Complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with ligand formed by condensation reaction of isatin with glutamic acid

12 Eclética Química Journal, vol. 45, n. 3, 2020, 12-27 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v45.3.2020.p12-27 Complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with ligand formed by condensation reaction of isatin with glutamic acid Wiesława Ferenc , Dariusz Osypiuk, Jan Sarzyński, Halina Głuchowska Maria Curie-Skłodowska University, Institute of Chemical Sciences, Faculty of Chemistry, Department of General and Coordination Chemistry and Crystallography, 20-031 Lublin, Poland


Introduction
A type of ligand such as isatin (1H-indole-2,3dione) creates an interesting basis of current research in inorganic and coordination chemistry due to its wide applications and diverse biological properties. It plays an important role such as: antibacterial, antifungal, anticonvulsant, anti-HIV and antiviral 1-7 . From literature survey, it appears that isatin heterocycles exhibit manifold importance in the field of medicinal chemistry. It is used for the design and development of anticancer drugs 1,2 . There are over ten thousand biologically active compounds containing indole core. More of them are approved as commercially available drugs or are undergoing clinical trials. Antioxidant potential of the thiosemicarbazone derivatives was analysed by in vitro free radical scavenging assay 8 . Therefore, derivatives of isatin based on the thiosemicarbazone are potential compounds with wide range of promising biological properties which may be explored further for the treatment of several diseases. Likewise, isatin structural scaffold could act as DNA inhibitors. The isatin compound hybrids have the potential to overcome the drug resistance 9 .
Isatin and its derivatives are one of the most important and broadly occurring structural units in several natural compounds. As a natural substance it occurs in plants while as metabolite in the human body [10][11][12][13][14][15][16] .
Isatin is sparingly soluble in ether. From water, alcohol and acetic acid it crystallizes in red needles form that melt at 200 -201ºC 17 . Its structure may cause the electrophilic substitution or nucleophilic addition on the carbonyl group carbon atom.
The metal complexes of isatin were found to have pharmacological properties and some of them, especially Co(II) compounds, can form the interesting group of single-molecule magnets (SMMs) with special magnetic applications [18][19][20][21][22] . They show magnetization hysteresis at low temperature occurring the special property of macroscopic magnets and possess a finite magnetization that can be frozen in the absence of an applied magnetic field. At low temperatures, these systems can be considered as a magnet since the relaxation of the magnetization becomes significantly slow 22 .
Isatin metal compounds with amino acids are little known. Only a few information about the synthesis of isatin and amino acid ligands are available in the literature 23,24 . Therefore, their studies are important for understanding the relationship between chemical structure and biological macrocycle activities that may indicate their practical use as models in biochemistry systems. Thus, the chemistry of metal complexes with multidentate ligands as isatin with amino acids can gain much interest. The toxicity towards various bacteria, the cytotoxic and inhibitory effects of the obtained compounds on some cells and examination of their impact on living organisms seem interesting.
The aim of our investigations was to synthesize the complexes of new ligand synthesized in reaction of condensation of isatin and glutamic acid, 2-(2-oxoindolin-3ylideneamino)pentanedioic acid (C13H12N2O5) with Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) ions and to characterize them by various physicochemical measurements.

Materials
All chemicals and solvents used for the syntheses were of commercially available reagent grade and applied without further purification.

Synthesis
Ligand (Scheme 1) was synthesized by refluxing glutamic acid (0.01 mol L -1 ) (1.47 g) with isatin (0.01 mol L -1 ) (1.47 g) in 100 mL methanol aqueous solution (99% pure, Aldrich Chemical Company). For the reaching of equilibrium state the solids in solutions were constant heating for 2-3 h in the presence of three drops of glacial acetic acid. They were filtered off, washed with water and methanol and dried at 303 K to constant masses.
The complexes of Mn(II), Co(II), Ni(II), Cu(II), Zn(II) with ligand were prepared by adding the equivalent quantities of methanol solutions of metal chlorides (3.6 mmol): MnCl2·4H2O (710 mg), CoCl2·6H2O (860 mg), NiCl2·6H2O (860 mg), CuCl2·2H2O (610 mg) and ZnCl2 (490 mg) (analytically pure, Polish Chemical Reagents in Gliwice -Poland) to a warm methanol solution of H2L (3.6 mmol, 1000 mg) in the round bottom flask. The pH of solution was adjusted by water ammonia solution dropwise to pH value about 6.5 -7. Then the reaction mixture was refluxed for 6 -8 h. It was cooling at room temperature and after partial evaporation filtered off, washed with water and methanol and dried at 303 K to constant mass. Scheme 1. a) Ligand synthesis, b) FTIR spectra of components used for the synthesis.

