Nano-Azo Ligand and Its Superhydrophobic Complexes: Synthesis, Characterization, DFT, Contact Angle, Molecular Docking, and Antimicrobial Studies

Metal complexes of the 2,2’-(1,3-phenylenebis(diazene-2,1-diyl))bis(4-aminobenzoic acid) diazo ligand (H 2 L) derived from m phenylenediamine and p -aminobenzoic acid were synthesized and characterized by diﬀerent spectral, thermal, and analytical tools. The H 2 L ligand reacted with the metal ions Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) as 1:1 stoichiometry. All complexes displayed an octahedral geometry according to the electronic and magnetic moment measurements. The IR spectra revealed the binding of the azo ligand to the metal ions via two azo nitrogen atoms and protonated carboxylate O in a neutral tetradentate manner. Both IR and 1 H NMR spectra documented the involvement of the carboxylate group without proton displacement. The thermal studies pointed out that the complexes had higher thermal stability comparable with that of the free ligand. SEM images revealed the presence of the diazo ligand and its Cd(II) complex in a nanostructure form. The contact angle measurements proved that the Cd(II) complex can be considered as a superhydrophobic material. The molecular and electronic structure of H 2 L and [Cd(H 2 L)Cl 2 ].H 2 O were optimized theoretically, and the quantum chemical parameters were calculated. The biological activities of the ligand, as well as its metal complexes, have been tested in vitro against some bacteria and fungi species. The results showed that all the tested compounds have signiﬁcant biological activities with diﬀerent sensitivity levels. The binding between H 2 L and its Cd(II) complex with receptors of the crystal structure of S. aureus (PDB ID: 3Q8U), crystal structure of protein phosphatase (PPZ1) of Candida albicans (PDB ID: 5JPE), receptors of breast cancer mutant oxidoreductase (PDB ID: 3HB5), and crystal structure of Escherichia coli (PDB ID: 3T88) was predicted and given in detail using molecular docking.


Introduction
Azo compounds were highly colored compounds. ey had widespread use as dyes and pigments in a variety of applications including fibers, coloring a large variety of leather, clothing, food, toys, medical devices, plastics, cosmetics, and dyeing of textile, as well as nonlinear and photo electronics, especially in optical information storage, biological medical studies, and advanced applications in organic synthesis [1][2][3]. In the development of metal complexes, the design and synthesis of a ligand was the most important step. ey exhibited unique properties and novel reactivity due to electron-donor, electron-acceptor properties, structural, functional groups, and the position of the ligand in the coordination sphere [4,5]. Azo dyes with hydrophobic characters have wide applications especially when these dyes exhibit superhydrophobic characters. Some applications of water repellency include clothing that will be both breathable and water repellant, umbrellas that stayed completely dry, building materials, paints, epoxies, and silicones. Superhydrophobic paints and epoxies could greatly reduce the cost of transporting goods, also eliminate many of the effects of ice storms and aircraft icing. Low-permeability paint, used as a water and vapor barrier, is the standard way of protecting metal surfaces from corrosion. Superhydrophobic biomaterials are also being evaluated in more demanding biological applications, such as the prevention of blood coagulation and drug delivery [6,7].
New diazo ligand (H 2 L) has the IUPAC name 2,2'-(1,3phenylenebis(diazene-2,1-diyl))bis(4-aminobenzoic acid) (Scheme 1). is piece of work had devoted with the aim to synthesize Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) complexes with the new diazo ligand and to examine their physical properties involving spectral behaviors and the electrical conductance values, and to determine the efficiency of the synthesized complexes against pathogenic bacteria. e molecular structure and molecular docking were also carried out in order to illustrate the way of bonding of the azo ligand and its complexes with receptors of the crystal structure of S. aureus (PDB ID: 3Q8U), crystal structure of protein phosphatase (PPZ1) of Candida albicans (PDB ID: 5JPE), receptors of breast cancer mutant oxidoreductase (PDB ID: 3HB5), and crystal structure of Escherichia coli (PDB ID: 3T88). In view of these findings, the contact angle measurements were performed to study the hydrophobic characters of the prepared compounds.

2.1.2.
Solutions. 1 × 10 −3 M stock solutions of complexes were prepared by dissolving the appropriate amount of the complexes in DMF. It was used for conductivity measurement. For UV-Vis spectra measurement, 1 × 10 −4 or 1 × 10 −5 M solutions of the diazo ligand and metal complexes were prepared by accurate dilution from the previously prepared stock solutions.

