Synthesis, Spectroscopic Characterization, and Biological Activities of Metal Complexes of 4-((4-Chlorophenyl)diazenyl)-2-((p-tolylimino)methyl)phenol

Azo Schiff base complexes of VO(II), Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) have been synthesized from 4-((4chlorophenyl)diazenyl)-2-((p-tolylimino)methyl)phenol (CDTMP). e nature of bonding and the structural features of the complexes have been deduced from elemental analysis, molar conductance, magnetic susceptibility measurements, IR, UV-Vis, H-NMR, EPR, mass, SEM, and �uorescence spectral studies. Spectroscopic and other analytical studies reveal square-planar geometry for copper, square-pyramidal geometry for oxovanadium, and octahedral geometry for other complexes.e EPR spectra of copper(II) complex in DMSO at 300K and 77K were recorded, and its salient features are reported. Antimicrobial studies against several microorganisms indicate that the complexes are more potent bactericides and fungicides than the ligand. e electrochemical behavior of the copper(II) complex was studied by cyclic voltammetry. All the synthesized compounds can serve as potential photoactive materials as indicated from their characteristic �uorescence properties. e second harmonic conversion efficiency of the synthesized azo Schiff base was found to be higher than that of urea and KDP (potassium dihydrogen phosphate). SEM image of copper(II) complex implies the crystalline state and surface morphology of the complex.


Introduction
Azo compounds are the largest class of industrially synthesized organic dyes due to their versatile application in various �elds, such as dyeing textile �ber, biomedical studies, and advanced application in organic synthesis and high technology areas such as laser, liquid crystalline displays, and electrooptical devices [1].e oxidation-reduction behaviors of these compounds play an important role in its biological activity [2,3].e architectural beauty of these coordination complexes arises due to the interesting ligand systems containing different donor sites [4].Azomethine derivatives are widely applied in biological systems and dyes [5].In recent years, there is considerable interest in the synthesis and modeling of new organic materials with optical nonlinearities because of their potential use in development of photonic devices [6].
Azo ligands containing salicylaldimine-based side chains can be used in the production of chemical sensors because they show a signi�cant change of color of the solution and maxima of the absorption band when they interact with transition metal atoms.Transition metal-complexed azomethine compounds have been studied in great detail in the literature for understanding their optical and electronic properties, structure-redox relationships, and mesogenic characteristics [7,8].It could be expected that design and synthesis of transition metal complexes derived from azo Schiff base ligands might be a possible way to obtain the applicable nonlinear optical materials [9].In particular p-nitroaniline, m-nitroaniline, p-nitro-N,N-dimethylaniline, and other similar choromophores were found to exhibit rather large NLO properties [10].Although many Schiff bases derived from aromatic aldehydes and amines and their corresponding metal complexes have been extensively studied, relatively few azo-linked Schiff bases and their metal complexes have been reported [11,12].
1.1.Scope.Design and synthesis of transition metal complexes derived from azo Schiff base ligands might be a possible way to obtain the applicable nonlinear optical materials, endowed with antiviral activity, optical activity, electronic activity, and so forth.e electron-pull and electron-push substituents on the benzene rings have much in�uence on the electronic structure of azo compounds to possess a larger nonlinear optical susceptibility.Because of the importance of azo-containing compounds and our interest in syntheses of nonlinear optical materials, we report herein the syntheses, spectroscopic studies and biological activities of azo Schiff base derivative 4-((4-chlorophenyl)diazenyl)-2-((p-tolylimino)methyl)phenol (CDTMP) which have donor and acceptor property due to electron-pull and electron-push substituents (Cl and CH 3 groups) and their VO(II), Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes.We are also interested to examine the antimicrobial, �uorescence, NLO, and redox properties of these azo Schiff base compounds.

Experimental
e chemicals and solvents were puri�ed by standard methods.Elemental analyses were carried out using a Perkin-Elmer 2400 II elemental analyzer.Molar conductance of the complexes was measured in DMSO at room temperature using a Systronic Conductivity Bridge 304.Magnetic susceptibility of the complexes was performed on a Sherwood MSB mark 1 Gouy balance.Infrared spectral studies were carried out on a Shimadzu FT IR 8000 spectrophotometer using KBr discs.UV-Vis spectra were obtained using a ermo Spectronic 6 Hexios , and �uorescence spectra were determined with an Elico SL174 Spectro�uorometer.NMR spectra were recorded on Bruker DRX-300, 300 MHz NMR Spectrometer using TMS as reference.EPR spectra of the Cu(II) and VO(II) complexes were recorded in Varian E-112 machine at 300 and 77 K using TCNE (Tetracyanoethylene) as the g-marker.Cyclic voltammetric measurements for Cu(II) and VO(II) complexes in DMSO were carried out on Electrochemical Analyzer CH Instruments (USA) using a three-electrode cell containing an Ag/AgCl reference electrode, Pt wire auxiliary electrode, and glassy carbonworking electrode with tetrabutylammonium perchlorate as supporting electrolyte.Electron-ionization (EI) mass spectra were recorded by JEOL-GC Mass Spectrometer MATE-2.

