1. Introduction

Recently, there has been renewed attention in the preparation and studies of mixed ligand transition metal complexes^{1, 2} due to their new useful properties such as magnetic exchange, photoluminescence, nonlinear optical property, electrical conductivity and antimicrobial activity^3-5.

Mixed ligand complexes containing amino acid as co-ligand are potential biomimetic models for metal-protein interaction⁶. Research has shown significant progress in utilization of transition metal complexes as drugs to treat a lot of human diseases like carcinomas, infection control, anti-inflammatory, diabetes and neurological disorders⁷.

Urea, carbamide or carbonyldiamide CO(NH₂)₂ (Figure 1a), which has a remarkable role in many biological processes in decomposition of proteins and amino acid catabolism, was discovered in 1828 by Wöhler when evaporating a solution containing a mixture of potassium isocyanate and ammonium sulphate⁸.

The mode of urea bonding with metal ions seems to be dependent upon the type and nature of the metal, lead(II) coordinates to the nitrogen atom, whereas iron(III), zinc(II) and copper(II) coordinate to the oxygen of urea⁹. Also, there are different types of coordination of urea in its complexes with rare-earth iodides and perchlorates¹⁰.

Figure 1.
Structures of urea (a) and asparagine (b) molecules.

The amino acids are the main building units of all various forms of life and were earlier discovered as ingredient of natural products even before they were recognized as components of proteins¹¹. The amino acid L-asparagine or 2-amino-3-carbamoylpropanoic acid (Fig. 1b) is a structural analog of L-aspartic acid. It was the first amino acid to be isolated from plants 200 years ago and because it has an N:C ratio of 2:4, this makes it an efficient molecule for the storage and transport of nitrogen in living organisms¹². There are some similar thermal studies of various types of mixed ligands with transition metals^{1, 2, 13-15}, however, no previous studies on the synthesis, characterization and thermal studies of the mixed ligand complexes of urea and asparagine acid have been reported. Hence, the present work reports the preparation, characterization and thermal study of new mixed ligand complexes of urea and asparagine with Co(II), Ni(II) and Cu(II) ions.

2. Materials and methods

2.1 Chemicals

All chemicals such as solvents, metal(II) chlorides (i.e. CoCl₂.6H₂O, NiCl₂.6H₂O and CoCl₂.2H₂O) were commercially available from BDH and were used without further purification.

2.2 Instrumentation

The melting points of the metal complexes were measured in glass capillary tubes with a Stuart Scientific Electrothermal melting point apparatus. TLC was carried out on silica gel GF₂₅₄ plates (mn-kieselgel G., 0.2 mm thickness) with a 3:1 v/v ethyl acetate / petroleum ether solution as eluent mobile at room temperature. The plates were scanned under ultraviolet light lamp of 254 nm. The CHN elemental analysis of the complexes was carried out by Vario ELFab. Chloride was determined volumetrically by silver nitrate. The amount of H₂O was determined gravimetrically using weight loss method. Perkin-Elmer 2380 flame atomic absorption spectrophotometer was used for the determination of metal content. Jenway conductivity meter model 4510 was used for measuring the molar conductance of the freshly prepared metal complexes solutions (10^-3 mol L^-1 in DMSO) at room temperature. IR spectra of the metal complexes were measured in the range 200-4000 cm^-1 with a FT/IR–140 (Jasco, Japan). Varian FT-300 MHz spectrometer was used for recording ¹HNMR spectra in d₆DMSO solvent and TMS as internal standard. Mass spectra were recorded in a Jeol JMS600 spectrometer. The electronic spectra of the complexes were measured in the range 400–800 nm, using UV–VIS spectrophotometer Specord 200, Analytilk Jena (Germany). The magnetic susceptibility of the solid complexes was measured at room temperature using Gouy’s method by a balance from Johnson Metthey and Sherwood model. The Differential Thermal Analysis (DTA) and Thermogravimetric Analysis (TGA) experiments were performed under nitrogen atmosphere using a platinum sample pan at a flow rate of 30 mL min^-1 and a 10 °C min^-1 heating rate for the temperature range 25–800 °C in Shimadzu DTA-50 and Shimadzu TGA-50H thermal analyzers, respectively, at Micro Analytical Center, Cairo University, Egypt.