Measurements.
Electronic and 1 H NMR spectra were carried out at room temperature using Shimadzu 3101PC spectrophotometer and 300 MHz Varian-Oxford Mercury, respectively. For 1 H NMR spectra, solution of DMSO-d 6 was used, and TMS was used as an internal standard. FT-IR spectra were recorded on a PerkinElmer 1650 spectrometer (4000-400 cm −1 ) in KBr pellets. Electron spin resonance spectra were also recorded on the JES-FE2XG ESR spectrophotometer at Microanalytical Center, Tanta University. Microanalyses of carbon, hydrogen, and nitrogen were carried out at Microanalytical Center, Cairo University, Egypt, using CHNS-932 (LECO) Vario Elemental Analyzer. Analyses of the metals were conducted by dissolving the solid complexes in concentrated HNO 3 and dissolving the residue in deionized water. e metal content was carried out using inductively coupled plasma atomic absorption spectrometry (ICP-AES), Egyptian Petroleum Research Institute. Mass spectra were recorded by the EI technique at 70 eV using the MS-5988 GS-MS Hewlett-Packard instrument at Microanalytical Center, National Center for Research, Egypt. e molar magnetic susceptibility was measured on powdered samples using the Faraday method. e diamagnetic corrections were made by Pascal's constant, and Hg[Co(SCN) 4 ] was used as a calibrant. Molar conductivities of 10 −3 M solutions of the solid complexes in DMF were measured using the Jenway 4010 conductivity meter. e thermogravimetric analyses (TG and DTG) of the solid complexes were carried out from room temperature to 800°C using a Shimadzu TG-50H thermal analyzer. e X-ray powder diffraction analyses were carried out by using Philips Analytical X-ray BV, diffractometer-type PW 1840. Radiation was provided by the copper target (Cu anode 2000 W) high-intensity X-ray tube operated at 40 KV and 25 mA, and SEM (scanning electron microscopy) for these samples was performed at the Egyptian Mineral Resources Authority, using SEM Model Quanta 250 FEG (field emission gun) attached with an EDX unit (energy-dispersive X-ray analyses), with accelerating voltage 30 KV, magnification 14x up to 1,000,000 and resolution for Gun.1n. Contact angle was measured by the contact-angle Attension eta model (version 2.7), Biolin Scientific company, Finland, Egyptian Nanotechnology Center (EGNC). e heavy phase was water, where the volume of drop was 4 mL, and the light phase was air. Divergence and the receiving slits were 1 and 0.2, respectively. e antimicrobial activities were carried out at Microanalytical Center, Cairo University, Egypt.
2.2. Synthesis of 2,2'-(1,3-Phenylenebis(diazene-2,1-diyl)) bis(4-aminobenzoic Acid) (H 2 L). m-Phenylenediamine (2 g/ 0.01 mol) was dissolved in 50 ml of ethanol, while 10 ml concentrated hydrochloric acid was diluted with about 60 g of crushed ice; then, dropwise addition of m-phenylenediamine solution to the crushed ice was carried out. To this cold solution, sodium nitrite (5 g/20 mL water) was added and stirred for about 1 h till dark reddish colored solution was obtained. e coupling agent p-aminobenzoic acid (5.6 g/0.02 mol) was dissolved in 50 ml of ethanol and then was added to the cold mixture. e resulting solution was stirred well, and sodium acetate (3 g) was added to neutralization.

Spectrophotometric Studies.
e absorption spectra were recorded for 1 × 10 −4 M solutions of the free diazo ligand and its metal complexes dissolved in DMF. e spectra were scanned within the wavelength range from 200 to 700 nm.

Antimicrobial Activity.
e antimicrobial activity was carried out using the diffusion agar technique [7,8]. LB agar media surfaces were inoculated with four investigated bacteria (Gram-positive bacteria: Bacillus subtilis and Staphylococcus aureus; Gram-negative bacteria: Neisseria gonorrhoeae and Escherichia coli) and one strain of fungi (Candida albicans) and then transferred to a saturated disk with a tested solution in the center of Petri dishes (agar plates). DMSO served as a control. All Petri dishes were incubated at 25°C for 48 h where clear or inhibition zones were detected around each disk. Antibacterial activity was calculated [9,10] by subtracting the diameter of the inhibition zone resulting with dimethylformamide from that obtained in each case. Amikacin and ketokonazole were used as reference compounds for antibacterial and antifungal activities, respectively. All experiments were performed as triplicate, and data plotted were the mean value.

Computational Methodology.
e electronic structure calculations of H 2 L and Zn(II) complex were carried out using the Gaussian09 suite of program [11]. ey were fully optimized employing DFT-based B3LYP method along with the LANL2DZ basis set. In order to incorporate the effect of the solvent around the molecule, the TD-DFT method (along with the LANL2DZ basis set) was used to calculate the electronic absorption spectra of the ligand and its Zn(II) complex.
e contribution of the molecular orbital to HOMO and LUMO was also calculated.

