Synthesis, Spectral Characterization, Cyclic Voltammetry, Molecular Modeling and Catalytic Activity of Sulfa-Drug Divalent Metal Complexes

Safaa N Abdou1, Abeer A Faheim1,2 and Abdel-Nasser MA Alaghaz3,4* 1Chemistry Department, College of Education and Science (Khurma), Taif University, Al-khurma, Taif, Saudi Arabia 2Chemistry Department, Faculty of Science (Girl’s), Al-Azhar University, P.O. Box 11754, Nasr-City, Cairo, Egypt 3Chemistry Department, Faculty of Science (Boy’s), Al-Azhar University, P.O. Box 11754, Nasr-City, Cairo, Egypt 4Chemistry Department, Faculty of Science, Jazan University, Jizan, Saudi Arabia


Introduction
Currently available methods for the removal of phenol/phenolic compounds from wastewaters (chemical oxidation, reverse osmosis, adsorption and others) are expensive, have regeneration problems and may produce themselves wastewaters with a high environmental impact [1,2]. Particularly contaminating waste waters are those generated by textile and paper mill industries. These wastewaters include medium to low concentrations of dyes or pigments. The degradation of dyes is one of the most important research fields in wastewater treatments. Researchers have recently focused on enzymatic treatments. Many peroxidases such as lignin peroxidase, manganese peroxidase, soybean peroxidase, horseradish peroxidase (HRP), laccase, polyphenol oxidases, micro peroxidases and azo peroxidases have been used for the removal of dyes in industrial effluents [3,4]. One of the most studied substrates is phenol, which is frequently used as a simple model compound of more complex pollutants such as dyes, pigments and others. Among the most toxic phenolic compounds are the chloro-or nitro-substituted phenols. The compounds are used as pesticides and anti-bacterial [5]. Phenol is present in wastewaters discharged by resin manufacturing, petrochemical, oil-refining, paper mill, coking, and irons melting industries [6]. Phenol derivatives include anthraquinone dyes, an important group of dyes. Phenolic groups, besides being part of many dyes and pigments, are also the main moiety of lignin. Nowadays, high amounts of ligno-cellulosic wastes from paper and wood industries are generated, of which only 1-2% are reused. Therefore, their accumulation represents a serious environmental problem. Moreover, high-valuable products potentially obtainable from lignin degradation are misspent [7]. The enzymatic complex (Liperoxidase, Mn-peroxidase and laccase) produced by white-rot fungi is able to degrade lignin up to mineralization. Hence, the application of well-known, commercially available and robust enzymes such as HRP is an attractive approach for lignin degradation. Recent studies on totally chlorine-free processes for pulping and bleaching involve the use of oxygen, ozone or hydrogen peroxide as oxidants, and enzymes or biomimetics as catalysts [8]. There are three main research fields in the heterogeneous catalytic degradation of phenols: the catalytic wetperoxide oxidation [9], the catalytic ozonation [10] and the catalytic wet oxidation [11]. The catalysts used in wet-peroxide oxidation include metal-exchanged zeolites, hydrotalcite-like compounds, metalexchanged clays and resins. The catalysts used in catalytic wet oxidation are transition metal oxides and supported noble metals [12].
The development of new methodologies for the deposition of high-j materials based on HfO 2 is of increasing importance for applications in microelectronics [13]. Generally, these depositions are performed using metal-organic chemical vapor deposition (MOCVD), which requires precursors with very special characteristics as thermal stability, high volatility, and low cost/toxicity [14]. The precursor of choice for the deposition of HfO 2 is most often tetrakis (dimethylamido) hafnium(IV) [14]; however, its high reactivity and premature decomposition issues have led to interest in the development of new precursors with the potential of fine-tuning their volatility characteristics.
Thus, in this paper we synthesized a new tridentate Schiff base containing N 2 O donor atoms and its cobalt(II), nickel(II), copper(II), zinc(II) hafnium(II) complexes. The HL ligand and its complexes were characterized by the FT-IR, 1H NMR, 13C NMR, mass, SEM, EDX, UV-Vis spectroscopy, elemental analysis, magnetic susceptibility, molar conductance and thermal analysis. The geometry of the complexes is characterized by means of spectral and magnetic measurements.