Synthesis of Azo Ligand (CDTMP)
. e diazonium salt, 5-((4-chlorophenyl)diazenyl)-2-hydroxybenzaldehyde (CPDA Z HB) [13] (2.3 g, 8.7 mmol), was condensed with p-toluidine (0.94 g, 8.7 mmol) in ethanol and re�uxed for 3 h.Aer the reaction was over, the reaction mixture was concentrated to one-half of the volume and refrigerated for one day.e obtained bright orange-colour solid product was �ltered and recrystallized from ethanol.e yield obtained was 77%, m.p. 212 ∘ C. (Scheme 1).   .e solution was stirred for 1 h at room temperature, and the coloured complexes precipitated upon concentration was �ltered, washed with ethanol, and dried under vacuum.e oxovanadium complex was synthesized from the ligand and oxovanadium sulfate by adopting the above method under re�ux for 3 h and concentrating the solution through rotoevaporation.e yield obtained was 60-70%.

Results and Discussion
e analytical data and physical properties of CDTMP and metal(II) complexes are presented in Table 1.e complexes are partially soluble in ethanol, chloroform, and acetonitrile but soluble in polar coordinating solvents such as DMF and DMSO.e molar conductivities of all the complexes are in the 8.9-11.2Ω −1 cm 2 mol −1 range indicating a nonelectrolytic nature.e obtained data showed that the stoichiometry of the chelates is 1 : 2 (M : Ligand) and the analytical data of the complexes are in good agreement with the general formula ML 2 for Cu(II), VO(II), and ML 2 ⋅2H 2 O for Mn(II), Ni(II), Co(II), and Zn(II) complexes.

Mass Spectral Studies
. e mass spectra of the ligand CDTMP (  ) and its complexes were recorded at room temperature, and they are used to compare their stoichiometry composition.e azo Schiff base shows a molecular ion peak at 349 .Molecular ion peak for the nickel complex observed at 790  con�rms the stoichiometry of metal chelates as [ML 2 ⋅2H 2 O] type, whereas the molecular ion peak for the Cu(II) and VO(II) complexes observed at  759, 764 con�rms the stoichiometry of metal chelates as [ML 2 ] type.e mass spectra of the ligand and its nickel(II) complex are shown in Figure 1.3.2.IR Spectral Studies.e characteristic IR spectral data of the ligand and its complexes are given in Table 2. e detected band at 1616 cm −1 is assigned to the stretching vibration of the azomethine group of the ligand CDTMP.is band is shied in the complexes toward lower frequencies (1608-1600 cm −1 ) because of the participation of the nitrogen atom to coordinate with metal ion.is fact can be explained by the withdrawing of electrons from nitrogen atom to the metal ion due to coordination process [14].e bands around 3130 cm −1 and 1284 cm −1 are assigned to (O-H) vibration and the phenolic C-O vibration, respectively.However, in the spectra of the metal complexes the C-O bands underwent a shi towards higher frequencies ∼ 33 cm −1 , and this shi con�rms the participation of oxygen in the C-O-M bond.Mn(II), Co(II), Ni(II), and Zn(II) complexes show a broadband at ∼3400 cm −1 due to the vibrations of coordinated water molecules [15].e band at 1483 cm −1 is assigned to the stretching vibration of the diazo group of the azo-linked Schiff base ligand, and the spectra of the complexes did not show any frequency shi of the -N=N-band, which may be explained by nonparticipation of azo group in complex formation [16].e band of medium intensity observed for the complexes in the region 525-594 cm −1 is attributed to  (M-N) and in the region 425-490 cm −1 to  (M-O) [17].All the IR data of the Schiff base and its metal complexes imply that the Schiff base behaves as bidentate and is bonded to the metal ion through phenolic oxygen and imino nitrogen of azomethine group.e IR spectra of the ligand and cobalt complex are depicted in Figure 2.  of this complex. 1H NMR spectrum of CDTMP is shown in Figure 3.