2.3 Synthesis of mixed ligand complexes

Generally, the solid complexes were prepared by adding dropwise an ethanolic solution of hydrated metal(II) chlorides (0.01 mol) to an ethanolic solution of urea (0.01 mol) with stirring. The mixture was refluxed for 12 h with persistent stirring. A hot solution of 0.01 mol asparagine in 1:1 ethanol / water mixture ratio with drops of 1 mol L^-1 NaOH was used to adjust the pH at 7-7.5 and to deprotonate NH³⁺ in the asparagine to NH₂. The mixture was refluxed for 2 h until the formation of colored precipitate occurred. All the solutions were in 1:1:1 molar ratio. The end products were filtered off and washed with distilled water to remove NaCl, followed by absolute ethanol until the solution became clear, and after that the product was washed with DMF and left to dry¹⁶. The yield was 56%, 52% and 42% for Co, Ni and Cu-complexes, respectively.

3. Results and discussion

Complexes of Co(II), Ni(II) and Cu(II) with urea and asparagine are studied. Some physical properties, molar conductivity and analytical data are summarized in Tables 1 and 2. The elemental analysis proves that the complexes of Co(II), Ni(II) and Cu(II) with urea (ur) and asparagine (A_asn) ligands are of 1:1:1 (metal:ur:asn) molar ratio. The molar conductivity values indicate that the chloride anions are in the outer-sphere of these complexes.

Table 1.
Some properties of the complexes.

Table 2.
The elemental analysis of the complexes.

3.1 IR Spectra of urea-asparagine complexes

In these complexes, urea acts in two ways: as a monodentate ligand through oxygen of C=O, or as a bidentate through nitrogen of two NH₂ groups, while asparagine acts as an anion bidentate molecule, through COO^- group and NH₂ group. The assignment of the distinctive bands is summarized in Table 3 and the IR spectra of complexes are shown in Figures 2, 3 and 4.

The IR spectra of the complexes show additional broad bands in the range 3386- 3430 cm^-1 due to the υ(OH) stretching of water molecule. Coordinated water is also identified by the appearance of ρr (rocking) and ρw (wagging) approximately at 875 cm^-1 and 521 cm^-1, respectively. These results agree with the elemental analysis and thermogravimetric studies¹⁷. The υ(NH₂) stretching vibrations of free urea at υ_s3353 cm^-1 and υ_as3466 cm^-1 were shifted to lower wave numbers in the spectra of the complexes of Co(II) and Ni(II). This fact shows that the υ(NH₂) group must be involved in coordination while υ(CO) shifted to higher frequency¹⁸.

Table 3.
Main IR bands (cm^-1) of the urea-asparagine complexes.

Figure 2.
IR spectrum of [Co(ur)(asn)(H₂O)₂]Cl.

Figure 3.
IR spectrum of [Ni(ur)(asn)(H₂O)₂]Cl.

Figure 4.
IR spectrum of [Cu(ur)(asn)(H₂O)₃]Cl.

The IR spectrum of the Cu(II)-complex showed a new band at 1629 cm^-1 assigned to υ(C=O-Cu(ΙΙ)) with slight change in υ(NH₂) vibration¹⁹. In comparison with asparagine, υs(COO^-) and υas(COO^-) shift to lower wave numbers, confirming the monodentate nature of the coordinated carboxylate group²⁰.

The υ(NH₃⁺) band at 3110 cm^-1, which is specific for the zwitterion in asparagine, vanished in the spectra of the complexes after the deprotonation of NH₃⁺ to NH². Therefore, the higher wave numbers shift of the bands assigned to υ_as(NH₂) and υ_s(NH₂) indicates that the NH₂ group is imminently involved in the coordination²⁰.

IR of the prepared complexes showed weak bands in the range of 476-454 cm^-1 and 422- 413 cm^-1, attributed to υ(M-O) and υ(M-N), respectively²¹. Other bands are listed in Table 3.

3.2 ¹HNMR spectra of urea-asparagine complexes

¹HNMR spectra of [Co(ur)(asn)(H₂O)₂]Cl, [Ni(ur)(asn)(H₂O)₂]Cl and [Cu(ur)(asn)(H₂O)₃]Cl complexes show various signals which were summarized in Table 4. Urea shows a new signal at 5 and 5.1 ppm in Co(II) and Ni(II) complexes, respectively, for the amide groups coordinated to the metal atom without proton displacement²², while in Cu(II) complex only carbonyl group is coordinated to metal²². The signals at 3.2, 3.1 and 2.85 ppm are assigned to CH group, whereas signals at 2.9, 2.45 and 2.5 ppm of CH₂ group are observed for Co(II), Ni(II) and Cu(II) complexes, respectively . The appearance of a new signal around 2.6-2.7 ppm is attributed to NH₂ group of asparagine and the amide group shows signals in the range 6.3-6.7 ppm²³. The coordinated H₂O shows a new signal around 3.5-3.7 ppm²⁴.