Materials and Methods
Metal salts, Sulfathiazole, phenyldichlorophosphine, were either Aldrich, BDH or Merck products. EDTA disodium salt, ammonium hydroxide, mureoxide and nitric acid were either BDH or Merck products. Organic solvents (methanol, absolute ethanol, diethylether, dimethylformamide (DMF) and dimethylsulfoxide (DMSO)) were reagent grade chemicals and were used without further purification.

Analytical and physical measurements
The percentage of carbon, hydrogen, nitrogen and sulfur contents were analysed using Carlo Erba 1108 model elemental analyser using sulphanilamide as a reference standard. Analysis of the metal(II) ions followed the dissolution of the solid complexes in concentrated HNO 3 , neutralizing the diluted aqueous solutions with ammonia and titrating the metal solutions with EDTA. Infrared spectra were recorded by using a 1% of the sample on KBr pellet with 16 scans and 2 cm -1 resolutions in a Jasco FT-IR/4100 spectrophotometer equipped with ATR accessory in the range of 4000-400 cm -1 . The electronic spectra of the complexes in UV-Vis region were obtained in DMSO solutions using a Shimadzu UV-1601 spectrophotometer in the range of 200-800 nm. Magnetic susceptibility measurements were computed on a modified HertzSG8-5HJ model Gouy magnetic balance using CuSO 4 .5H 2 O as the calibrant. Molar conductance of the complexes were studied in 10 -3 M DMSO solution, using a deep vision model 601 digital direct reading deluxe conductivity meter at room temperature. The 1H NMR spectrum of OIAC was investigated with a Bruker 300 Avance DRX 300 FT-NMR spectrometer in 1% HCl/D 2 O solution using TMS as standard. 31P NMR spectra were run, relative to external H 3 PO 4 (85%), with a Varian FT-80 spectrometer at 36.5 MHz. Thermal studies (Thermo gravimetric (TG) analysis) were manipulated under a dynamic N2 atmosphere in the 20-800ºC range at a heating rate of 10ºC min -1 on a Mettler Toledo star system. X-ray powder diffraction determinations were accomplished using an X-ray diffractometer ((XPERT PRO PAN alytical, Netherland)) for phase identification. The patterns were run with Cu Ka radiation with a secondary monochromator (k=0.1545 nm) at 40 kV and 30 mA. The surface morphology of chitosan, OIAC and the complexes were observed using a scanning electron microscope of model SEM-JSM 6390 at an accelerating voltage of 18 kV with a magnification range of 5 KX at liquid nitrogen temperature. Cyclic voltammograms were obtained on a CHI-600A electrochemical analyzer using a three electrode set-up comprising of a glassy carbon working, platinum wire auxiliary and a Ag/AgCl reference electrode under oxygen free conditions. A ferrocene/ferrocenium(1+) couple was used as an internal standard and E1/2 of the ferrocene/ferrocenium (Fc/Fc+) couple under the experimental condition is found to be 470 mV. TBAP was used as a supporting electrolyte. Molar conductivity was measured by using an Elico digital conductivity bridge model CM-88 using freshly prepared solution of the complex in acetonitrile. The hydrolysis of 4-nitrophenylphosphate by the metal(II) complexes were studied in a 10 −3 M dimethylformamide solution. Scanning electron microscopy (SEM) images were taken in Quanta FEG 250 equipment. The reaction was followed spectrophotometrically and the hydrolysis of 4-nitrophenylphosphate was monitored by following the UV-Vis absorbance change at 420 nm (assigned to the 4-nitrophenolate anion) as a function of time. A plot of log (Aα/Aα −At) versus time was made for all the complexes. EPR spectra of the complexes were recorded at Elexsys, E500, Bruker company.

Isolation of ligand
Sulfathiazole [N1-4-amino-thiazolylbenzenesulfonamide] (0.1 mol) in 100 ml cold dry benzene was added in small portions to a well stirred cold solution of dichlorophenylphosphine (0.l mol) in 100 ml cold dry benzene during half an hour at 15°C under dry conditions. After completion of the reaction (HCl gas ceased to evolve), the reaction mixture was filtered while hot and the solid obtained was washed several times with dry benzene, diethyl ether and dried in vacuo to give the corresponding 4-(phenylphosphinylideneamino-Nthiazolyl benzene-sulfonamide (HL) (Figure 1).