Molecular
Docking. Crystal structure of S. aureus (PDB ID: 3Q8U), crystal structure of protein phosphatase (PPZ1) of Candida albicans (PDB ID: 5JPE), receptors of breast cancer mutant oxidoreductase (PDB ID: 3HB5), and crystal structure of Escherichia coli (PDB ID: 3T88) were used in this study. Molecular docking studies were performed using MOE 2008 software [12] in order to find out the possible binding modes of the most active compounds against the above receptors. It is an interactive molecular graphics program for calculating and displaying feasible docking modes of a receptor and ligand and complex molecules. It necessitates the ligand and the receptor as the input in the PDB format. e amino acid chain was kept, and the water molecules and cocrystallized ligands were removed. e structure of complexes in the PDB file format was created by Gaussian03 software. Crystal structure of HSA (human serum albumin) (PDB ID: 5FUO), crystal structure of protein phosphatase (PPZ1) of Candida albicans (PDB ID: 5JPE), receptors of breast cancer mutant oxidoreductase (PDB ID: 3HB5), and crystal structure of Escherichia coli (PDB ID: 3T88) were downloaded from the Protein Data Bank (http://www.rcsb.org./pdb).

Characterization of the Free Diazo Ligand (H 2 L).
e synthesized diazo ligand (H 2 L) was soluble in DMF and DMSO solvents and of high melting point which indicated its stability. From the elemental analysis data of the ligand (Supplementary Table 1), it was found that the theoretical values were in agreement with the found values. e IR spectrum of the free diazo ligand was carried out in the range of 4000-400 cm −1 , and the most effective bands are listed in Supplementary Table 2. e broad band observed at 3433 cm −1 can be attributed to the ](O-H) stretching vibration. Also, band observed at 1660 cm −1 and sharp band at 1603 cm −1 can be attributed to C�O stretching vibration [13] and N�N [14], respectively. Also, the ](COO) asym and ](COO) sym stretching vibration bands were observed at 1543 and 1384 cm −1 , respectively.
Scheme 2: Synthesis of metal complexes of the diazo ligand (H 2 L). 1 H NMR spectrum of the free diazo ligand (H 2 L) in DMSO-d 6 exhibited the following signals: 7.25-7.91 ppm (m, 10H, Ar-H) [13], 4.78 ppm (s, 4H, NH 2 ) [15][16][17], and 10.12 ppm (s, 2H, COOH). ese signals indicated that three different types of protons were present in the diazo ligand. e position of NH 2 was confirmed from carrying out the spectra in the deuterated solvent. e mass spectrum of the ligand (H 2 L) showed an accurate parent molecular ion peak at m/z 404 amu, matched with the theoretical molecular weight. Also, the mass spectrum displayed multiple peaks corresponding to successive degradation of the ligand. e base peak appeared at m/z � 134 due to the [C 7 H 6 NO 2 ] + ion. e other fragments gave the peaks at 104, 132, 136, 164, 240, and 268 amu with various intensities.

Elemental Analyses.
e stoichiometry and formulation of the free diazo ligand (H 2 L) and its metal complexes were confirmed by their elemental analysis (Supplementary Table 1). e metal : ligand ratio was found to be 1 : 1 in all complexes, which has been arrived by estimating the carbon, hydrogen, nitrogen, chloride, and metal contents of the complexes. e elemental analyses of the ligand and its complexes revealed good agreement with the proposed structures. e ligand and its metal complexes have high melting points, and they were found to be air-stable. e diazo ligand was soluble in common organic solvents, and all the complexes were freely soluble in DMF and DMSO but insoluble in methanol, ethanol, and water.

Molar Conductance.
e results given in Supplementary Table 1 showed that the Cr(III) and Fe(III) complexes had molar conductivity values of 59 and 77 Ω −1 mol −1 cm 2 , respectively. It is obvious from these data that these chelates were ionic in nature, and they were electrolytes [18], but the molar conductivity values of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) chelates fall in the range of 11-31 Ω −1 mol −1 cm 2 indicative of their nonelectrolytic nature [14].

IR Spectral Studies.
e infrared spectra of the H 2 L ligand and its metal complexes taken in the region 4000-400 cm −1 are listed in Supplementary Table 2 which gave the most useful assignments for bands diagnostic to the mode of coordination of the ligand. e band observed at 1603 cm −1 in the free diazo ligand (H 2 L) was assigned to the ](N�N) stretching vibration. is band was found in the spectra of the complexes at 1610-1628 cm −1 . Also, another band observed at 1660 cm −1 assigned to carbonyl of carboxylate ](C�O) at the free diazo ligand (H 2 L) was found in the spectra of the complexes at 1649-1656 cm −1 (Supplementary Table 2). e shift in the azo groups of the diazo ligand confirmed the participation of the azo group in chelation.
e broad band at 3433 cm −1 observed in the diazo ligand was shifted to 3423-3450 cm −1 , which was attributed to the ](O-H) stretching vibration. is shift can be attributed to the bond formation with metal ions and also the presence of hydrated water molecules. e broadening of this band may be attributed to the presence of the intramolecular hydrogen bond [19]. e ] asym (COO − ) and ] sym (COO − ) stretching vibrations were observed at 1543 and 1384 cm −1 , respectively, for the free diazo ligand (H 2 L) [19][20][21][22][23][24][25][26]. Coordination with metal ions via the carboxylate O atom and nitrogen atom was indicated by the shift in the position of the ] asym (COO) and ] sym (COO) stretching vibration bands to 1534-1555 and1384-1402 cm −1 , respectively, for ligand-metal complexes.