Methods and apparatus applied
The contents of carbon, hydrogen and nitrogen were determined by elemental analysis using a CHN 2400 Perkin-Elmer analyser. The amounts of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) metals were established by X-ray fluorescence XRF method with the use of spectrophotometer of X-ray fluorescence with energy dispersion EDXRF-1510 (Canberra-Packard).
The FTIR spectra of complexes and the products of the intermediate and final complex decompositions were recorded over the range of 4000 -400 cm -1 using an M-80 Perking-Elmer spectrometer. The samples were prepared as KBr discs. The 1 H-NMR spectrum for ligand and Zn(II) compound in DMSO-d6 was recorded on a Bruker Avance 300 MHz NMR spectrometer at 298.1 K.
The X-ray diffraction patterns of compounds and of their residues after final decomposition processes were taken on a HZG-4 (Carl-Zeiss. Jena) diffractometer with Ni filtered CuKα radiation. The measurements were made within the range of 2θ = 4º -80º by means of Bragg-Brentano method.
The thermal stability and decomposition of the complexes were studied in air using a Setsys 16/18 (Setaram) TG, DTG and DSC instrument. The experiments were carried out under air flow rate of 20 mL min -1 in the range of 297 -1173 K at a heating rate of 5 K min -1 . The initial masses of samples used for measurements changed from 7.85 to 4.09 mg and were heated in Al2O3 crucibles.
The TG-FTIR measurements of Mn(II), Co(II) and Zn(II) complexes were performed to identify their gaseous decomposition products on the Q5000 TA apparatus coupled with the Nicolet 6700 spectrophotometer. The experiments were carried out under a dynamic nitrogen atmosphere in flowing nitrogen of 20 mL min −1 in open platinum crucibles. The complexes were heated up to 1273 K at a heating rate of 20 K min −1 . The gaseous decomposition products were analysed over the range of 4000 -400 cm -1 using the Nicolet 6700 spectrophotometer.
Magnetic susceptibility of polycrystalline samples of transition metal compounds was investigated at 76 -303 K and for some of them at 2 -300 K. The measurements in the range of 76 -303 K were carried out using the Gouy's method. Mass changes were obtained from Cahn RM-2 electrobalance. The calibrate employed was Hg[Co(SCN)4] for which the magnetic susceptibility was assumed to be 1.644·10 -5 cm 3 g -1 . The measurements were made at a magnetic field strength of 9.9 kθe. Correction for diamagnetism of the constituent atoms was calculated by the use of Pascal's constants 25 .
The effective magnetic moment values were calculated from the Eq. 1: where: µeff -effective magnetic moment, mmagnetic susceptibility per molecule and Tabsolute temperature.
The measurements in the range of 2 -300 K were carried out with the use of Quantum Design SQUID -VSM magnetometer at magnetic field 0.1 T. The superconducting agent may generally operate at a field strength ranging from 0 to 7 T. The SQUID magnetometer was calibrated with the palladium rod sample.

Results and Discussion
All synthesized compounds were obtained as powders with the different colours changing from brownish pink, via brown to brick-red ones. Elemental analysis, XRF and thermogravimetric experimental data confirmed them to be hydrated. The molar conductance, magnetic and infrared spectroscopy measurements were made to estimate their structure. The compounds were obtained with general formulae M(LH)2·nH2O for Mn(II), Ni(II) and Zn(II) and ML·nH2O for Co(II) and Cu(II), where LH=C13H11N2O5 -, L=C13H10N2O5 2-, n = 1 for Mn(II), Cu(II) and Zn(II), n = 2 for Co(II) and n = 3 for Ni(II). Some predictions about the compositions of compounds may be compatible with those in literature [26][27][28][29][30] . In the cited papers the ways of Cu(II) and Pd(II) ion coordination with ligand formed by the glutamic acid and various organic compound condensation reactions are presented. Not having identical compositions to those used by us, these arrangements show that one deprotonated glutamic carboxylic acid group may coordinate to metal ions. As we did not determine the complex structures this fact let us also state the formulae of compounds, especially for Cu(II) and Co(II) ones.
The results of elemental and XRF analyses are as follows: Ligand The complexes are insoluble in most of the organic solvents except methanol and acetonitrile.
The 1 H-NMR spectrum for H2L was recorded in DMSO-d6. The experimental results were not suitable for fair interpretation due to their poor quality, high ratio of background noise to signals and d-electron nature so the results were not presented in this article.
In order to estimate the electrolytic properties of these compounds dissolved in methanol their molar conductance were measured and they were found to be in the range of 8.23 -33.67 S cm 2 mol -1 indicating the analysed complex solutions not to be electrolytes since their molar conductance worths are less than 70 S cm 2 mol -1 23,24 .