Oxidation reactions
The aerobic oxidation reactions were carried out in a 25 ml flask at room temperature and at 70 ºC under atmospheric pressure conditions. 0.05 g of the catalyst ([Co(L) 2 ) was taken in 10 ml of acetonitrile. To this, 10 mmol of the oxidant, 30% H 2 O 2 solution and 5 mmol of cyclohexane were added successively and the reaction solution was magnetically stirred for 8 and 12 h. Aliquots of the reaction mixture were taken separately at 8 h and 12 h for product analysis. The blank experiments were also run individually without catalyst and oxidant by following the same reaction procedure. All samples were analysed by Hewlett-Packard gas chromatography (HP 6890) having FID detector, a capillary column (HP-5), with a programmed oven temperature from 50 to 200ºC and a 0.5 cm 3 min -1 flow rate of N 2 as a carrier gas. The conversion of cyclohexane and selectivity for cyclohexanol and/or cyclohexanone was calculated as follows: Conversion % of cyclohexane=100×[Initial% -Final%]/ Initial%Selectivity)100)×[GC Peak area % of cyclohexanol and/or cyclohexanone] ×Σ Peak area of total products.

Results and Discussion
The ligand In the 13C NMR spectrum three peaks at 165.9 ppm and 168.3 ppm, 25.6 ppm and 7.7 ppm were observed, being assigned to C=N and C-S respectively. In the 31P NMR spectrum the 31P (C=N) signal is observed at 25.6 ppm.
Further insight concerning the structure of the ligand is obtained from IR, UV-vis. The IR and UV-vis measurements, of HL ligand will be discussed with its metal complexes.

Metal complexes and characterization
IR spectra and mode of bonding: In the absence of a powerful technique such as X-ray crystallography, IR spectra have proven to be the most suitable technique to give enough information's to elucidate the nature of bonding of the ligand to the metal ion. The IR spectra of the free ligand and metal complexes were carried out in the range 4000-400 cm -1 .
The ν (C=N) of the thiazole ring occurs at 1620 cm −1 after complexation indicating the coordination of the thiazole nitrogen to metal ions [15]. In addition, the ligand exhibits two bands at 1314 and 1166 cm −1 due to ν asym (SO 2 ) and ν sym (SO 2 ) stretching vibrations, respectively. Also, it has a band at 3412 cm −1 which attributed to ν (NH). The bands due to asymmetric and symmetric SO 2 group are shifted to lower frequencies upon complexation. While the ν (NH) is disappeared or hidden under the broad bands at 3450-3300 cm −1 in the spectra of the complexes as the result of the presence of coordinated molecules which is turns make it difficult to confirm the enolization of the sulfonamide group. In the far IR spectra of all the complexes, the non-ligand bands observed at 500-503, 455-459 and 412-415cm -1 regions can be assigned to ν (M-O), ν (M-N) and ν (M-Cl), respectively [16]. From the infrared spectra, it is apparent that, the chelation of the divalent metal ions to the ligand occurs from the HL ligand through the oxygen atom of the sulfonamide group and the nitrogen atom of the thiazole ring in the ligand.
NMR spectra: Unfortunately, the insolubility of either Zn(II) or Hf(II) complexes in CDCL 3 , CD 3 COCD 3 or DMSO-d6 make it difficult to carry out 1 H NMR, 13C NMR and 31P NMR spectra of the complexes to further clarify the way of binding of HL ligand to the metal ions.

Molar conductance measurements:
The observed very low molar conductance of the complex in DMSO (10-2M) solution at room temperature was consistent with non-electrolyte nature of the complexes [17]. Thus the complexes may be formulated as [M(L) 2 (Cl) 2 ], where M=Co(II), Ni(II), Cu(II), Zn(II) and Hf(II), L=ligand.