Table 4.
¹HNMR chemical shift of free urea and asparagine ligands and their complexes

3.3 Mass spectra of urea – asparagine complexes

The mass spectra of Co(II), Ni(II) and Cu(II) complexes with urea and asparagine ligands exhibited the molecular ion peaks at m/z (calc. 321.58, found 321.61; calc. 321.34, found 321.36 and calc. 344.21, found 344.23), respectively.

The molecular ion of [Co(ur)(asn)(H₂O)₂]Cl complex loses NH₄Cl and H₂NCH₂COO^- leaving ions at m/z 268.13 and 247.07, respectively; then loses NH₂ and H₂O fragments, giving an ion at m/z 212.08. The spectrum of [Ni(ur)(asn)(H₂O)₂]Cl complex shows a peak at m/z 267.89, indicating the loss of H₂O and ½Cl₂. The molecule of [Cu(ur)(asn)(H₂O)₃]Cl loses H₂O + NH₂ and ½Cl₂ + H₂O, leaving ions at m/z 310.07 and 290.76, respectively. The ion at m/z 310.07 loses CO₂, leaving an ion at m/z 266.09, which further loses ½N₂ to leave an ion at m/z 252.01. Afterwards, this last ion gives a new peak at m/z 234, indicating loss of H₂O and the remaining fragment loses another CO, leaving an ion at m/z 206.05.

3.4 Magnetic and electronic spectral studies

The electronic spectra of the Co(II), Ni(II) and Cu(II) complexes as well as their magnetic moment data have provided good evidence for the structures of these complexes as shown in Table 5. For [Co(ur)(asn)(H₂O)₂]Cl, hexa-coordination is suggested as in Figure 5a, based on the appearance of bands at 18248 cm^-1 and at 14534 cm^-1 (Figure 6), which were attributed to the ⁴T_1g→⁴T_1g(P) (_υ3) and ⁴T_1g→ ⁴A^2g (υ₂) transitions, respectively²⁵. The third band, υ₁, could not be observed due to the limited range of the instrument used (200-1100 nm). Also, the magnetic moment of 4.81 B.M is within the range reported for a high-spin octahedral geometry around the Co(II) ion²⁶.

Table 5.
Magnetic moments and electronic spectral data in DMSO solution for the complexes.

[Ni(ur)(asn)(H₂O)₂]Cl complex has a magnetic moment of 2.9 B.M, which is within the range reported for an octahedral geometry around the Ni(II) ion with a ³A_2g ground term²⁷. In addition, this complex has three bands in the UV-VIS spectrum (Figure 7): the band at 22727 cm^-1 may be attributed to ³A_2g→ ³T_1g (υ₃); 16181 cm^-1 due to ³A_2g→ ³T_1g (υ₂) and υ₁ at 14619 cm^-1 in accordance with an octahedral structure around the Ni(II) ion (Fig. 5a)^{25, 28}. The electronic spectrum of [Cu(ur)(asn)(H₂O)₃]Cl (Fig. 5b and Fig. 8) shows a strong band at 16026 cm^-1. This band is due to ²E_g → ²T_2g transition and a distorted octahedral geome­try is suggested²⁵. The broadness of this band may be due to Jahn-Teller effect²⁵, which confirms the distorted octahedral geometry. The magnetic moment value (1.84 B.M) is also within the range reported for the d⁹-system containing one unpaired electron²⁹. The bands at 23585 cm^-1, 24510 cm^-1 and 22727 cm^-1 should be attributed to the charge transfer transitions in the complexes [Co(ur)(asn)(H₂O)₂]Cl, [Ni(ur)(asn)(H₂O)₂]Cl and [Cu(ur)(asn)(H₂O)₃]Cl, respectively³⁰.

Figure 5.
Suggested structure of the complexes.

Figure 6.
UV-VIS spectrum of [Co(ur)(asn)(H₂O)₂]Cl complex in DM.