Thermal analysis
The thermal stability of complexes was studied in air at 293 -1173 K (Tab. 1, Fig. 1). When heated to 1173 K they decompose in three stages. First they dehydrate in one step and next being gradually decomposed form ultimately the oxides of appropriate metals with the intermediate formations of oxycarbonates, M2OCO3 or their mixtures with the metal oxides.  HL -=C13H11N2O5 -, L=C13H10N2O5 2-, ∆T1 -temperature range of dehydration process, nnumber of water molecules lost in one step, ∆T2 -temperature range of anhydrous complex decomposition, TKfinal temperature of decomposition process, ∆Hº -enthalpy value for one molecule of water.
The complexes are stable up to 310-323 K. In the range of 310 -414 K they dehydrate with an endothermic effect losing all water molecules. The found enthalpy values for one water molecule of dehydration process change from 10.31 to 37.78 kJ mol -1 . On further heating of compounds at 413 -1013 K the oxycarbonates, M2OCO3 of the corresponding metals were formed 31   The residue masses calculated from TG curves are equal to 10.10-14.98%, while those theoretical were 11.01-14.53%.
The coordination number values of central ion depend mainly on the kind of cation 32,33 . In order to determine them it is necessary to estimate the positions of water molecules in analysed compounds. Not having monocrystal structure data of compounds we can only estimate them taking into account their initial dehydration process temperature values. If water molecules are lost in one step below 423 K they may form outer sphere water 32 . However, if they are coordinated weakly with central atom, they may be released also at low temperature indicating this way their outer sphere character. Then in the presented complexes it is difficult to state if water molecules are in outer or inner coordination positions.

TG-FTIR Analysis
The TG-FTIR coupled technique was applied for analysed complexes of Mn(II), Co(II) and Ni(II) to identify their gaseous decomposition products. For example the FTIR spectra of the volatile components of mixture evolved during destruction of Co(II) complex are presented in Figs. 3 and 4. Their interpretations reveal that H2O, CO2, CO and hydrocarbons are released during heating to 1173 K 34,35 . The heating of Co(II) complex leads to the release of water molecules up to about 393 K. The FTIR spectra show characteristic bands in the regions: 4000-3600 and 1700-1400 cm −1 (Fig. 3) due to stretching and deformation vibrations of water molecules 34 . Next, the intensity of gases evolved during heating increases. It is connected with the great liberation of CO2 molecules. The FTIR spectra show bands at 3800 -3500 cm −1 , 2400 -2300 cm −1 and at about 900 cm −1 coming from stretching and deformation vibrations of carbon dioxide. The maximum amounts of CO2 are observed after 20 minutes of heating at about 580 K. The FTIR spectra show as well the characteristic bands derived from N2O at 1250 -1100 cm -1 and ammonia molecules at about 1000 -800 cm -1 . Additionally, the bands at 1200 and 1000 cm -1 derived from the rocking vibrations of C-H resulting from the (CH3)2N moiety are observed 34,35 . Another volatile products of Co(II) compound thermal decomposition in nitrogen are hydrocarbons. Therefore FTIR spectrum contains the bands at 3300 -3100 cm −1 , 1700 -1650 cm −1 and 1500 -1400 cm −1 coming from their molecule stretching vibrations 34,35 . Above 700 K the bands at 2200 -2100 and 800 cm −1 resulting from carbon monoxide vibrations occur. Their highest intensities appear after 30 minutes of heating 34 .

