Synthesis of Bivalent Ni(II), Cu(II) and Zn(II) Complexes of Azodicarbonamide in Mixture of Methanol and Aqueous Solvents: Spectral Characterizations and Anti-Microbial Studies

Abstract

Three new transition-metal complexes were produced by refluxing azodicarbonamide (ADCA) with nickel(II), copper(II), and zinc(II) solutions in a mixture of 50% (v/v) methanol and water. The magnitude of chelation between metal ions and ligand molecules was assessed by FT-IR, UV, elemental analysis, TGA, conductivity, mass, and magnetic susceptibility measurements. FT-IR analysis suggested a bi-dentate chelation in all complexes, which takes place through the N-azo and O-carbonyl groups. Based on the measurement of magnetic moments and spectral analysis, a distorted octahedral geometry was proposed for Ni(II) and Cu(II) complexes, whereas zinc complex showed a hexa-coordinated geometry. The optical band gaps of the nickel(II), copper(II) and zinc(II) complexes were found to be 1.91, 2.50, and 1.96 eV, respectively, which means that they can be employed as semiconductors and that they are in the same range as highly effective photovoltaic materials. The Urbach energy parameters were also estimated from other optical parameters. The biological activity of azodicarbonamide and its synthesized complexes has been screened against the selected gram bacteria (+ve) and fungi.

1. Introduction

Azodicarbonamide (ADCA; Figure 1) is a yellow powder that is frequently employed in industry as a foaming agent [1,2,3] in the creation of numerous products; it is non-toxic, odorless, and non-polluting when decomposed. Among the gases produced by the decomposition of ADCA are 65% N₂ gas, 24% CO, 5% CO₂, and 5% ammonia [4,5,6]. The structure of azodicarbonamide is as follows [7]:

The current procedure for the decomposition of ADCA is as follows: Hydrazodicarbonamide (biurea) is synthesized from hydrazine hydrate and urea by means of an acid condensation reaction [8], and ADCA is produced via an oxidation process from biurea. By oxidizing biurea, there are various ways to make azodicarbonamide, and these include the chlorate method, the chlorine method, the hydrogen peroxide method, and the dichromate oxidizing agent framework [8,9]. Due to its advantages, including its improvement of gluten products and its low cost, azodicarbonamide is employed as a dough conditioner [10,11,12]. Generally, azodicarbonamide will not interact with flour directly, but when wheat is mixed with water to form dough, it may produce reactive oxygen. The sulfhydryl group (-sh) of protein amino acids in wheat will be oxidized by the reactive oxygen forming disulfide bonds (-s-s), which will connect the protein chains to one another to form a mesh structure. It is well known that the addition of initiators, specifically the salts of zinc and calcium, can lower the temperature at which azodicarbonamide decomposes, according to several researchers [13,14], when azodicarbonamide interacts with calcium- or zinc-based salts, the resulting salts of azodicarboxylic acid serving as the catalyst for temperature breakdown. Additionally, the Lewis acid-base reaction, in which the metal of the activating addition functions as a Lewis acid by accepting electrons, is the activation mechanism of the azodicarbonamide decomposition process, and azodicarbonamide acts as the base—that is, it is an electron pair donor. The possibility that metals possessing filled pre-outer d-electron orbitals could form complexes with azodicarbonamide molecules as ligands was also assumed [15]. A break in the -c-n= bond is made easier by the synthesis of complexes, which results in a lack of electron density around the n-c bond of the azo-group.

2. Materials and Methods

2.1. Equipment’s

Azodicarbonamide and its metal complexes were characterized using different techniques. CNH elemental analysis was replicated three times using a LECO analyzer (Micro TruSpec model) to ensure the reproducibility of the results. On a Perkin-Elmer 1000 FTIR spectrophotometer, the FT-IR spectra of the complexes were measured using KBr-disc. The measurement of UV-Visible spectra for compounds was carried out in DMF solutions on a UNICAM UV-300 spectrophotometer (thickness of cuvette, 1 cm). Using a TGA Perkin Elmer thermal analyzer, a thermal analysis (TG and DTG) was carried out at 25–950 °C with the heating rate 10 °C min⁻¹. The measurement of magnetic susceptibilities was carried out at 25 °C using Balance, Sherwood Scientific, Cambridge Science Park Cambridge, England.

2.2. Synthesis of Azodicarbonamide Systems

All the chemicals utilized in this study were ultra-fine and did not require additional purification. Azodicarbonamide purchased from (Fluka Chemical Co. 99.9%), Cu(NO₃)₂·6H₂O, ZnCl₂ and NiCl₂·xH₂O (99.9%, Aldrich Chemical Co., USA). NiCl₂·6H₂O, Cu(NO₃)₂·6H₂O, and ZnCl₂ (1 mmol) were added to azodicarbonamide (2 mmol) in a mixture of 50% (v/v) methanol and water to create 40 mL of three azodicarbonamide systems. Colored precipitates were produced by refluxing the reaction mixtures for about 3–5 h at suitable temperatures. The precipitates were filtered-off and washed with MeOH and water, after which it was dried under CaCl₂ in a desiccator. About 73–79% of the yields were produced.