Mass spectra:
The mass spectrum of the Ni complex showed a molecular ion NiC 30 H 24 Cl 2 N 6 NiO 4 P 2 S 4 ]+ peak at m/z 852.36 amu and Hf(II) complex showed molecular ion peak at m/z 676 amu. The calculated mass of Hf(II) complex was 972.15 amu, therefore, the molecular ion peak may be corresponding to the M+ peak. The observed data were in good agreement with the proposed molecular formula that is [M (C 30 H 24 Cl 2 N 6 NiO 4 P 2 S 4 ) Cl 2 ] and suggest the monomeric nature of the complexes. In addition to the molecular ion peaks, the spectra exhibit other peaks assignable to various fragments arising from the thermal cleavage of the complexes.
The observed magnetic moment of the Cu(II) complex is 2.21 B.M., which confirms the octahedral structure of this complex [15,18]. For octahedral Cu(II) complex, the expected transition is 2B1g→2A1g with respective absorption at 15,398 cm −1 . Due to Jahn-Teller distortions, Cu(II) complexes give a broad absorption between 600 and 700 nm.
Ligand field parameters: Various ligand field parameters are calculated for the complexes ( Table 1). The value of Dq in Co(II) complexes were calculated from transition energy ratio diagram using the υ3/υ2 ratio [18]. The nephelauxetic parameter β was readily obtained by using the relation β=B (complex)/B (free ion), where B (free ion) for Ni(II) is 1042 cm −1 and for Co(II) is 1117 cm −1 [18]. The value of β lies in the range 0.47-0.95. These values indicate the appreciable covalent character of metal ligand σ bond. The g values are almost equal to free electron g value.

EPR spectrum of Cu(II) complex:
The spectrum of the Cu(II) complex exhibits two broad band with g||=2.18 and g ┴ 2.06 so, g|| >g ┴ > 2.0023, indicating that the unpaired electron of Cu(II) ion is localized in the dx 2 -y 2 orbital. In axial symmetry, the g values are related to the G-factor by the expression G=(g||-2)/ (g┴-2)=4. The G values of the Cu(II) complex are<4 suggesting that the considerable exchange interaction in the solid state. Further, the shape of the ESR spectrum of Cu(II) complex indicates that the geometry around the Cu(II) ions are elongated octahedron. The lower value of α2 (0.46) compared to β2 (1.04) in Cu(II) complex indicate that the covalent in-plane σ-bonding is more pronounced than the covalent in-plane π-bonding character [18].

Kinetics of thermal decomposition:
In order to characterize the metal complexes more fully in terms of thermal stability, their thermal behaviors were studied. In the present investigation, the correlations between the different decomposition steps of the complexes with the corresponding weight losses are discussed in terms of the proposed formula of the complexes. The weight losses for each complex are calculated within the corresponding temperature ranges.
The [Hf(L) 2 (Cl ) 2 ] complex with the molecular formula [Hf C 30 H 24 Cl 2 HfN 6 O 4 P 2 S 4 ] is thermally decomposed in four successive steps. The first estimated mass loss 24.42% (calculated mass loss=25%) within the temperature range 58-186ºC can be attributed to the loss of (C 12 H 10 P 2 N 2 ) fragment. The DTG curve gives an exothermic peak at 214ºC (the maximum peak temperature). The second estimated mass loss of 15.39% (calculated mass loss=15.63%) within the temperature range 186-324ºC could be attributed to the liberation of (C 12 H 8 ) fragment. The DTG curve gives an exothermic peak at 333ºC (the maximum peak temperature). The third estimated mass loss 38.98% (calculated mass loss=39.22%) within the temperature range 324-589ºC can be attributed to the loss of (C 6 H 6 N 4 S 4 O 3 Cl 2 ) fragment. The DTG curve gives an exothermic peak at 563ºC (the maximum peak temperature). The fourth step occurs within the temperature range 589-798ºC with an estimated mass loss 19.94% (calculated mass loss=19.95%), which is reasonably accounted for the loss of rest of the ligand molecule (C 11 H 13 N 2 ), leaving HfO as residue with total estimated mass loss of 78.52% (calculated mass loss=79.86%). The DTG curve gives an exothermic peak at 675ºC (the maximum peak temperature).
The thermodynamic activation parameters (Table 2) of decomposition processes of the metal (Co(II), Ni(II), Cu(II) and Zn(II)) complexes namely activation energy (E * ), entropy (ΔS * ) and Gibbs free energy change of the decomposition (ΔG * ) were evaluated graphically by employing three methods, Coats-Redfern [19] (CR), Horowitz-Metzger [20] (HM), and Piloyan-Novikova [21] (PN). From the results obtained, the following remarks can be pointed out: (1) The high values of the energy of activation, Ea of the complexes reveal the high stability of such chelates due to their covalent bond character [22].
(2) The positive sign of ΔG for the investigated complexes reveals that the free energy of the final residue is higher than that of the initial compound, and all the decomposition steps are non spontaneous processes. Also, the values of the activation, ΔG increases significantly for the subsequent decomposition stages of a given complex. This is due to increasing the values of TΔS significantly from one step to another which overrides the values of ΔH [23].
(3) The negative ΔS values for the decomposition steps indicate that all studied complexes are more ordered in their activated states [24].
Powder XRD: Single crystals of the complexes could not be prepared to get the XRD and hence the powder diffraction data were obtained for structural characterization. Structure determination by X-ray powder diffraction data has gone through a recent surge since it has become important to get to the structural information of materials, which do not yield good quality single crystals. The indexing procedures were performed using (CCP4, UK) CRYSFIRE program [18] giving tetragonal crystal system for [Co(L) 2 (Cl) 2 ] having M(9)=11, F(6)=8, cubic crystal system for [Ni(L) 2 (Cl) 2 ] having M(6)=13, F(6)=7 and tetragonal crystal system for [Cu(L) 2 (Cl) 2 ] having M(6)=19, F(6)=8, as the best solutions. Their cell parameters are shown in Table 3.    that the residues majorly consist of metal (cobalt, nickel, copper and hafnium), carbon, phosphorus and sulfur. The SEM micrographs of ligand and its complexes are shown in Figure 3a-f. The Co(II) complex shows bar like structure. The Ni(II) complex shows faceted microcrystal. Agglomerated morphology was seen for the Cu(II) complex. For Hf(II) complex bar with layered structure was present.