Figure 7.
UV-VIS spectrum of [Ni(ur)(asn)(H₂O)₂]Cl complex in DMSO solution.

Figure 8.
UV-VIS spectrum of [Cu(ur)(asn)(H₂O)₃]Cl complex in DMSO solution.

3.5 The Thermal degradation study

The TGA and DTA curves of the prepared Ni and Cu complexes are given in Figures 9 to 12. These curves characterize and compare the thermal degradation of these two complexes at 10 °C min^-1 heating rate, under nitrogen and between 20-800 °C. For the evaluation of the thermal degradation kinetics parameters at a single heating rate (10 °C min^-1), the activation energy (E_a) and pre-exponential factor (Z) are determined by using the Coats-Redfen method for the reaction order n ≠ 1. When the Coats-Redfern method is linearized for a correctly-chosen order of reaction (n) yields the activation energy (E_a) from the slope of the equation:

(1)

where: α = fraction of weight loss, T = temperature (K), Z = pre-exponential factor, R = molar gas constant, q = heating rate and n = reaction order estimated by Horovitz-Metzger method.

The thermodynamic parameters of the thermal degradation step: enthalpy (ΔH*), entropy (ΔS*), and Gibbs energy (ΔG*) of activation are calculated using the following standard equations:

(2)

The characteristics of the thermal degradation of these two complexes recorded on the TG/DTG/DTA curves, their kinetics and thermodynamics parameters extracted from these curves are given in Tables 6-9.

Figure 9.
TG and DTG curves of [Ni(ur)(asn)(H₂O)₂]Cl complex.

Figure 10.
DTA curve of [Ni(ur)(asn)(H₂O)₂]Cl complex.

Figure 11.
TG and DTG curves of [Cu(ur)(asn)(H₂O)₃]Cl complex.

Figure 12.
DTA curve of [Cu(ur)(asn)(H₂O)₃]Cl complex.

3.5.1 Thermal analysis of [Ni(ur)(asn)(H₂O)₂]Cl

The thermolysis of [Ni(ur)(asn)(H₂O)₂]Cl (Tables 6 and 7) and (Figures 9 and 10) involves several successive steps at 24-186, 186-274, 274-351, 351-428 and 428-552 °C. The first step represents the elimination of 100% coordinated H₂O molecules and 12.5% of a chloride atom (calc. 12.59%, found 12.58%) with activation energy E_a = 93 kJ mol^-1 and a T_DTG peak at 132 °C. The second step corresponds to the loss of the remaining 87.5% Cl atom (calc. 12.12%, found 12.11%) which has E_a of 124 kJ mol^-1 with T_DTG at 242 °C and exothermic peak T_DTA at 248 °C. According to the DTG curve (Figure 8), the third step is assigned to the removal of 42.86% urea molecule (calc. 8.01%, found 8.00%) and with E_a and reaction order (n) of 114 kJ mol^-1 and 0.1, respectively. The fourth step which corresponds to the loss of the remaining urea molecule and 3.52% of asparagine (calc. 12.12%, found 12.11%) has E_a = 129 kJ mol^-1. The final step corresponds to the 59.82% loss of asparagine (calc. 24.41%, found 24.44%) with E_a = 131 kJ mol^-1 and a T_DTG peak at 491 °C. The final residue is NiO and carbon (2.67%) as ash (O=12.2%asn, C=24.46%asn) (calc. 33.21%, found 33.21%). The values of ΔS*, ΔH* and ΔG* are: -12.4, -117.9, -165, -120.4 and -144.5 J K^-1 mol^-1; 91.9, 122, 111.3, 125.7 and 126.9 kJ mol^-1; and 93.5, 150.5, 164.3, 172.8 and 197.8 kJ mol^-1, respectively, for the steps observed in the thermal decomposition of the complex.

Table 6.
Characteristic parameters of thermal decomposition (10 °C min^-1) for [Ni(ur)(asn)(H₂O)₂]Cl.

Table 7.
Kinetic and thermodynamic parameters of the thermal decomposition of [Ni(ur)(asn)(H₂O)₂]Cl.

Table 8.
Characteristic parameters of thermal decomposition (10 °C min^-1) for [Cu(ur)(asn)(H₂O)₃]Cl.

Table 9.
Kinetic and thermodynamic parameters of the thermal decomposition of [Cu(ur)(asn)(H₂O)₃]Cl.