X-Ray powder diffraction
X-Ray powder diffraction of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes indicated them to be a crystalline compounds. A number of crystallization trials to obtain the single crystals of analysed complexes with several different solvents (such as H2O, alcohols, DMSO, DMF and acetonitrile) have been carried out. The compounds were dissolved in pure solvent as well as in solvent mixture. Crystallization test were performed both at room temperature and at 8 ºC.
Due to the lack of monocrystals of analysed compounds suitable for measurements, estimation of the unit cell parameters was carried out applying the Dicvol06 programme 36,37 (Tab. 2) using the Xray powder diffraction data. According to the obtained results all analysed complexes were found to form low symmetry compounds. The complexes of Mn(II), Co(II), Cu(II) and Zn(II) crystallized in monoclinic system while that of Ni(II) in triclinic one 36,37 . All experimental data (angular values and lattice constants of primitive cell) are showed in Tab

Infrared spectra
The IR spectrum of ligand exhibits a broad band at 3260 cm -1 assigned to the N-H stretching vibration mode, ν(H-N) 34,35 . Also in its spectrum the strong sharp ketonic band of indole group vibration, ν(C=O), appears at 1710 cm -1 . A clear band of stretching vibration due to C=N, ν(C=N), appears at 1620 cm -1 and band at 1484 cm -1 comes from stretching vibration of HC-N group, ν(HC-N). The stretching vibration of C=O from COOH group yields the band at 1712 cm -1 , ν(C=O). In the spectrum of ligand, the bands of C=C stretching vibrations, ν(C=C), and C-H scissoring vibrations, δ(C-H), are at 1104 and 752 cm -1 , respectively.
In the spectra of analysed complexes, the new wide bands at 3440 -3332 cm -1 appear indicating the presence of water molecules in compounds. It is in good agreement with the results of the elemental analysis and thermogravimetric data. The stretching vibration bands of N-H group, ν(N-H), are present in the region of 3340 -3200 cm -1 . The ketonic bands of indole C=O group vibrations, ν(C=O), occur in their spectra from 1725 to 1704 cm -1 . A strong band due to C=N stretching vibrations, ν(C=N), appears at 1620 cm -1 . In complex spectra the HC-N vibration band is present at 1468 cm -1 . In the spectra of Co(II) and Cu(II) complexes there is not characteristic band of carboxylic acid stretching vibration νCOOH at 1712 cm -1 that is splitted into two band peaks of asymmetric and symmetric carboxylate stretching vibrations, νasCOO-and νsCOO, at 1692 and 1696 cm -1 and 1336 and 1328 cm -1 , respectively, (Tab. 3). It indicates that two carboxylate anions take part in the metal ion coordination. In the case of Mn(II), Ni(II) and Zn(II) complex spectra there are bands of carboxylic acid stretching vibrations, νCOOH, at 1712, 1692 and 1688 cm -1 resulting from one carboxylic group that does not coordinate with metal ions. There are also seen the bands of asymmetric and symmetric carboxylate stretching group vibrations, νasCOOand νsCOO-, at 1690, 1680 and 1680 cm -1 and at 1296, 1328 and 1340 cm -1 , respectively (Tab. 3). It suggests that only one carboxylate anion coordinates with central ion. In the spectra of complexes the C=C stretching vibration bands, ν(C=C), and C-H scissoring vibration bands, δ(C-H), are in the ranges of 1104 -1088 cm -1 and 756 -748 cm -1 , respectively.
There are some new bands present in the spectra of compounds not being seen in the ligand IR spectrum. The bands at 680 -580 cm -1 confirmed the presence of the metal ion-oxygen bonds in complexes 38,39  There are the differences between the spectrum of ligand and the spectra of its compounds. In the spectra of complexes are bands at 3440 -3332 cm -1 , characteristic for ν(OH) stretching vibrations confirming the presence of water molecules in the compounds. These bands are not in the ligand spectrum. Ketonic bands of indole C=O stretching vibrations, ν(C=O), in the spectra of compounds occur in the range of 1725 -1704 cm -1 , while in the H2L spectrum it appears at 1710 cm -1 . The different values of those band frequencies may suggest the C=O group coordination with metal ions in the complex molecules 34,35 . A band due to azomethine nitrogen stretching vibration, ν (HC-N), occurs at 1484 cm -1 in the ligand spectrum whereas in all the complex spectra it is observed only at 1468 cm -1 . Therefore, azomethine nitrogen was found to coordinate with metal ions.
In the spectrum of ligand and in the spectra of compounds the bands resulting from C=N and C=C, stretching vibrations, ν(C=N), ν(C=C), and C-H scissoring vibrations δ(C-H), occur at 1620, 1104 -1088 and 756 -748 cm -1 , correspondingly.