Electronic Spectral and Magnetic Susceptibility Studies.
e electronic spectra of CDTMP (  ) and its complexes were recorded in DMSO.e absorption spectrum of the ligand shows strong peaks at 26100 and 33670 cm −1 which may be assigned to    * (transitions of the -CH=Nand -N=N-groups) and    * (transition of -N=Nazo group) transitions, respectively.e spectral data of the ligand and its complexes with molar extinction coefficient or molar absorption coefficient  (L mol −1 cm −1 ) are calculated and given in Table 4.
e Zn(II) complex (  ) has been found to be diamagnetic in nature.On the basis of analytical, conductance, and spectral data, the Zn(II) complex of CDTMP is assigned to octahedral geometry [26].e Mn(II) complex (  ) under study has the magnetic moment value of 5.84 B.M. which supports an octahedral geometry around the metal ion [27].e ligand-�eld-splitting energy (10Dq), interelectronic repulsion parameter (), and covalency factor (nephelauxetic ratio) () for the octahedral Co(II) and Ni(II) complexes have been calculated using the secular equation given by Konig are shown in Table 3 [28].e ligand-�eld parameters of Ni(II) and Co(II) complexes are shown in Table 5, and the values support the proposed geometry for the synthesized Ni(II) and Co(II) octahedral complexes.e covalent factor  equal to / ′ for the complexes is less than one suggesting considerable amount of covalent character of the metal-ligand bonds.

Electrochemical
Behavior.e redox behavior of the copper(II) complex (  ) was performed in DMSO solution at room temperature with tetrabutylammonium perchlorate (TBAP) as supporting electrolyte; glassy carbon as working electrode; Pt wire as auxiliary electrode; Ag/AgCl as reference electrode in order to monitor spectral and structural changes accompanying electron transfer.e copper(II) complex exhibited one-step reduction peak at 0.122 V and two oxidation peaks at 0.812 and 0.352 V at the scan rate of 100 mV as shown in Figure 4.It has been observed that the  pc and  pa values change with scan rate as shown in Table 6.e Δ  values increase with increase in scan rate and is found to be greater than 400 mV which indicates that the nature of the electron transfer process is irreversible [29,30].
3.6.EPR Spectral Studies.e EPR spectrum of the copper(II) complex (  ) was recorded at 300 K and 77 K, and the spin Hamiltonian parameters of the Cu(II) complex are listed in Table 7. e observed spectral parameters for this compound are  ⟂ = 2.07,  ‖ = 2.15, and  iso = 2.09 which are evident that the unpaired electron lies predominantly in the   2 - 2 orbital as  ‖ >  ⟂ >   , and this shows the characteristic of square planar geometry [31].e  iso value is less than 2.3 indicating the covalent character of the metal-ligand bond [32].In the axial spectra, the -values are related with exchange interaction coupling constant () by the expression    ‖ − 2.0023/ ⟂ − 2.0023.According to Matsubayashi [33], if the value of  is larger than four, the exchange interaction is negligible because the local tetragonal axes are aligned parallel or slightly misaligned.If its value is less than four, the exchange interaction is considerable, and the local tetragonal axes are misaligned.For present copper complex, the  value is 2.1, which suggests that the exchange interaction is considerable, and the local tetragonal axes are misaligned.e  2 values fall in the range 0.72 indicating the presence of appreciable in-plane covalency [34].e magnetic moment of the copper(II) complex calculated using the relation  2  3/4|| 2 is found to be 2.01 B.M., and the orbital reduction factors  ‖ and  ⟂ were estimated from the expression  ‖  ( ‖ − 2.0023)Δ/8,  ⟂  ( ‖ − 2.0023)Δ/2,   −2 cm −1 (spin-orbit coupling constant for the free ion).For this complex, the observed order  ‖ (0.32) <  ⟂ (0.46) implying the presence of in-plane bonding.e EPR spectrum is shown in Figure 5.
Based on the previous discussions such as elemental analysis, molar conductance, EI mass, IR, magnetic susceptibility measurements, and electronic and EPR spectral studies, the following structure may be proposed for the complexes.e synthesized azo-azomethine ligand was coordinated to the metal ion through the azomethine nitrogen and oxygen atom of the hydroxyl group in salicylaldehyde.e geometry around the Cu(II) complex is square planar, VO(II) is square pyramidal, and the other metal(II) ion is in octahedral geometry and depicted in Scheme 2.