3.5.2 Thermal analysis of [Cu(ur)(asn)(H₂O)₃]Cl

The TG and DTG curves of [Cu(ur)(asn)(H₂O)₃]Cl (Tables 8 and 9) and (Figures 11 and 12) show five steps of a continuous mass loss with DTG peaks indicating slow mass losses. The first step (22-178 °C) at T_DTG 143 °C is attributed to the release of 19.05% of coordinated H₂O (calc. 2.99%, found 2.98%). The remaining loss of H₂O, and the loss of 100% Cl and 36.36% of urea occur in the second step (178-306 °C). Third (306-449 °C), fourth (449-666 °C) and fifth (666-785 °C) steps are due to the release of [63.64%ur+16.46%asn], [43.20%asn] and [14.40%asn] fragments (calc. 17.37%, found 17.35%; calc. 16.46%, found 16.44% and calc. 5.49%, found 5.51%), respectively, at the T_DTG peaks at 362, 514 and 725 °C (Figure 9), respectively. The E_a calculated of these five steps are 98, 119, 134, 123 and 137 kJ mol^-1, respectively, and the values of ΔS*, ΔH* and ΔG* are: -192.6, -102.7, -82.3, -126.3 and -146.9 J K^-1 mol^-1; 96.8, 117.2, 135.0, 118.7 and 134.0 kJ mol^-1, and 124.3, 138.7, 165.2, 183.6 and 240.5 kJ mol^-1, respectively, for the steps observed in the thermal decomposition of the complex.

3.5.3 General remarks of thermal degradation:

1. Thermal analysis confirms the presence of coordinated water molecules.

2. Sharp peak in Ni-complex means that the leaving parts move away faster than that in Cu-complex.

3. The releasing of the urea ligand before asparagine ligand may be due to non-ionic bonding of this ligand with the metal ions.

4. The first step which represents the dehydration of coordinated water is faster in Ni complex (E_a = 93 kJ mol^-1) than in Cu complex (E_a = 98 kJ mol^-1) (Figure 11a). The Cu complex is more stable than Ni complex (Figure 11b), which is indicated by the higher T_max value.

5. The chloride evolution starts at the first step in Ni complex and in the second step the Cu complex, in accordance with literature³¹. On the other side, the high value of T_DTG (211 °C) of Cu complex reflects its higher stability compared to Ni complex of T_DTG (132 °C).

6. Ni complex has a faster complete decomposition of its backbone (E_a = 131 kJ mol^-1) than Cu complex (E_a = 137 kJ mol^-1), making clear the higher stability of Cu complex.

7. The values of ΔG* for a given complex, generally, increase significantly for the subsequent decomposition steps, consequence of the increase of TΔS* values from one step to another which exceed the ΔH* values.

4. Conclusions

In this paper, some new complexes containing urea and asparagine ligands were prepared and characterized. The complexes have the following molecular formulae: [M(L₁)(L₂)(H₂O)_n]Cl where M = Co(II), Ni(II) and Cu(II), L₁ = urea, and L₂ = asparagine. These complexes were characterized by elemental analysis, conductance measurements, IR, ¹HNMR and mass spectroscopy. Electronic spectra and magnetic measurements suggested an octahedral geometry for the complexes. Thermal analysis study showed faster decomposition reactions of the Ni complex and higher thermal stability of Cu complex.

Acknowledgements

This work was supported by Sana'a University, Faculty of Science. Authors extend the thanks to Inorganic Chemistry Section for using their Laboratory.

6. References

[1] Singh, M. K., Laskar, R., Sutradhar, S., Paul, B., Bhattacharjee, S., Das, A., Synthesis and Characterization of mixed ligand complexes of Co (II) ion with some N and S donor, IOSR Journal of Applied Chemistry 7 (4) (2014) 24-29. https://doi.org/10.9790/5736-07412429.

[2] Bindu, G. H., Rao, G. N., Mixed ligand complexes of essential metal ions with L-glutamine and succinic acid in sodium dodecyl sulfate–water mixtures, J. Serb. Chem. Soc. 77 (4) (2012) 453–463. https://doi.org/10.2298/JSC110201177B.

[3] Li, Y. T., Yan, C. W., Zhu, Y., Guan, H. S., Synthesis and Magnetic Studies of Μ-Oxamidobridged Copper (II)–Manganese (II) Heterobinuclear Complexes, Synth. React. Inorg., Met-Org. Nano-Met. Chem. 34 (2005) 1165–1179. https://doi.org/10.1081/SIM-120039264.