Magnetic measurements
The magnetic susceptibility of Mn(II), Co(II), Ni(II) and Cu(II) compounds was measured in the ranges of 77 -303 K and 2 -300 K.
The effective magnetic moment values experimentally determined in the range of 77 -303 K changed from: 4.24 to 4.34 µB for Mn(II), 3.05 -3.25 µB for Co(II), 3.09 -2.65 µB for Ni(II) and 1.21 to 1.47 µB for Cu(II) complexes while those at 2 -300 K varied from: 3.09 to 5.02 µB for Mn(II), 2.22 to 3.12 µB for Co(II), 3.12 to 3.87 µB for Ni(II), 0.79 to 1.31 µB for Cu(II) complexes. The molar susceptibility measurements for helium temperatures were carried out in an applied magnetic field of 0.1 T.
All analysed compounds demonstrate paramagnetic properties and obeyed the Curie-Weiss law. Their magnetic susceptibility values decreased with increasing temperature [42][43][44][45][46][47][48] . The dependences of magnetic susceptibility, m corr , its reciprocal values and also m corr *T worths as a function of temperature for Mn(II) and Cu(II) complexes are presented in Figs. 7 and 8.
At a high temperature region, the magnetic moment values approached the theoretical values Tab. 4. They seem close to spin only values which were calculated at room temperature from the equation µeff=[4s(s + 1)] 1/2 for Mn(II), Co(II), Ni(II) and Cu(II) ions and are equal to 5.92; 3.88; 2.83 and 1.73 µB, respectively.
The obtained data indicated that there is no significant orbital contribution to the magnetic moments of the complex, or its contribution may be essential.
Values of µeff = 3.09 -5.02 µB for Mn(II) compound may suggest that it is high-spin compound with weak ligand field 42-43 and its sp 3 d 2 hybridization 44 . The electronic configuration in this case for Mn(II) ion is t2g 3 eg 2 . The magnetic moment values changed from 3.09 µB (at 2 K) to 5.02 µB (at 300 K). At 300 K the χm·T value is equal to 3.1416 cm 3 K mol -1 (Fig. 7). With a lowering of a temperature its value decreases to 2.6774 cm 3 K mol -1 (at 43.35 K) which perhaps results from the antiferromagnetic interaction between the magnetic centres of complex. At the temperature values lower than 43.35 K, the worths of χm·T drastically decrease to 1.1914 cm 3 K mol -1 (at 2 K). It indicates the antiferromagnetic interaction around magnetic centres as well. The values of magnetic moments are lower than that theoretically calculated. Probably the vectors L and S are aligned by the strong field of the heavy atom in opposite directions which diminishes the resultant magnetic moment. The analysed Mn(II) complex seems highspin with octahedral symmetry of Mn(II) ion. In the coordination sphere of Mn(II) ion may be four oxygen atoms, two of them coming from monodentate carboxylate groups, two others from indole groups and there are also two nitrogen atoms of azomethine groups. However due to the lack of crystallographic data this interpretation seems rather speculative based only on magnetic moment values and IR spectra data interpretations.    For Co(II) complex the values of magnetic moment values changed from 2.22 (at 2 K) to 3.71 µB (at 7.88 K) showing magnetic saturation and then with the increasing temperature it changed to 3.12 µB (at 300 K). At 300 K the χm corr ·T value was equal to 1.2117 cm 3 K mol -1 (µeff = 3.12 µB). Upon temperature lowering the χm corr ·T and the magnetic moment values at first gradually increase and next evenly decrease to 0.9215 cm 3 K mol -1 at 20.8982 K. Next with the cooling of the sample the χm corr ·T values increase rapidly, changing their values from 0.9215 to 1.7149 cm 3 K mol -1 (at 7.8803 K) reaching the saturation paramagnetic state and gain the magnetic moment value 3.71 µB, (Neel temperature). At 7.8803 K the complex shows the paramagnetic properties, while next changing temperature from 7.8803 to 2 K, the χm corr ·T values again rapidly decrease to 0.6159 cm 3 ·mol -1 . It results from the antiferromagnetic order between Co(II) centres or from the possible intermolecular hydrogen bonds in the compound crystal lattice 49 . This drastic decrease of χm corr ·T indicates a negative θ value which may confirm the antiferromagnetic intermolecular interaction. The magnetic moment values are lower compared to that theoretically calculated. The electronic configuration in Co(II) ion under ligand field influence is t2g 5 eg 2 .