1 H NMR Spectral Studies.
e 1 HNMR spectra of the diazo ligand and its Zn(II) complex were recorded in DMSO-d 6 solution using tetramethylsilane (TMS) as the internal standard. 1 H NMR spectrum of the ligand revealed the following signals at δ � 4.78 (m, 4H, NH 2 ), 7.05-7.94 (m, 10H, Ar-H), and 10.18 ppm (s, 2H, COOH). e shift in the carboxylic acid protons (10.40 ppm) indicated that oxygen of (COOH) groups was coordinated to the Zn(II) ion. However, the singlet signal of the amino group that appeared at δ 4.78 ppm (s, 4H, NH 2 ) means that the two singlet signals assigned to the two NH 2 protons remained in their position as in the free diazo ligand (H 2 L), confirming the remaining of these groups inert towards coordination. e upfield shift in the spectrum of the complex relative to the free ligand can be attributed to the effect of metal ions in chelation.

Mass Spectroscopy.
e electronic mass spectrum of the [Zn(H 2 L)Cl 2 ].H 2 O complex confirmed formation of this complex by showing a molecular ion peak at m/z � 540.81 g/ mol corresponding to (M-H 2 O), which was very close to the expected molecular weight which was 540.50 g/mol, and it also showed a peak at m/z � 403 amu, respectively, corresponding to the ligand moiety. e intensities of these peaks showed the stability of the complex.
3.2.6. Spectrophotometric Studies. UV-visible spectral data of the ligand and its chelate (10 −4 M) solutions in DMF were recorded from 200 to 700 nm at room temperature using the same solvent as blank. e free ligand (H 2 L) exhibited broad bands in the UV-visible region at 375 and 271 nm. e first band was assigned to the intraligand n ⟶ π * electronic Journal of Chemistry transition. e second broad band around 271 nm was assigned to the π ⟶ π * electronic transition. e n ⟶ π * and π ⟶ π * transition bands were shifted to 339-395 and 261-285 nm, respectively, indicating that the ligand coordinated to metal ions [29,30].

Magnetic Susceptibility and Electronic Spectral Studies.
A comparison of the electronic spectra of the free diazo ligand (H 2 L) coordinated with those of the corresponding metal complexes was discussed. is can be considered as evidence for the complex formation. Additionally, the diffused reflectance spectra of metal complexes showed different bands at different wavelengths; each one was corresponding to a certain transition which suggested the geometry of the complexes. e diffused reflectance spectra of the complexes were dominated by intense intraligand charge-transfer bands.
e diffused reflectance spectrum of the Cr(III) complex exhibited three bands at 28,489, 24,780, and 13,449 cm −1 which may be assigned to the 4 respectively. e magnetic moment value was found to be 4.81 BM which indicated the presence of the Cr(III) complex in the octahedral geometry [31,32]. e diffused reflectance spectrum of the Mn(II) complex showed three bands at 26,645, 19,490, and 15.660 cm −1 assignable to 4 T 1g ⟶ 6 A 1g , 4 T 2g (G) ⟶ 6 A 1g , and 4 T 1g (D) ⟶ 6 A 1g transitions, respectively [31]. e magnetic moment value was found to be 5.54 BM which indicated the presence of the Mn(II) complex in the octahedral structure.
From the diffused reflectance spectrum, it is observed that the Fe(III) chelate exhibited bands at 21,490 and 20,890 cm −1 , which may be assigned to the 6 A 1g ⟶ T 2g (G) and 6 A 1g ⟶ 5 T 1g transitions in the octahedral geometry of the complex [33]. e observed magnetic moment value of the Fe(III) complex was found to be 5.30 BM. e reflectance spectrum of the Co(II) complex displayed bands near 23,110, 15,240, and 12.845 cm −1 , assigned to 4 T 1g (F) ⟶ 4 T 2g (F), 4 T 1g (F) ⟶ 4 A 2g (F), and 4 T 1g (F) ⟶ 4 T 1g (P) transitions, respectively. ese bands suggested an octahedral geometry for the Co(II) complex. e magnetic moment value of the Co(II) complex was 5.16 BM which was consistent with the high-spin octahedral geometry. e diffused reflectance spectrum of the Ni(II) complex showed absorption bands at 20,680, 14,757, and 13,120 cm −1 which are attributed to 3 3 A 2g (F) ⟶ 3 T 1g (F), and 3 A 2g (F) ⟶ 3 T 1g transitions, respectively, and assignable to the octahedral geometry for the Ni(II) complex [34]. e magnetic moment value of the Ni(II) complex was 3.80 BM corresponding to the octahedral complex. e diffused reflectance spectrum of the Cu(II) complex exhibited broad bands in ranges 24,729 and 13,290 cm −1 assigned to 2 B 1g ⟶ 2 B 2g , 2 B 1g ⟶ 2 E g and 2 B 1g ⟶ 2 A 1g transitions, respectively. ese bands suggested distorted octahedral geometry for the Cu(II) complex. e magnetic moment value was found to be 1.78 BM indicating octahedral geometry [35]. e Zn(II) and Cd(II) complexes were diamagnetic. According to the empirical formulae, an octahedral geometry was proposed for these chelates.