2.3. Anti-Microbial Assay

The diffusion method [16,17] was utilized to determine the inhibition zone of azodicarbonamide and its complexes against G (+ve) bacteria (Bacillus subtilis and Aspergillus oryazae) and fungi (Penicillium sp.). Dimethyl sulfoxide was used as the control solvent. Two different concentrations, 3 mg/mL and 6 mg/mL, were used in agar plates that were incubated at 37 ± 0.5 °C for 24 h.

3. Result and Discussion

3.1. Elemental and Conductance Data and Magnetic Moment

A new series of azodicarbonamide complexes have been produced from refluxing azodicarbonamide with alcoholic Ni(II), Cu(II), and Zn(II) solutions. The obtained compounds are air-stable and soluble in dimethylsulphoxide and dimethylformamide organic solvents. The molar conductance measurements of ADCA and its complexes were carried out in a DMSO solution and revealed the non-electrolytic nature as well as the neutral form of the prepared compounds. The stoichiometry of the compounds was confirmed by the elemental analysis results, which closely matched the calculated values. The electronic spectra as well as the characteristic infrared bands were taken into consideration as additional evidence of the suggested geometry of compounds. The microanalytical investigation information of the complexes found a 1:2 stoichiometry (metal: ligand) with molecular formulae [Ni(ADCA)₂(Cl)₂]·1.5H₂O (1), [Cu(ADCA)₂(NO₃)₂]·H₂O (2), and [Zn(ADCA)₂(Cl)₂]·H₂O (3), where ADCA is C₂H₄N₄O₂ (Figure 2). The physical data: the conductance, magnetic, and elemental analysis results are listed in

Table 1. Micro analytical and physical data for azodicarbonamide and its complexes.

Compounds	Yield%	Mp/°C	Color	Conductance (ohm−1·cm2·mol−1)	µeff (B.M)	Element	Found	Calc.
NH2CON=NCONH2	-	245	White	11	-	%C	20.69	20.69
						%H	3.47	3.45
						%N	48.57	48.27
[Ni(ADCA)2(Cl)2]·1.5H2O	79	˃350	Light green	10	3.46	%C	12.52	12.34
						%H	2.86	2.83
						%N	28.38	28.79
[Cu(ADCA)2(NO3)2]·H2O	75	˃350	Dark green	7	1.93	%C	10.97	10.96
						%H	2.21	2.28
						%N	31.71	31.96
[Zn(ADCA)2(Cl)2]·H2O	73	˃350	Yellow	13	Dia	%C	13.05	12.43
						%H	2.60	2.59
						%N	28.36	29.02

.

3.2. Magnetic Properties

All the complexes under consideration were found to be paramagnetic on the basis of the magnetic moment measurements taken from Table 1, with the exception of the diamagnetic zinc(II) complex, which exhibited a hexa-coordinated geometry because of the measuring temperature affecting the precise value of the magnetic moment and the spin-orbit coupling’s magnitude. All observations were made at ambient temperature. At room temperature, the magnetic moment of the solid nickel (II) complex was found to be 3.46 B.M, which lies within the range of experimental data (3.32 B.M) and is indicative of two unpaired electrons per Ni(II) ion in an octahedral environment. The Cu(II) complex showed an μ_eff value of 1.93 B.M, which is in agreement with the experimental range (1.96 B.M) and is indicative of one unpaired electron per Cu(II) ion, suggesting these complexes within the range are consistent with spin-free distorted octahedral geometry.

3.3. IR Spectra of Azodicarbonamide and Its Complexes

The comparison of the IR spectra of the complexes with those of azodicarbonamide determine the kind and site of coordination that may be involved in chelation. The most important experimental and theoretically calculated IR spectral main bands of azodicarbonamide and of its metal complexes are given in

Table 2. Vibrational assignment of important IR bands of the azodicarbonamide ligand and its metal complexes.