[4] Aydogdu, Y., Yakuphanoglu, F., Aydogdu, A., Cukurovah, E., Solid State Electrical Conductivity Properties of Copper Complexes of Novel Oxime Compounds Containing Oxolane Ring, Material Letters 57 (2003) 3755-3760. https://doi.org/10.1016/S0167-577X(03)00174-5.

[5] Rahaman ,S. K. H., Fun, H. K., Ghosh, B. K., A Study on Copper(II)-Schiff Base-Azide Coordination Complexes: Synthesis, X-Ray Structure and Luminescence Properties of [Cu(L)(N₃)]X (L = Schiff Bases; X = ClO₄, PF₆), Polyhedron 24 (2005) 3091-3097. https://doi.org/10.1016/j.poly.2005.06.028.

[6] Carballo, R., Castineiras, A., Covelo, B., Martinez, E. G., Niclós, J., Lopez, E. M. V., Solid state coordination chemistry of mononuclear mixed ligand complexes of Ni (II), Cu (II) and Zn (II) with α-hydroxycarboxylic acids and imidazole, Polyhedron 23 (9) (2004) 1505-1518. https://doi.org/10.1016/j.poly.2004.02.028.

[7] S. Rafique, S., Idrees, M., Nasim, A., Akbar, H., Athar, A., Transition metal complexes as potential therapeutic agents, Biotechnol. Mol. Biol. Rev. 52 (2) (2010) 38-45.

[8] Saffran, E. K., Kinne, R. K. H., Vitalism and Synthesis of Urea - From Friedrich Wöhler to Hans A. Krebs, American Journal of Nephrol 19 (1999) 290–294. https://doi.org/10.1159/000013463.

[9] Penland, R. B., Mizushima, S., Curran, C., Quagliano, J. V., Infrared Absorption Spectra of Inorganic Coordination Complexes. X. Studies of Some Metal-Urea Complexes, J. Am. Chem. Soc. 79 (7) (1957) 1575–1578. https://doi.org/10.1021/ja01564a014.

[10] Savinkina , E. V., Golubev, D. V., Podgornov, K.V., Albov, D. V., Grigoriev, M. S., Davydova , M. N., Different Types of Coordinated Urea Molecules in its Complexes with Rare‐Earth Iodides and Perchlorates, Zeitschrift für anorganische und allgemeine Chemie 1 (39) (2013) 53-58. https://doi.org/10.1002/zaac.201200267.

[11] Amenda, J. P., Helgesonb, H. C., Solubilities of The Common L-A-Amino Acids as A Function of Temperature Solution PH, Pure & Appl. Chem. 69 (5) (1997) 935-942. https://doi.org/10.1351/pac199769050935.

[12] Lea, P. J., Sodek, L., Parry, M. A. J., Shewry, P. R., Halford, N. G., Asparagine in plants, Ann. Appl. Biol. 150 (2007) 1–26. https://doi.org/10.1111/j.1744-7348.2006.00104.x.

[13] Alabdali, A. J., Ibrahim, F. M., Synthesis and Thermal Study of Co (II), Ni (II), Cu (II) Mixed Ligand Complexes Using Histidine As Tridentate Ligand, IOSR Journal of Applied Chemistry (IOSR-JAC) 6 (6) ( 2014) 60-63. https://doi.org/10.9790/5736-0666063.

[14] Mihsen, H. H., Shareef, N. K., Synthesis, characterization of mixed- ligand complexes containing 2,2-Bipyridine and 3-aminopropyltriethoxysilane, Journal of Physics: Conf. Series 1032 (2018) 012066. https://doi.org/10.1088/1742-6596/1032/1/012066.

[15] Leelavathy, C., Arul Antony, S., Structural elucidation and thermal studies of some novel mixed ligand schiff base metal (II) complexes, International Journal of Basic and Applied Chemical Sciences 3 (4) (2013) 88-95. http://www.cibtech.org/jcs.htm.

[16] Pop, V., David, L., Simuţ, C., Drăgan, S. F. M., Spectroscopic Investigation of Copper (II) Complex with Mixed Ligands, Aspartic Acid and Urea, Anal. Univ. Din Orad. 17 (2003) 17- 21. http://www.fizicaoradea.ro/docs/2003_a_2.pdf.