ESR Studies of the Cu(II)-H 2 L Complex.
e ESR spectrum of the Cu(II) complex was recorded in DMSO at 300 and 77 K (Figure 1). e observed order for the Cu(II) complex (A ║ (197) > A ┴ (98.88); g ║ (2.445) > g ┴ (2.333)) indicated that the complex exerted an octahedral geometry [35][36][37]. e observed value of G for the Cu(II) complex (G �1.421) implied that the exchange coupling was not present, and misalignment was appreciable. e trend g || > g ⊥ > g e (2.0023) showed that the unpaired electron was localized in the d x2-y2 orbital of the Cu(II) ion in the complex [35][36][37][38]. e g iso (2.0023) value was less than 2.3 which indicated the covalent character of the metal-ligand bond, and the α 2 value (0.76) suggested the presence of in-plane covalency.

Powder X-Ray Diffraction and SEM.
e XRD patterns indicated crystalline nature for the H 2 L diazo ligand and the Cd(II) complex only. It can be easily seen that the pattern of the H 2 L diazo ligand differed from its metal complexes, which may be attributed to the formation of a well-defined distorted crystalline structure. Probably, this behavior was due to the incorporation of water molecules into the coordination sphere. On comparing the XRD spectra of the chelates with the XRD spectra of the free ligand, it was concluded that all chelates under study can be considered to have amorphous structures as they lack sharp peaks except the Cd(II) complex where it had a crystalline structure. erefore, the nonsimilarity of the XRD pattern between the metal ions and chelates suggested that these chelates had different phase structures than the free diazo ligand (H 2 L). Such facts suggested that H 2 L and the Cd(II) complex were crystalline, while its Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) complexes were amorphous. e average crystallite size (ξ) can be calculated from the XRD pattern according to the Debye-Scherrer equation [39,40]: e equation uses the reference peak width at angle θ, where λ is the wavelength of the X-ray radiation (1.541874Å), K is a constant taken as 0.95 for organic compounds [37], and β1/2 is the width at half maximum of the reference diffraction peak measured in radians. e dislocation density, δ, is the number of dislocation lines per unit area of the crystal. e value of δ is related to the average particle diameter (ξ) by the following relation [41,42]: e value of δ was calculated and found to be 33.78 and 26.77 nm, and the value of δ was 8.76 × 10 −4 and 1.39 × 10 −3 nm −2 for H 2 L and Cd(II) complex, respectively, giving evidence for the formation of the free azo ligand and its cadmium(II) complex in a nanosize. e SEM micrographs of H 2 L and its Cd(II) are given in Figure 2. ere is a significant difference in the morphological structures, and the particle size is in the nanometer range. e particle size of the ligand was 64-67 nm, while the Cd(II) complex was 20-32 nm.

Contact Angle.
For two centuries or more of research, wetting phenomena still remain popular topics of investigations. Young's static and dynamic contact angle equations [6,7] are the most common and relevant parameters describing the wetting property of a surface with respect to liquid water: where θ Y � Young's contact angle, σ sv � surface tension (energy per unit surface) of the solid-vapor interface, σ sl � surface tension of the solid-liquid interface, and σ lv � surface tension of the liquid-vapor interface. e mean contact angles were measured for H 2 L and its metal complexes (Supplementary Figure 3). e data listed in Table 1 proved that the diazo ligand and all the complexes are hydrophobic, and the Cd(II) complex can be considered as a superhydrophobic material. Superhydrophobic surfaces are actively studied across a wide range of applications and industries and are now finding increased use in the biomedical areas. A partial list includes clothing that will be both breathable and water repellant, umbrellas that stayed completely dry, building materials, paints, epoxies, goods transport, and anticorrosion. Superhydrophobic materials such as this promising nano-Cd(II) complex exhibit a number of unique properties that arise from the high roughness of a low surface energy material that stabilizes a nonwetted state [6,7].