Assignments	Compounds
	NH2CON=NCONH2	[Cu(ADCA)2(NO3)2]·H2O	[Zn(ADCA)2(Cl)2]·H2O	[Ni(ADCA)2(Cl)2]·1.5H2O
δN-C=O + δNCN + δC-N=N	636	608	563	620
δC=O + δH-N-H + δN = N	752	777.2	713	764
δN-H	856	840	831	997
δH-N-H + vC-N + δN-C=O	1116	1043	1042	1110
δH-N-H + vC-N + δN-C=O	1330	1344	1387	1419
vC=O + vC-N	1727	1621	1556	1679
ν(M-O)	-	438	465	407
ν(M-N)	-	585	572	551

and shown in Figure S1a–d with their tentative assignments. The bands at 1727 cm⁻¹ in the IR spectrum are assigned to a superposition of the stretching vibrations of C=O and N-C, which are more strongly infrared-active. The bands of azodicarbonamide observed at 1116 cm⁻¹ in the IR spectrum are assigned to the same vibration, namely a superposition of the deformation bands of NH₂ and the stretching vibrations of C=O and N-C. The bands at 1330 cm⁻¹ in the IR spectrum and at 1332 cm⁻¹ are assigned to a superposition of the deformation bands of NH₂, the N-C stretching vibration, and the N-C=O bending vibration. The strong characteristic band can be observed in the IR spectrum of azodicarbonamide (Figure 3a) located at 1728 cm⁻¹, which is assigned to the stretching vibration of C=O [18]. Azodicarbonamide has two probable sites for the coordination to metals containing azo and C=O groups [19]. The observed wavenumber shifts and the broadening in the azodicarbonamide spectrum were considered the result of intermolecular hydrogen bonding interactions [20]. However, these intermolecular hydrogen bonds break upon coordination to metals. The strong carbonyl stretching peak at 1728 cm⁻¹ in azodicarbonamide shifts to 1621, 1556, and 1679 cm⁻¹, which indicates the interaction of the carbonyl and copper (II) ion and the Zinc (II) and nickel (II) ion within the coordination compound. The coordination of the azodicarbonamide with the metal ions was further confirmed by the appearance of new bands between 551–585 and 407–465 cm⁻¹, which were designated to the metal nitrogen (M-N) and metal-oxygen (M-O) extending vibrations individually. These bands were not present in the spectra of the free ligand, thus affirming the participation of O and N in the coordination with transition metal ions [21,22]. Therefore, the IR spectra shows that the ligand azodicarbonamide is bidentate and coordinates to the metal through the nitrogen atom of the azo group and the oxygen atom of carbonyl groups [19].

3.4. Thermal Studies

Thermogravimetric analysis is used to investigate azodicarbonamide and its metal complexes at 25–1000 °C in a nitrogen environment (

Table 3. The maximum temperature, T_max (°C), and weight loss values of the decomposition stages for azodicarbonamide metal complexes.

Compounds	TGA Range (°C)	Number of Peaks	Weight Loss (%)		Lost Species
			Calc.	Found
[Ni(ADCA)2(Cl)2]·1.5H2O Mw = 379.95	0–265 265–353 353–800	1 1 Residue	38.12 25.75 36.13	38.66 25.66 35.761	0.5HCNO + 1.5H2O + N2+ 2Cl C2N2O2+ 0.5HCN 1.5NH3 + 0.5HCN + 0.5HCNO+ NiO↓
[Cu(ADCA)2(NO3)2]·H2O Mw = 439.74	10–139 135–250 250–600	1 1 Residue	9.514 41.675 48.811	8.981 41.065 48.095	H2O + HNCO HNCO + C2H3N5O2 + N2 + 0.5NH3 0.5NH3 + 2NO3 + CuO↓ + 0.5C↓
[Zn(ADCA)2(Cl)2]·H2O Mw = 383.94	0–265 265–353 353	1 1 1 Residue	8.37 14.03 28.742 48.858	8.02 15.55 25.13 51.3	H2O + N2 NH3 + HNCO 2Cl + HCN C2H3N3O2 + ZnO↓ + 7.5C↓

& Figure 3). The DTG curves show the rate of weight loss versus the temperature scale, while the TGA curves show the percentage mass loss as a function of the temperature. When azodicarbonamide breaks down between 165 and 195 °C, gas is released along with the formation of a residue. The gas is made up of N₂, CO gases, and a third substance that, depending on the temperature, is either ammonia or HNCO acid. The residue is a combination of biurea, HNCO, and H₃N₃C₂O₂, whereas the sublimate is made up of HNCO, cyamelide, and urea [23,24,25,26,27]. Two major reactions, (i) and (ii), seem to occur simultaneously in the first mode of decomposition, in which azodicarbonamide breaks down to create biurea. HNCO and N₂ in the second mode of decomposition breakdown to create ammonia, N₂, HNCO, and H₃N₃C₂O₂. The first mode of decomposition occurs twice as frequently as the second at 171.5 °C, while it appears that isocyanic acid’s secondary reactions produce cyanuric acid, cyamelide, carbon monoxide, and urea. The biurea that was initially created breaks down into urazole and ammonia at higher temperatures [27,28]. The sequence for the thermal degradation of azodicarbonamide is given (Scheme S1).

