ΔlogR T verses ΔlogW gives a linear relationship whose slope and intercept provides the value of n and E a , respectively.The order of decomposition reaction and the activation energy are found to be 1.22 and 4.728 J/mol, respectively.The calculated value of activation energy is found to be low attributable to auto-catalytic effect of the metal ion on the thermal decomposition of the complex [38].The correlation factor(r) is found to be 0.99 and the thermal decomposition of the complex fits well with the experimental results.The non-volatile nature of the complex is indicated from the thermal stability of the complex.
The single point energy and dipole moment of the ligand and its Cu(II) and Zn(II) complexes were calculated using DFT/B3LYP 6-31G (d, p) and DFT/B3LYP LANL2DZ basic sets.The single point energy of both the complexes were found to be less than the energy of the ligand which confirms more stability of complexes then the free ligand.The higher dipole moment of the complexes then the free ligand indicates stronger dipole-dipole interaction after complexation.Also it is observed that the Zn(II) complex possess higher dipole moment values in both the methods than the Cu(II) complex that indicates that Zn(II) complex is found to be less covalent than the Cu(II) complex.

Chemical reactivity study
The E HOMO and E LUMO energy of the ligand and its Cu(II) and Zn(II) complexes are calculated to evaluate the global reactivity descriptors as chemical hardness, electrochemical potential, electrophilicity and chemical hardness.Calculation of molecular orbital coefficients indicates that the possible coordination sites of the ligand are -N=N-(azo) and C-O (phenolic) group [41,42] that binds the metal ion, Since the E HOMO and E LUMO values are found negative that indicate the ligand and its complexes are stable [41,42].The decrease of E HOMO values of the complex compounds with the value of the ligand that confirms the weakening of metal-ligand sites.The HOMO level for the ligand is localized on the N 8 , N 13 , O 12 , N 40 , N 52 and O 51 (Figure 5) indicates that these are the preferred bonding sites for the neuclephilic attack.The energy gap between Figure 6.Molecular orbital energy level diagram with HOMO and LUMO orbital of Cu(II) complex.
The DFT/B3LYP method indicates the chemical reactivity and the selection of active sites of the molecular system.The energy of FMOs and the energy band gap explain the charge transfer interaction within the molecule.The chemical reactivity values as electronegativity (χ), chemical potential (μ), global hardness (η), global softness (S) and global electrophilicity index (ω) [44,45] are listed in (Table 5).The E HOMO and E LUMO plots and the MO diagram (Figure 5, Figure 6 and Figure 7) of the ligand, Cu(II) and Zn(II) complexes show the exact energy gap between the orbitals.The chemical softness (S) of the ligand is higher than the complexes therefore it reacts effectively with the metal ion forming the stable complexes which is confirmed from their chemical potential (μ) value.The electrophilicity () is a positive quantity that measures the tendency of a system to accept electron from the environment.The Zn(II) complex is found to be more stable than the Cu(II) complex as it possess less electrophilicity value than Cu(II) complex (Table 6).

CONCLUSION
The tetradentate ligand (4,4'-(1E,1'E)-(5,5'-(propane-2,2-diyl)bis(2-hydroxy-5,1-phenylene)bis(diazene-2,1-diyl)dibenzenesulfonic acid) and its six dimeric stable complexes were synthesized and their structures have been confirmed by elemental analysis, IR, electronic, 1 H NMR and ESR spectra.The stability of the complexes was confirmed from the thermal study.The kinetic parameters such as activation energy (E a ) and order of reaction (n) were determined from the thermal decomposition data.The XRD study shows that the Cd(II) complex is orthorhombic in nature.The theoretical study using DFT/B3LYP supports experimental evidences of the bonding sites of the ligand, geometry and the stability of the complexes.The quantum chemical parameters reported here will be helpful to the future investigators working in the field of drug designing.

Figure 1 . 1 H
Figure 1. 1 H NMR Spectrum of LH 2 .Powder XRD studiesThe XRD (powder pattern) of the complex [Cd 2 LCl 2 (H 2 O) 2 ] was indexed in X-ray diffractometer with Cu as anode material, K -alpha [Å] = 1.54060 and the generator settings 30 mA, 40 kV.The prominent peaks (Figure2) of the diffraction pattern were indexed and analysed by the computer programme LSUCRPC[34].The lattice parameters (a, b, c, α, β, γ and V (volume) are shown in (Table2) along with Miller indices h k l.The indexing is confirmed by comparing with observed and calculated 2θ values that are evident from the figure of merit 5.8 as suggested by de Woulff[35].The observed and calculated 2θ values are also in good agreement.The density (d) 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 (n) is calculated from the relation: n = dNV/M where d = density of the compound, N = Avogadro's number, V = volume of the unit cell and M = molecular weight of the complex.The value of 'n' is found to be '1' that agrees well with the suggested orthorhombic structure of the complexes.The mean particle size of the same complex can be measured from the Debye Scherrer equation:

Table 1 .
Electronic spectra and magnetic moment data.

Table 2 .
X-ray diffraction data of the complex.

Table 5 .
Calculated quantum chemical parameters for ligand and its metal complexes.