3.2.11.
ermal Analyses. ermogravimetric analyses of the metal complexes under investigation were used to get information about the thermal stability of these new complexes, to decide whether the water molecules (if present) were inside or outside the inner coordination sphere of the central metal ion, and to suggest a general scheme for thermal decomposition of these chelates. In the present investigation, heating rates were suitably controlled at 10°C min −1 under nitrogen atmosphere, and the weight loss was measured from the ambient temperature up to ∼1000°C. e TG curves of the complexes are listed in Table 2. e weight losses for each chelate were calculated within the corresponding temperature ranges. e thermal decomposition process of H 2 L involved three decomposition steps. Decomposition of H 2 L started at 30°C and finished at 100°C with three stages. e first stage of decomposition involved the removal of four molecules of NH 3 and two molecules of carbon dioxide gas in the 30-355°C temperature range and was accompanied by a weight loss of 36     contaminated with carbon as a final product. ey were accompanied by a weight loss of 25.37% (calcd. � 24.63%) and corresponded to the loss of the C 11 H 4 O 0.5 molecule. e actual weight loss from these five steps was 76.62% which was close to the calculated value of 76.03%. e thermal degradation of the [Co(H 2 L)Cl 2 ] complex took place in three degradation stages within the temperature range from 30 to 990 o C. e first stage of decomposition occurred at maximum 177°C and was accompanied by a weight loss of 12.23% (calcd. � 12.45%) corresponding to the loss of 2NH 3 and 1/2Cl 2 gases. e second and third steps of decomposition occurred at two maxima 364 and 640°C and were accompanied by a weight loss of 6.33% (calcd. � 6.64%) and 60.30% (calcd. � 60.03%), respectively, corresponding to the loss of 1/2Cl 2 gas and C 18

Geometry Optimization.
e geometric parameters (bond lengths and bond angles) of the optimized structures of the H 2 L azo ligand and its Cd(II) complex ( Figure 3) are listed in Supplementary Table 5. From the analysis of these data, the following remarks were found: (1) ere was an elongation in the coordination bonds after complexation, and a large variation in N (11) (3) e bond angles of the free ligand moiety were altered somewhat upon coordination; also, the bond angles surrounding the central cadmium atom in the complex lie in the range reported for octahedral geometry [43]. (4) e decrease in the metal-chloride angles may be attributed to the intramolecular hydrogen bond [43].

Molecular Electrostatic Potential (MEP).
e MEP is a plot of electrostatic potential mapped onto the constant electron density surface. It is also very useful in the research of the molecular structure with its physiochemical property relationship. It is also can be used to understand the molecular interactions and predict the reactive sites for electrophilic and nucleophilic attacks [43]. e MEP is directly related to chemical reactivity. Red color represented the regions of negative electrostatic potential. e negative regions are related to electrophilic reactivity. However, the electron-poor region has blue color (favor site for the nucleophilic attack) [43,44], but the region with green color points to the neutral electrostatic potential region. Figure 4 represents 3D plots of the MEP for the parent ligand and its Cd(II) complex. It was found that oxygen and nitrogen atoms of free H 2 L were surrounded by a greater negative charge surface, making these sites potentially more favorable for the electrophilic attack (Figure 4(a)), where a greater negative charge was surrounded to the metal center (Figure 4(b)) confirming the complexation process. Table 6 and Supplementary Figure 6 clarify Mulliken atomic charges for some important atoms. e calculated charge value for N13 and N14 is −0.08 and −0.007 a.u., respectively, in contrast to the expected one (−1 a.u.), which indicated electron transfer from these atoms to the metal center. Furthermore, decrease of oxygen and chlorine formal charge from the expected values is a reason for migration of electrons to the cadmium center in order to forming the complex. Finally, this charge transfer from donor atoms to the metal center causes the calculated value to decrease to −0.756 a.u. against the formal charge of +2 for the cadmium atom [43,44]. Figure 5 shows the molecular orbital representation of the H 2 L ligand and its Cd(II) complex along with their HOMO, LUMO energies and energy band gaps. Both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were the main orbitals that participate in chemical stability [11,44,45]. e HOMO represented the ability to donate an electron; LUMO as an electron acceptor represents the ability to obtain an electron. In other words, the energy of the HOMO is directly related to the ionization potential; LUMO energy is directly related to the electron affinity. Hence, these frontier energies play an important role in the electric and optical properties. e energy gap between the HOMO and LUMO energies is a critical parameter in determining molecular electrical transport properties due to providing measurement of electron conductivity. In addition, this energy gap characterizes the molecular stability and spectroscopic properties of the molecular systems. e smaller energy gap describes a chemically soft molecule which can be easily polarizable. e HOMO-LUMO energy gap has proved to be an important tool in determining the kinetic stability and chemical reactivity of a molecule. e lower energy gap is a suitable condition where a molecule can be exited easily, whereas a higher energy gap leads to higher kinetic stability but lower chemical reactivity of the molecule. e difference between the energy gap for the ligand reflects the presence of the complexation status. e increase in the global electrophilicity value attributed to the highest capacity of accepted electrons, so the ligand has a highly powerful donation ability. e calculation of both absolute hardness (η) and absolute softness (σ) parameters is useful to recognize the molecular stability and reactivity. e calculations of the binding energy indicated that the increase of the value of the calculated binding energy of the complex compared to that of the free ligand indicated that the stability of the formed Cd(II) complex was higher than that of the free diazo ligand.