The [Ni(C₂H₄N₄O₂)₂(Cl)₂]·1.5H₂O thermogram reveals three main decomposition steps. In the first step, T_max = 265 °C, and the residue is 0.5HCNO + 2Cl⁻ + N₂ +1.5H₂O with a weight loss percent of 39.12 (calc 38.95%). In the second step, T_max = 353.51 °C, and the residue is C₂N₂O₂ + 0.5HCN with a weight loss percent of 25.75 (calc 25.66%). In the final step, the final residue is 1.5NH₃ + 0.5HCN + N₂ + NiO with a weight loss of 36.13% (calc 35.05%). The [Cu(C₂H₄N₄O₂)₂(NO₃)₂]·H₂O thermogram reveals three degradation steps within the temperature range 45–1000 °C. In the first step, T_max = 45 °C with a weight loss of 8.981% (calc 9.541%), corresponding to the loss of H₂O + HNCO. In the second step, T_max = 250 °C and is accompanied by a weight loss of 41.065% (calc 41.675%), corresponding to the loss of HNCO + C₂H₃N₃O₂ + N₂ + 0.5NH₃. Then, the final thermal decomposition product obtained is 0.5NH₃ + 2NO₃ + CuO + 0.5C. In the [Zn(C₂H₄N₄O₂)₂(Cl)₂]·H₂O thermogram, decomposition occurs in four steps. In the first step, T_max = 158 °C, with a weight loss of 8.371% (calc. 8.02%), corresponding to the loss of H₂O + N₂. In the second step, T_max = 197 °C, corresponding to the losses of NH₃ + HCN with weight losses of 14.034% (calc. 15.55%). In the third step, T_max = 538 °C, corresponding to the loss of Cl⁻ + HCN with a weight loss of 28.19% (calc 25.13%). Finally, C₂H₃N₃O₂ + ZnO + 2C is obtained through the final decomposition step.

3.5. Electronic Spectral Measurements

The absorption spectra of the azodicarbonamide ligand and its metal complexes in DMF were recorded over the wavelength range of 200 to 800 nm. As seen in

Table 4. Absorption data and band assignment of azodicarbonamide ligand and its complexes.

Compound	Electronic Transition, λmax (nm, DMF)	Band Assignments	Eg (eV)	Eu (eV)
C2H4N4O2	341, 331, 433	π → π*, π → π*, n → π*	3.72	22.94
[Ni(ADCA)2(Cl)2]·1.5H2O	717 564 520	3A2g (F) → 3T2g (F) 3A2g (F) → 3T1g (F) 3A2g (F) → 3T2g (P)	1.91	9.38
[Cu(ADCA)2(NO3)2]·H2O	521 400	2Eg → 2T2g intra-ligand transitions	2.50	4.29
[Zn(ADCA)2(Cl)2]·H2O	236, 327, 386	π → π*, n → π*, L→ M (LMCT)	1.96	19.83

and Figure 4, ADCA has three strong absorption bands at 241, 331, and 433 nm, which may be attributed to the π → π* and n → π* electronic transitions [29]. The electronic spectrum of the Ni(II) complex exhibited three absorption bands at 717, 564, and 520 nm, which may be assigned to spin-allowed transitions ³A_2g (F) → ³T_2g (F), ³A_2g (F) → ³T_1g (F), and ³A_2g (F) → ³T_2g (P), as is characteristic of the distorted octahedral geometry of the Ni(II) ion [30,31]. The spectrum of the Cu(II) complex shows two bands in the ultraviolet and visible regions at about 521 and 400 nm, which can be attributed to ²E_g → ²T_2g and intra-ligand transitions, respectively [31]. The zinc(II) complex displays an absorption band at 386 nm assignable to the LMCT transition and hexa-coordinated geometry, and this is further supported by its diamagnetic nature and the absence of the d-d band due to its complete d¹⁰ electronic configuration.

3.6. Optical Band Gap Energy

Tuac’s equations [32,33] were used to predict the optical band gap,

αhν = (hν − E_g)ⁿ

where hν = the photon energy, h = Plank constant, n = ½ and 2 for direct and indirect transitions respectively, α = the absorption coefficient, A = an energy-independent constant.