Biological
Activities. e antimicrobial activity of the azo Schiff base compound CDTMP and its complexes were tested in vitro by the well diffusion method [35] against the bacterial strains (Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus) and the fungal strains (Candida albicans and Aspergillus niger).Amikacin and Ketoconozole were used as reference compounds for antibacterial and antifungal activities, respectively.Stock solution (10 −3 M) was prepared by dissolving the compounds in DMSO.e diameter of the inhibition zones was measured in millimeters.Antimicrobial activities were performed in triplicate, and the average was taken as the �nal reading.e growth of inhibition zones aer incubation is shown in Table 8 and Figure 6.
e observed result indicates that most of the complexes have higher activity than the free ligand.Such increased activity of the complexes can be explained on the basis of Overtone's concept [36] and Tweedy's Chelation theory [37].Among the complexes, copper complex was found to be the most active and could inhibit the fungal growth at higher concentration.e greater activity of Cu(II) complex can be explained on the basis of their particle size [38].From scanning electron microscopy it is clear that copper complex has nanoparticle size.is smaller size may be the reason for increased biological activity.It has been suggested that the mode of action of the compounds may involve the formation of hydrogen bond through azomethine group with the active center of cell constituents resulting in interference with normal cell process.ese complexes also disturb the respiration process of the cell and thus block the synthesis of the proteins that restricts further growth of the organism.3.8.Fluorescence Studies.e photoluminescence properties of CDTMP and their complexes were studied at room temperature, and the data are reported in Table 9. e excitation spectra of ligand show a maximum at 353 nm and an emission maximum at  em = 376 nm.e metal(II) complexes show strong �uorescence with high-�uantum yield, and the excitation at 426-799 nm gives an emission at 442-804 nm, assigned to - * intraligand �uorescence �39, 40].Emission intensity of the metal(II) complexes is higher than that of free ligands, and the enhanced �uorescence e�ciency of the complexes is attributed to coordination increasing the rigidity, thereby reducing energy loss by thermal vibration  decay [41].Introduction of the substituent on the azo ligand has an interesting effect on the intensity of emission spectra.e �uorescence quantum yields [42] of the ligand and its complexes were also obtained.From this �uorescence study it is evident that these azo compounds might possess potential molecular sensing or photoconductive applications.Fluorescence spectrum of copper(II) complex is depicted in Figure 7.
3.9.NLO Property.e recent recognition is that the organic materials with conjugated -electron systems exhibit extremely large optical nonlinear responses, and this leads to rapid growth of interest in a �eld which is currently dominated by inorganic materials [43].e SHG (second harmonic generation) efficiency of the azo compound (CDTMP) was determined by modi�ed version of powder technique developed by Kurtz and Perry [44].e efficiency of the sample was compared with microcrystalline powder of KDP and urea.e input energy used in this particular setup is 2.2 mJ/pulse.Azo Schiff base shows the better second harmonic efficiency than urea and KDP.4-((4-Chlorophenyl)diazenyl)-2-((p-tolylimino)methyl)phenol shows 1.6 and 3.9 times more activity than urea and KDP, respectively.e electron-pull and electron-push substituents on the benzene rings have much in�uence on the electronic structure of azo compounds.e conjugative and induction effects of Cl atom as electron-pull groups and CH 3 as electron-push groups make electron density on the benzene rings change, and the band of N=N become an effective electronic passage.e delocatized -electron distribution of the azo dye makes it possible to possess a larger nonlinear optical susceptibility.Figure 8 depicts the SEM photograph of the synthesized Cu(II) complex.We noted that there is a uniform matrix of the synthesized complex in the pictograph and the image of copper complex shows a �ower-like shape with 6� nm particle size, clearly indicating that the Cu(II) complex is amorphous in agreement with the powder XRD results.e smaller grain sizes found from XRD and SEM suggest that these complexes are polycrystalline with nanosized grains.property in comparison with urea and KDP; hence the compound may be studied for optical sensing material.e ligand and all metal(II) complexes possess appreciable antibacterial and antifungal activities, and the copper(II) complex was found to be more potent than the other complexes due to the nanoparticle size.In general, all the synthesized compound can serve as potential photoactive materials, as indicated from their characteristic �uorescence properties.Based on these results, it could be proposed that these novel azo materials can be better accommodated for technological and pharmaceutical usage.

F 6 :
e inhibition zones formed for Pseudomonas aeruginosa and Candida albicans.

3. 10 .
Scanning Electron Microscopy.e morphology and particle size of the Schiff base metal complex have been illustrated by the scanning electron micrograph (SEM).SEM uses a focused beam of high-energy electrons to generate a variety of signals from the surface of a solid specimen.e signals reveal information about the sample, including external morphology, topography, chemical composition, crystalline structure, and orientation of materials making up the sample.