Molecular Parameters.
Additional parameters such as chemical potentials Pi, global electrophilicity ω, global softness S, electrophilicity index (χ), and additional electronic charge ∆N max were calculated for the free H 2 L ligand and the Cd-H 2 L complex (Table 3). e high ω value of both free H 2 L and Cd(II) complex suggested a great chance and priority for biological activity which is further confirmed by the experimental data. e mentioned quantum chemical parameters were calculated with the help of the following equations [43,44]:

Vibrational Properties.
In order to acquire the spectroscopic signature of the diazo ligand and its complexes, a frequency calculation analysis was performed. Vibrational properties had been investigated to determine vibrational modes connected with the molecular structure of the cadmium (II) complex using FT-IR spectra based on the optimized geometry. It is well known that the vibrational frequencies computed at quantum chemical methods such as DFT levels contain well-known systematic errors. e scaling factor of 0.96 for the LanL2DZ level was used to correct the effects of anharmonicity and neglected part of electron correlation [11,44,45]. e theoretical and the experimental spectra are shown in Supplementary Figure 8.
It is also noteworthy that experimental results belong to the solid phase, while theoretical calculations belong to a gas phase.
e stretching vibration of a free or nonhydrogenbonded OH group of H 2 L appears at 3664 cm −1 [11,45]. υ(COO − ) asy and υ(COO − ) sy stretching of the free ligand appeared at 1597 cm −1 and 1392 cm −1 , respectively. e theoretical spectrum of the free azo ligand showed υNH 2 and υN � N modes at 3637 and 1694 cm −1 , respectively. In the Cd(II) complex, the chelating ring occurred by means of the ligand bonding to the cadmium (II) ion with a tetradentate N,O chelated mode. It was observed from Figures 6 that the wavenumber interval between the υ(COO − ) asy and υ(COO − ) sy stretching modes is an effective indicator of forming complexation.
e wavenumber value of υN � N for the complex was lower than free H 2 L which was an indicator of the coordination through the nitrogen atom. e spectrum of the Cd(II) complex showed υNH 2 at a wave number very close to free H 2 L at 3639 cm −1 confirming that this group did not participate in the complex formation process. υM-O, υM-N, and υM-Cl modes of the Cd(II) complex were found at 564, 487, and 326 cm −1 , respectively. e calculated and experimental data were very close to each other (Supplementary Figure 8).

Structural Interpretation of Metal Complexes.
e structures of the complexes of the H 2 L diazo ligand with Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) ions were confirmed by the elemental analyses, IR, molar conductance, magnetic, mass, solid reflectance, and thermal analysis data. erefore, from the IR spectra, it was concluded that H 2 L behaved as a neutral tetradentate ligand coordinated to the metal ions via two nitrogen atoms of azo groups (N�N) and two oxygen atoms of protonated carboxylate groups (COO). From the molar conductance data, it was found that the complexes were electrolytes. On the basis of the above observations and from the magnetic and solid reflectance measurements, octahedral geometry was suggested for the investigated complexes ( Figure 6).

Antifungal Activities.
e preliminary fungi toxicity screening of the H 2 L diazo ligand and complexes was performed against Candida albicans in vitro by the diffusion technique [46]. H 2 L free ligand and Cr(III), Mn(II), and Fe(III) complexes showed no fungal growth inhibition, but Co(II), Ni(II), Cu(II), Zn(II), and Cd(II) complexes showed fungal growth inhibition at different rates (Figure 7). e Cd(II) and Ni(II) complexes in this study were nearly two times more active than the ketokonazole standard against the Candida albicans microorganism used (Table 4).

Antibacterial Activities.
e antibacterial activities of the H 2 L diazo ligand and its complexes against Bacillus subtilis, Staphylococcus aureus, Neisseria gonorrhoeae, and Escherichia coli are presented in Table 4. e H 2 L ligand was found to have no activity at all towards both different types of bacteria (Grampositive Bacillus subtilis and Staphylococcus aureus and Gramnegative Neisseria gonorrhoeae and Escherichia coli). is can be attributed to their very versatile nutritional capability, adaptability to various hydrocarbon rings, and the possession of the pump mechanism which ejects metal complexes as soon as they enter the cell [47]. In addition, Bacillus subtilis, Staphylococcus aureus, Neisseria gonorrhoeae, and Escherichia coli were sensitive to all the complexes, and an inhibitory zone was obtained in the range of 9.0-23.0 mm (Figure 7).
In all cases, the metal complexes were found to be more active than the H 2 L ligand expectedly due to chelation, which reduced the polarity of the metal atom, mainly  because of partial sharing of its positive charge with donor groups of the ligand and possible π-electron delocalization on the aromatic rings. is increased the lipophilic character, favouring its permeation into the bacterial membrane, causing the death of the organisms [48]. Higher activity of the metal complex was probably due to greater lipophilic nature of the complex. It increased the activity of the metal complex and can be explained on the basis of Overtone's concept and Tweedy's chelation theory [49][50][51][52]. According to Overtone's concept of cell permeability, the lipid membrane that surrounds the cell favours the passage of only lipid-soluble materials due to which liposolubility was considered to be an important factor that controls the antimicrobial activity. On chelation, the polarity of the metal ion will be reduced to a greater extent due to the overlap of the ligand orbital and partial sharing of the positive charge of the metal ion with donor groups [53,54]. Furthermore, it increased the delocalization of the π electrons over the whole chelate ring and enhanced the lipophilicity of the complex.
is increased lipophilicity enhanced the penetration of the complexes into the lipid membrane and thus blocked the metal-binding sites on enzymes of microorganisms [55]. ese metal complexes also disturb the respiration process of the cell and thus block the synthesis of proteins, which restricts further growth of the organism [56].