Figure 5 depicts the plotting of (αhν)² and (αhν)^1/2 against (hv). A band gap is obtained by extrapolating the linear component of the curve to (αhν)^1/2 = 0. The following equation has been used to compute the absorption coefficient (α): α = 1/dln(1/T), where d = the optical path length of cuvette and T = estimated transmittance. In the DMF solvent, the band gaps for azodicarbonamide and its Ni(II), Cu(II), and Zn(II) complexes were 3.79, 1.91, 2.50, and 1.96 eV, respectively, and these are listed in Table 4 and Figure 6. To provide insight into the data listed in Table 4, complexation minimizes the E_g values rather than azodicarbonamide ligand. This decrease in E_g values is owed to the electron transfer from ligand to metal ion [34]. It is speculated that the presence of Ni or Cu ions in the given complex enhances the mobility of ligand electrons by accepting them in their empty shell. This results in the expansion of the localized levels in the resultant complex, and, as a result, the band gap is narrower and widely utilized in optics, electronics, and energy conversion devices [35]. In fact, a small energy difference facilitates the electron transition between HOMO–LUMO so that the molecule becomes more electro-conductive [36]. The low E_g values for the studied compounds are in a good agreement with the reported values and, consequently, can be employed as semiconductors as well as highly effective photovoltaic materials [34,37,38,39].

According to the Urbach formula:

α(hν) = α_o exp(hν/E_u)

where α_o = constant, E_u = the Urbach energy that is interpreted as the width of the localized states.

The absorption coefficient, α, exponentially depends on the photon energy when hv ≤ E_g. From the plot of lnα versus $\mathrm{h}\mathsf{\nu }$ (Figure 5), the Urbach energy (E_u) can be computed and should equal the reciprocal of the slope of the straight line of the linear portion of the curve. The estimated E_u values are 22.94, 9.377, 4.29, and 19.53 meV, corresponding to azodicarbonamide and its Ni(II), Cu(II), and Zn(II) complexes, respectively. The low E_u values indicate the minimal defects in the complex structure.

3.7. Mass Spectra of the Azodicarbonamide and Its Metal Complex

The anticipated formulae shown in Figure 2 have been confirmed using the following mass spectral fragmentations of the azodicarbonamide and its Ni(II), Cu(II), and Zn(II) complexes (Figure S2). The mass spectrum of azodicarbonamide having a molecular ion peak at m/z = 116.9 corresponds to C₂H₄N₄O₂. Meanwhile, another peak at m/z = 100.2 belongs to the [C₂H₂N₂O₂]⁺ of the nitroacetonitrle ion. For the structure of guanidine, the hydroxide ion, [CH₆N₃O]⁺, belongs to the peak at m/z = 76.1. Meanwhile, the last peak at m/z = 59.2 belongs to the diazenylmethanolate ion, [CH₃N₂O]⁺. Additionally, the last peak at m/z = 44 belongs to the aminomethanone ion, [CH₂NO]⁺. Furthermore, the last peak at m/z = 30.4 belongs to the diazenium ion, [N₂H₃]⁺ [40] (Scheme S2). The mass spectrum of the Ni(II) complex (Figure S2) shows that the parental ion peak at m/z = 279 belongs to [${\mathrm{C}}_4{\mathrm{H}}_5{\mathrm{N}}_7{\mathrm{NiO}}_5$]⁺. The other fragments of the complex give peaks with various intensities at different values such as 243 [C₄H₃N₅NiO₄]⁺, 226 [C₄N₄NiO₄]⁺, and 164 [C₂N₂NiO₂]⁺. The mass spectral fragmentation pattern of Ni(II) complex (Scheme S3) is in agreement with the anticipated structure. The mass spectrum of the Cu(II) complex C₄H₁₀CuN₁₀H₁₁ (Figure S2) shows the molecular ion peak at m/z = 375 corresponding to [C₄H₇CuN₉O₉]⁺. The other fragments of the complex show the peaks at different values; for example [C₃H₇CuN₇O₃]⁺ has a peak at 251, [H₆CuN₆O₂]⁺ has a peak at 211, [C₂H₃CuN₅O₂]⁺ has a peak at 191, and [C₂H₃N₅O]⁺ has a peak at 122. The fragmentation pattern in Scheme S4 is in agreement with the proposed structure of Cu(II) complex. The mass spectra of the Zn(II) complex (Figure S2) show that the molecular ion peaks at m/z = 386.6 belong to the [C₄H₁₀N₈O₅Cl₂Zn]⁺. The other fragments of the complex show that the peaks at m/z = 358.46, 340.46, 323.46, 253, 210.46, and 183.46 correspond to [C₄H₁₀Cl₂N₆O₅Zn]⁺, [C₄H₈Cl₂N₆O₄Zn]⁺, [C₄H₅Cl₂N₅O₄Zn]⁺, [C₄H₅N₅O₄Zn]⁺, [C₃H₄N₄O₃Zn]⁺, and [C₂H₃N₃O₃Zn]⁺, respectively. The fragmentation pattern in Scheme S5 is in agreement with the proposed structure of the Zn(II) complex.

3.8. Biological Activity: Antibacterial Screening

Data listed in

Table 5. The inhibition diameter (cm) of azodicarbonamide ligand and its metal complexes.