[17] Gamo, I., Infrared Spectra of Water of Crystallization in Some Inorganic Chlorides and Sulfates, Bull. Chem. Soc. Jpn. 34 (1430) (1961) 760-764. https://doi.org/10.1246/bcsj.34.760.

[18] Abdul-Ameer, N., Synthesis and Characterization of Mn(П), Co(П), Ni(П) and Cu(П) Complexes with Mixed Ligands, Aceturic Acid and Urea, Tikrit Journal of Pure Science 17 (2) (2012) 55-61. ISSN: 18131662.

[19] Sagatys, D. S., Bott, R. C., Smith, G., Byriel, K. A., Kennard, C. H. L , The Preparation and Crystal Structure of A Polymeric (1:1)-Silver Nitrate-Urea Complex, [(AgNO₃)2(CH₄N₂O)₂]N, Polyhedron 11 (1) (1992) 49-52. https://doi.org/10.1016/S0277-5387(00)83258-1.

[20] Bellamy, L. J., The Infrared Spectra of Complex Molecules, 3rd Ed., London: Chapman and Hall, 1975.

[21] El-Metwally, N. M., Gabr, I. M., El-Asmy, A. A., Abou-Hussen, A. A., Spectral, Magnetic, Electrical and Thermal Studies on Malonyl Bis(Thiosemicarbazide) Complexes, Transition Metal Chemistry 31 (1) (2006) 71-78. https://doi.org/10.1007/s11243-005-6347-6.

[22] Gaballa, A. S., Teleb, S. M. Nour, E.-M., Synthesis and Spectroscopic Studies of Metal Complexes Formed in The Reaction of Metal Ions with Urea At High Temperature, Journal of The Korean Chemical Society 51 (4) (2007) 339-345. https://doi.org/ 10.5012/jkcs.2007.51.4.339.

[23] Amparo, C., Virtudes, M., Eiles, M., Carles, M., Methionine and Histidine Pd(II) and Pt(II) Complexes: Crystal Structures and Spectroscopic Properties, J. Inorg. Biochem. 48 (1992)135-152. https://doi.org/10.1016/0162-0134(92)80023-O.

[24] Hosny, W. M., Dioxouranium(VI) Mixed Ligand Complexes Containing 8-Hydroxyquinoline and Some Amino Acids, Synth. React. Inorg. Met. Org.Chem. 2 (1998) 1029-1052. https://doi.org/10.1080/00945719809351686.

[25] El-Asmy, A. A., Khalifa, M. E., Rakha, T. H., Hasanian, M. M., Abdallah, A. M., Mono and trinuclear complexes of oximinoacetoacetyl-pyridine-4-phenylthiosemicarbazone, Chem. Pharm. Bull. 48 (2000) 41-44. https://doi.org/10.1248/cpb.48.41.

[26] Saha, N. C., Butcer, R. J., Chaudhuri, S., Saha, N., Synthesis and spectroscopic characterisation of cobalt(III) and nickel (II) complexes with 5-methyl-3-formylpyrazole-N (4)-dibutylthiosemicarbazone (HMP z NBu 2): X-ray crystallography of [Co(MPz NBu₂)₂] NO₃· H₂O (I) and [Ni(HMPzNBu₂)₂](ClO₄)₂ (II), Polyhedron 22 (3) (2003) 383-390. https://doi.org/10.1016/S0277-5387(02)01343-8.

[27] El-Metwally, N. M., El-Asmy, A. A., Chelating Activity of Bis(Diacetylmonoxime) Thiocarbohydrazone Towards VO2+, Co(II), Ni(II), Cu(II) and Pt(IV) Ions, Journal of Coordination Chemistry 59 (14) (2006) 1591-1601. https://doi.org/10.1080/00958970600572743.

[28] B. N., Figgis, “Introduction To Ligand Field Theory”, Interscience, New York, 258, 319(1967).

[29] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam (1968).

[30] Al-Maydama, H., El-Shekeil, A., Khalid, M. A., Al-Karbouly, A., Thermal degradation behaviour of some polydithiooxamide metal complexes, Eclét. Quím. 31 (1) (2006) 45-52. https://doi.org/10.1590/S0100-46702006000100006.

[31] Komiyama, T., Igarashi, S., Yukawa, Y., Synthesis of polynuclear complexes with an amino acid or a peptide as a bridging ligand, Current Chemical Biology 2 (2) (2008) 122-139. https://doi.org/10.2174/187231308784220509.

Author notes

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