Journal of Chemistry
A look at the antibiotic, amikacin, activities (6.0-9.0 mm) against various bacterial isolates relative to the metal complexes (9.0-23.0 mm) showed that the activities of the former were much lower, with optimum activity being about half of metal complexes, against all the bacterial organisms.
When the antimicrobial activity of metal complexes is investigated, the following principal factors [57] should be considered: (i) the chelate effect of the ligands; (ii) the nature of the N-donor ligands; (iii) the total charge of the complex; (iv) the existence and the nature of the ion neutralizing the ionic complex; and (v) the nuclearity of the metal center in the complex. is is probably one of the reasons for the diverse antibacterial activity shown by the complexes, while the nature of the metal ion coordinated to the H 2 L ligand may have a significant role to this diversity. In general, all the complexes exhibited better inhibition than free H 2 L against Bacillus subtilis, Staphylococcus aureus, Neisseria gonorrhoeae, and Escherichia coli (Table 4). More specifically, Cd(II) and Co(II) complexes showed the best inhibition among all the complexes in this study, and they were one to twentythree times more active than the H 2 L diazo ligand against all the microorganisms used, indicating that the coordination of the H 2 L ligand to Cd(II) and Co(II) ions enhanced its antimicrobial activity (Figure 7). On the contrary, the rest complexes present higher antimicrobial activity to H 2 L against the four microorganisms.

Molecular Docking.
Targeting the minor groove of DNA through binding to a small molecule has long been considered an important tool in molecular recognition of a specific DNA sequence. DNA is considered a major biological target for metal complexes that have pharmacological activity. In this context, molecular docking between H 2 L and [Cd(H 2 L)Cl 2 ].H 2 O with four possible biological targets, receptors of the crystal structure of S. aureus (PDB ID: 3Q8U), crystal structure of protein phosphatase (PPZ1) of Candida albicans (PDB ID: 5JPE), receptors of breast cancer mutant oxidoreductase (PDB ID: 3HB5), and the crystal structure of Escherichia coli (PDB ID: 3T88), was performed. e structures of these macromolecules were obtained from the Protein Data Bank. e calculated binding energies for the interaction with these proteins are listed in Tables 5 and 6  From these data, the following can be concluded [58][59][60]: (1) First, H 2 L and its Cd(II) complex showed strong binding affinities for all proteins (2) e [Cd(H 2 L)Cl 2 ].H 2 O complex binds stronger than the free ligand to different proteins, see Figure 10 (3) e binding energy for the interaction of the cadmium (II) complex with the receptor protein 3Q8U was three times less than the free diazo ligand confirming the high activity of this compound [58] (4) A closer look at the interactions indicated that the presence of heteroatoms such as oxygen and nitrogen was critical to how the compound binds [59] (5) Generally speaking, H-acceptor interaction dominates the binding of the synthesized complexes to different receptors besides the metal binding to the protein backbone

Conclusion
A new diazo ligand, 2,2'-(1,3-phenylenebis(diazene-2,1diyl))bis(4-aminobenzoic acid), was synthesized and treated with different metal salts to give the corresponding metal complexes. e new ligand H 2 L reacted with the metal ions   ). e possible structures of the ligand and its metal-azo complexes were proposed based on elemental analyses, 1 H NMR, ESR, MS, IR, and UV-Vis electronic absorption. It was concluded that the H 2 L ligand behaved as a neutral tetradentate ligand with N coordination sites and protonated carboxylate oxygen upon coordination to the metal ions leading to the formation of octahedral geometries for all complexes. e thermal studies for the compounds showed a higher thermal stability for the complexes than that of the free ligand. is may be due to chelation, the type, and the number of solvents of crystallization in the metal complexes. Also, the ligand and its metal complexes were screened in vitro against microorganisms (bacteria or fungi). e results of biological activity showed that the Cd(II) complex had higher antibacterial activity against the (Gram-positive) bacteria and fungi, while Co(II) had higher antibacterial activity against the (Gramnegative) bacteria compared to the free ligand and other metal complexes. Molecular docking studies of the free ligand and the Cd(II) complex with four different receptors clarified that the Cd(II) complex showed the minimum binding ability with the binding energy of −18 kcal/mol with the crystal structure of S. aureus (3Q8U).

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare no conflicts of interest.

Supplementary Materials
Supplementary