No.	Aspergillus oryzae	Penicillium sp.	Bacillus subtilis
Dimethyl sulfoxide	−ve	−ve	−ve
Azodicarbonamide	−ve	−ve	−ve
Ni (II) complex	1.3 cm	0.8 cm	−ve
Cu (II) complex	0.6 cm	−ve	−ve
Zn (II) complex	−ve	−ve	−ve

and Figure 6 depict the antibacterial and antifungal properties of the azodicarbonamide ligand and its metal complexes. As an insight into the data, the following remarks can be concluded: The azodicarbonamide and its Zn(II) complex show no antimicrobial activity, and the Ni(II) complex gives significant antifungal activity against Aspergillus oryzae and Penicillium sp. with inhibition zones of 1.3 and 0.3 cm, respectively. However, it has no antibacterial activity. The Cu(II) complex has acceptable antifungal activity against Aspergillus oryzae with the inhibition zone of 0.6 cm.

3.9. Computational Studies

Theoretical studies have been performed using DMOL³ in the Materials Studio package [41,42,43,44]. DFT semi-core pseudopod calculations (dspp) were performed with the double numerical basis sets as well as the polarization function (DNP) [45]. The RPBE function is based on the generalized gradient approximation (GGA) as the best correlation function [46,47]. The structures of the ADCA complexes (Scheme 1, Scheme 2, Scheme 3, Scheme 4, Scheme 5, Scheme 6, Scheme 7, Scheme 8, Scheme 9, Scheme 10, Scheme 11 and Scheme 12) depict optimized molecular structures in conjunction with the numbering of the atoms for the azodicarbonamide ligand and its metal complexes. As an insight into data listed in

Table 6. Selected bond lengths (Å) and bond angles (°) of (Ligand) ligand using DFT-method from DMOL³ calculations.

Bond	Length (Å)	Angle	Degree (°)	Angle	Degree (°)
N(8)-H(12)	1.014	H(12)-N(8)-H(11)	120.799	O(6)-C(3)-N(2)	125.105
N(8)-H(11)	1.015	H(9)-N(7)-C(4)	120.259	C(3)-N(2)-N(1)	111.035
N(7)-H(10)	1.013	N(8)-C(3)-N(2)	107.587	C(4)-N(1)-N(2)	111.047
N(1)-C(4)	1.499	H(12)-N(8)-C(3)	118.942
N(7)-H(9)	1.014	H(11)-N(8)-C(3)	120.221
C(4)-N(7)	1.346	H(10)-N(7)-H(9)	120.443
O(5)-C(4)	1.231	H(10)-N(7)-C(4)	119.245
N(1)-N(2)	1.248	N(7)-C(4)-O(5)	127.374
C(3)-N(8)	1.345	N(7)-C(4)-N(1)	107.502
C(3)-O(6)	1.232	O(5)-C(4)-N(1)	125.113
N(2)-C(3)	1.499	N(8)-C(3)-O(6)	127.287

,

Table 7. Selected bond lengths (Å) and bond angles (°) of (Cu complex) ligand using DFT-method from DMOL³ calculations.

Bond	Length (Å)	Bond	Length (Å)	Angle	Degree (°)	Angle	Degree (°)
N(23)-O(25)	1.253	C(6)-O(7)	1.224	O(22)-Cu(17)-O(18)	86.068	N(11)-Cu(17)-O(4)	92.419
N(23)-O(24)	1.257	C(6)-N(8)	1.336	O(22)-Cu(17)-O(12)	92.414	N(11)-Cu(17)-N(3)	160.538
O(22)-N(23)	1.321	O(4)-Cu(17)	2.206	O(22)-Cu(17)-N(11)	94.457	O(4)-Cu(17)-N(3)	75.274
N(19)-O(21)	1.255	N(3)-Cu(17)	2.161	O(22)-Cu(17)-O(4)	172.959	Cu(17)-O(4)-C(1)	110.239
N(19)-O(20)	1.259	N(2)-N(3)	1.241	O(22)-Cu(17)-N(3)	98.473	Cu(17)-N(3)-C(6)	130.498
O(18)-N(19)	1.317	N(10)-N(11)	1.243	O(18)-Cu(17)-O(12)	176.060	Cu(17)-N(3)-N(2)	117.983
O(22)-Cu(17)	2.189	C(9)-O(12)	1.247	O(18)-Cu(17)-N(11)	101.856	O(12)-Cu(17)-N(11)	74.618
O(18)-Cu(17)	2.186	O(12)-Cu(17)	2.214	O(18)-Cu(17)-O(4)	91.040	O(12)-Cu(17)-O(4)	90.870
C(14)-N(16)	1.338	N(11)-Cu(17)	2.175	O(18)-Cu(17)-N(3)	93.520	O(12)-Cu(17)-N(3)	90.299
C(14)-O(15)	1.223

,

Table 8. Selected bond lengths (Å) and bond angles (°) of Ni complex ligand using DFT-method from DMOL³ calculations.

