Synthesis, characterization and comparative thermal degradation study of Co(II), Ni(ΙΙ) and Cu(II) complexes with Asparagine and Urea as mixed ligands

New Co(II), Ni(II) and Cu(II) complexes with urea and asparagine as ligands have been synthesized in (M:L1:L2) molar ratio (where M= Co(II), Ni(II) and Cu(II), L1 =urea, and  L2 =asparagine) then identified by micro analytical data, molar conductance measurements,  IR, 1HNMR, Mass, UV-VIS spectroscopies and magnetic susceptibility measurements. Thermal degradation studies were carried out by thermal analysis. These complexes have the general formula [M(L1)(L2)(H2O)n]Cl. The molar conductance values in DMSO solvent show the electrolytic nature of these complexes, indicating the outer-sphere coordination of the chloride anions with metal ions. The three complexes have an octahedral structure although urea has shown two modes of coordination. Thermal analysis study shows rapid decomposition reaction for Ni complex and the highest thermal stability for Cu complex. The kinetic parameters were determined from the thermal decomposition data using the Coats-Redfern method. Thermodynamic parameters were calculated using standard relations.


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][4][5] .
Mixed ligand complexes containing amino acid as co-ligand are potential biomimetic models for metal-protein interaction 6 . Research has shown significant progress in utilization of transition metal complexes as drugs to treat a lot of human diseases like carcinomas, infection control, antiinflammatory, diabetes and neurological disorders 7 .
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 sulphate 8 .
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) ABSTRACT: New Co(II), Ni(II) and Cu(II) complexes with urea and asparagine as ligands have been synthesized in (M:L1:L2) molar ratio (where M= Co(II), Ni(II) and Cu(II), L1 = urea, and L2 = asparagine) then identified by micro analyses, molar conductance measurements, IR, 1 HNMR, Mass, UV-VIS spectroscopies and magnetic susceptibility measurements. Thermal degradation studies were carried out by thermal analysis. These complexes have the general formula [M(L1)(L2)(H2O)n]Cl. The molar conductance values in DMSO solvent show the electrolytic nature of these complexes, indicating the outer-sphere coordination of the chloride anions with metal ions. The three complexes have an octahedral structure with urea molecule showing two modes of coordination. Thermal analysis study shows the rapid decomposition reaction for Ni complex and the highest thermal stability for Cu complex. The kinetic parameters were determined from the thermal decomposition data using the Coats-Redfern method. Thermodynamic parameters were calculated using standard relations. coordinate to the oxygen of urea 9 . Also, there are different types of coordination of urea in its complexes with rare-earth iodides and perchlorates 10 . 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 proteins 11 . The amino acid L-asparagine or 2-amino-3carbamoylpropanoic 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 organisms 12 . 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.

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 (mnkieselgel 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 1 HNMR 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.

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 dry 16 . The yield was 56%, 52% and 42% for Co, Ni and Cu-complexes, respectively.

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 (Aasn) 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.

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 COOgroup 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 to 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 studies 17 . The υ(NH2) stretching vibrations of free urea at υs3353 cm -1 and υas3466 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 frequency 18 Table 3. Main IR bands (cm -1 ) of the urea-asparagine complexes.   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) vibration 19 . In comparison with asparagine, υs(COO -) and υas(COO -) shift to lower wave numbers, confirming the monodentate nature of the coordinated carboxylate group 20 .
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 υas(NH2) and υs(NH2) indicates that the NH2 group is imminently involved in the coordination 20 .
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), respectively 21 . Other bands are listed in Table 3.  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 displacement 22 , while in Cu(II) complex only carbonyl group is coordinated to metal 22 . 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 ppm 23

Mass spectra of ureaasparagine 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.

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 4 T1g→ 4 T1g(P) (υ3) and 4 T1g→ 4 A2g (υ2) transitions, respectively 25 . 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) ion 26 . [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 3 A2g ground term 27 . In addition, this complex has three bands in the UV-VIS spectrum (Figure 7): the band at 22727 cm -1 may be attributed to 3 A2g→ 3 T1g (υ3); 16181 cm -1 due to 3 A2g→ 3 T1g (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 2 Eg → 2 T2g transition and a distorted octahedral geometry is suggested 25 . The broadness of this band may be due to Jahn-Teller effect 25 , which confirms the distorted octahedral geometry. The magnetic moment value (1.84 B.M) is also within the range reported for the d 9 -system containing one unpaired electron 29

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 (Ea) 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 (Ea) from the slope of the equation: 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: max ln kT 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.

2.
Sharp peak in Ni-complex means that the leaving parts move away faster than that in Cucomplex. 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 (Ea = 93 kJ mol -1 ) than in Cu complex (Ea = 98 kJ mol -1 ) (Figure 11a). The Cu complex is more stable than Ni complex (Figure 11b), which is indicated by the higher Tmax value. 5.
The chloride evolution starts at the first step in Ni complex and in the second step the Cu complex, in accordance with literature 31 . On the other side, the high value of TDTG (211 °C) of Cu complex reflects its higher stability compared to Ni complex of TDTG (132 °C). 6.
Ni complex has a faster complete decomposition of its backbone (Ea = 131 kJ mol -1 ) than Cu complex (Ea = 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.

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, 1 HNMR 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.

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