SEM and EDX spectra:
Cyclic voltammetry: The cyclic voltammogram (Figure 4) of metal(II) complexes were recorded in DMSO at room temperature. The Co(II) and Ni(II) complexes showed well distinguished cathodic peak in the range of -622 mV and the corresponding anodic peak at the range of -947 mV. The measured ΔEp values (ΔEp=-325) clearly indicated that these redox couples were found to be less stable. The Cu(II) complex also showed redox couples with anodic peak at -668 mV and cathodic peak -497 mV respectively and ΔEp: -188 mV. The ipa/ipc values for complexes cobalt and copper 1.12 and 2.07 respectively, which clearly confirmed the involvement of one and two electron redox process. The measured ΔEp values for these complexes are -325 and -188 mV which clearly indicated that the redox couples are quasi-reversible process.   [25]. Ligand containing metal ion was optimized using molecular mechanic methods. Several cycles of energy minimization had to be carried out for each of the molecules. The root mean square gradient for the molecules was less than one.
The Co(II) complex has octahedral geometry with auto optimized energy 1438.32 kJ/mol. The equatorial positions being occupied by the four N atoms. The equatorial Co(II)-N distance being 1.87 Å. The axial position is occupied by chlorine ligand at Co(II)-Cl distance of 2.37 Å. These values are close to the ideal distance of 1.86 Å and 2.32 Å, respectively. The N-Co-O and Cl-Co-Cl angles are 86.26 Å and 89.75 Å, respectively ( Figure 5).
The Hf(II) complex also has octahedral geometry with auto optimized energy 2014.74 kJ/mol. The equatorial Hf(II)-N distance being 1.81 Å. The axial position is occupied by chlorine ligand at Hf(II)-Cl distance of 2.38 Å. These values are close to the ideal distance of 1.86 Å and 2.37 Å, respectively. The N-Hf-O and Cl-Hf-Cl angles are 83.17 Å and 87.45 Å, respectively ( Figure 6).
Finally, the previous findings indicated that the coordination

Conclusion
In the present research studies, our successful efforts are in contribution in the field of mycology to design and development of novel molecular systems. On the basis of various physico-chemical and spectral data presented and discussed above, the complexes may tentatively be suggested to have octahedral geometry. The magnetic studies of the complexes account for their high spin nature due to the presence one unpaired electron metal(II) complexes. The catalytic activities of the divalent metal complexes have been studied in the oxidation of cyclohexane by hydrogen peroxide, an environmental friendly oxidant. The molecular parameters of the ligand and its Co(II) and Hf(II) complexes have been calculated.