Bond	Length (Å)	Bond	Length (Å)	Angle	Degree (°)	Angle	Degree (°)
Cl(19)-Ni(17)	2.386	N(3)-Ni(17)	2.090	Cl(19)-Ni(17)-Cl(18)	94.535	O(12)-Ni(17)-O(4)	86.609
Cl(18)-Ni(17)	2.386	N(2)-N(3)	1.259	Cl(19)-Ni(17)-O(12)	89.276	O(12)-Ni(17)-N(3)	88.580
C(14)-N(16)	1.340	C(9)-O(12)	1.255	Cl(19)-Ni(17)-N(11)	87.412	N(11)-Ni(17)-O(4)	87.896
C(14)-O(15)	1.229	C(6)-N(8)	1.338	Cl(19)-Ni(17)-O(4)	174.385	N(11)-Ni(17)-N(3)	159.054
O(12)-Ni(17)	2.128	C(6)-O(7)	1.229	Cl(19)-Ni(17)-N(3)	106.982	O(4)-Ni(17)-N(3)	76.757
N(11)-Ni(17)	2.108	O(4)-Ni(17)	2.131	Cl(18)-Ni(17)-O(12)	174.911	O(12)-Ni(17)-O(4)	86.609
N(10)-N(11)	1.259	C(1)-O(4)	1.255	Cl(18)-Ni(17)-N(11)	107.298	O(12)-Ni(17)-N(3)	88.580
				Cl(18)-Ni(17)-O(4)	89.804	Ni(17)-O(12)-C(9)	111.061
				Cl(18)-Ni(17)-N(3)	87.083	Ni(17)-N(11)-C(14)	130.322
				O(12)-Ni(17)-N(11)	76.200	Ni(17)-N(11)-N(10)	118.004
				Ni(17)-O(4)-C(1)	110.225	Ni(17)-N(3)-C(6)	130.593
				Ni(17)-N(3)-N(2)	117.939

and

Table 9. Selected bond lengths (Å) and bond angles (°) of (Zn complex) ligand using DFT-method from DMOL³ calculations.

Bond	Length (Å)	Bond	Length (Å)	Angle	Degree (°)	Angle	Degree (°)
Cl(19)-Zn(17)	2.330	C(9)-O(12)	1.244	Cl(19)-Zn(17)-Cl(18)	179.876	Cl(18)-Zn(17)-O(4)	92.703
Cl(18)-Zn(17)	2.328	C(6)-N(8)	1.348	Cl(19)-Zn(17)-O(12)	92.787	Cl(18)-Zn(17)-N(3)	89.164
C(14)-N(16)	1.348	C(6)-O(7)	1.227	Cl(19)-Zn(17)-N(11)	88.952	O(12)-Zn(17)-N(11)	69.365
C(14)-O(15)	1.227	O(4)-Zn(17)	2.309	Cl(19)-Zn(17)-O(4)	87.195	O(12)-Zn(17)-O(4)	179.937
O(12)-Zn(17)	2.310	N(3)-Zn(17)	2.368	Cl(19)-Zn(17)-N(3)	90.867	O(12)-Zn(17)-N(3)	110.695
N(11)-Zn(17)	2.367	N(2)-N(3)	1.246	Cl(18)-Zn(17)-O(12)	87.315	N(11)-Zn(17)-O(4)	110.574
N(10)-N(11)	1.246	C(1)-O(4)	1.244	Cl(18)-Zn(17)-N(11)	91.017	N(11)-Zn(17)-N(3)	179.812
				Zn(17)-O(12)-C(9)	114.409	O(4)-Zn(17)-N(3)	69.366
				Zn(17)-N(11)-C(14)	127.748	Zn(17)-N(11)-N(10)	119.568
				Zn(17)-O(4)-C(1)	114.448	Zn(17)-N(3)-C(6)	127.774
				Zn(17)-N(3)-N(2)	119.543

for the bond length as well as the bond angles of azodicarbonamide compounds, the following can be concluded:

• The bond lengths of azodicarbonamide moiety show significant change upon complexation. The remarkable changes were observed for the C(4)-N(1), C(3)-N(2), N(1)-N(2), C(4)-O(5), and O(6)-C(3) bond lengths, which are elongated or shortened depending on the coordination with metal ions [48].
• The bond angles in the Ni-complex are quite near to its octahedral geometry, predicting sp³d² or d²sp³ hybridization [49]. The Cu-complex shows a distorted Oh geometry. However, the bond angles in the Zn-complex predict the hexagonal environment around the Zn metal ion.
• The C(4)-O(5) and C(3)-O(6) bond distances of the carbonyl group in azodicarbonamide are slightly elongated due to the formation of a strong M-O bond which makes the C-O bond weaker [50].
• The bond distances of Ni-O and Cu-O in Ni- and Cu-complexes are shorter than that of Zn-O in the Zn-complex, reflecting the greater strength of the Ni-N and Cu-N bonds. The bond distances of Ni-N and Cu-N in Ni- and Cu-complexes are shorter than that of Zn-N in the Zn-complex, reflecting the greater strength of Ni-N and Cu-N bonds.
• The data recorded in

  Table 10. Some quantum chemical parameters of azodicarbonamide and its complexes.

  Compound	-EH	-EL	-EH-L	χ	µ	η	S	ω	ϭ
  Ligand	5.381	3.606	1.775	4.493	−4.493	0.443	0.2219	22.7510	2.2535
  Cu	5.918	4.748	1.170	5.333	−5.333	0.292	0.1462	48.6169	3.4188
  Zn	5.787	4.351	1.436	5.069	−5.069	0.359	0.1795	35.7866	2.7855
  Ni	5.474	4.523	0.951	4.998	−4.998	0.2377	0.1189	52.5447	4.2061

  reveals some quantum chemical parameters, including the energies of frontier molecular orbitals (E_HOMO, E_LUMO), the energy band gap (E_H-E_L), electronegativity ($\mathsf{\chi }$), chemical potential ($\mathsf{\mu }$), global hardness ($\mathsf{\eta }$), global softness ($\mathrm{S}$), and global electrophilicity index ($\mathsf{\omega }$), that have been computed according to the following equations [51,52,53].

$$\mathsf{\chi } \left(\mathrm{electronegativity}\right)=-\frac{1}{2} \left({\mathrm{E}}_{\mathrm{LUMO}}+{\mathrm{E}}_{\mathrm{HOMO}}\right)$$

$$\mathsf{\mu } \left(\mathrm{potential}\right)=-\mathsf{\chi }=\frac{1}{2} \left({\mathrm{E}}_{\mathrm{LUMO}}+{\mathrm{E}}_{\mathrm{HOMO}}\right)$$

$$\mathsf{\eta } \left(\mathrm{hardness}\right)= \frac{1}{2} \left({\mathrm{E}}_{\mathrm{LUMO}}-{\mathrm{E}}_{\mathrm{HOMO}}\right)$$

$$\mathrm{S} \left(\mathrm{softness}\right)= \frac{1}{2} \mathsf{\eta }$$

$$\mathsf{\omega } \left(\mathrm{electrophilicity}\right)=\frac{{\mathsf{\mu }}^2}{2\mathsf{\eta }}$$

• The σ = 1/$\mathsf{\eta }$ calculated energy band gap for azodicarbonamide is 1.775 eV higher than that of the corresponding metal complexes. In addition, the ΔE_H-L value for the Ni-complex is the smallest—ΔE_H-L = 0.957 eV—which is in a good agreement with the experimental data.
• The energetic parameters (total energy, binding energy, and dipole moment) have been computed and listed in

  Table 11. Various theoretical molecular parameters of azodicarbonamide and its complexes.

  	Dipole Magnitude (D)	Binding Energy (ev)	Total Energy (ev)
  Azodicarbonamide	0.1140	−53.5028	−12197.595
  Cu-complex	15.1645	−141.43307	−45715.839
  Ni- complex	13.1330	−117.59501	−54711.175
  Zn-complex	0.0270	−113.57451	−56368.463

  . The higher negative values of the binding and total energies for azodicarbonamide complexes indicate the higher stability of the prepared metal compounds compared to that of azodicarbonamide molecule.

4. Conclusions

Physical-chemical studies of novel azodicarbonamide complexes obtained from refluxing the ligand with alcoholic Ni(II), Cu(II), and Zn(II) salt solutions were presented. Elemental, IR, molar conductance, magnetic, UV–vis, mass, and thermal analyses were carried out to confirm the molecular structures of the studied compounds. The vibrational spectra indicate the neutral bidentate behavior of the ADCA ligand. The studied ligand coordinates to metal ions through the oxygen of (C=O)_amidic and the nitrogen of (N=N)_azo groups. The molar conductivity measurements prove the non-electrolytic nature of all compounds with 1:2 stoichiometry. Magnetic measurements and electronic spectra predict the paramagnetic and octahedral geometry of copper and nickel complexes, but zinc complex is diamagnetic and has a hexa-coordinated geometry. The energy band gap between HOMO and LUMO for the studied metal complexes is lower than that of the synthesized ADCA ligand, indicating the facility of electron transfer. Ni(II) and Cu(II) complexes can be used as fungicide. However, azodicarbonamide and its zinc complex have no biological efficacy.