For Ni(II) ion complex the magnetic moments change from 3.12 µB (at 2 K) to 3.87 µB (at 300 K). At 300 K the mT value is equal to 1.8659 cm 3 K mol -1 . With a lowering of a temperature its value increases to 18.4818 cm 3 K mol -1 at 8.9516 K and then drastically decrease to 1.7149 cm 3 K mol -1 at 2 K. It is connected with antiferromagnetic interactions that occur in Ni(II) centre, (θ has negative sign). At 8.9516 K (Neel temperature) the complex shows the paramagnetic properties. The magnetic moment values are higher than that calculated theoretically which may indicate the ferromagnetic interaction around the Ni(II) ion. These values may suggest the octahedral environment of Ni(II) ion 49 . The electronic distribution in Ni 2+ cation in the ligand field may be t2g 6 eg 2 .
For Cu(II) compound the magnetic susceptibility values decrease with rising temperature. The magnetic moment values change from 0.79 µB (at 2 K) to 1.31 µB (at 300 K). At 300 K the χm corr ·T value was equal to 0.2145 cm 3 K mol -1 (µeff = 1.31 µB). Upon temperature lowering the χm corr ·T and the magnetic moment values decrease very slowly to 0.1622 cm 3 K mol -1 (µeff = 1.14 µB) at 45.15 K (Fig. 8). Next their values drastically decrease from 0.1622 cm 3 K mol -1 at 45.15 K to 0.0773 cm 3 K mol -1 (0.79 µB) at 2 K. This sudden decrease could be caused by crystal field effect, as well as the antiferromagnetic interactions between neighbouring 3d metal ions (interaction between magnetic centres of complex) or intermolecular hydrogen bonds in molecular crystal structure 48 . The study of magnetostructural data may indicate that ferromagnetic and antiferromagnetic coupling between adjacent orbitals (d orbitals of the metal ions and the symmetry adapted linear contribution of the ligand orbitals) on Cu neighbouring ions 48 . The magnetic moment values experimentally determined for Cu(II) compound are lower than that theoretically calculated (µeff = 1.73 µB). This may suggest a bidentate character of carboxylate group. However, this formulation seems rather uncertain because of the lack of crystal data. The coordination sphere may have the shape of a trigonal pyramid with one nitrogen and three oxygen atoms in its corners. Two oxygen atoms derived from carboxylate groups from glutamic acid part and third one from indole group of isatin ring. The apex of the pyramid forms a nitrogen atom from the azomethine group. The complex may have a tetragonal geometry around Cu(II) ion 49 .
Their thermal stability was studied in air at 293 -1173 K. The complexes are decomposed in three steps. First, they dehydrate in one step releasing all water molecules and form anhydrous compounds to be next decomposed to the oxides of appropriate metals. The enthalpy values of dehydration processes were determined as well. With the rise of temperature, the hydrates release the water molecules the presence of which in the gaseous mixture was confirmed by the bands in the range of 4000-3600 and 1700-1400 cm −1 . During heating the complexes being decomposed release the CO2, CO, hydrocarbons gaseous molecules, the presence of which in the gaseous mixture was identified by FTIR spectra. The magnetic susceptibility of complexes was investigated in the ranges of 77 -303 K and 2 -300 K. All of them obey Curie-Weiss law showing the paramagnetic properties. In the molecular central ion, the ferromagnetic or antiferromagnetic interactions occur.
From the obtained results it appears that in Co(II) and Cu(II) complexes two carboxylate groups take part in the metal ion coordination while in those of Mn(II), Ni(II) and Zn(II) only one carboxylate anion coordinates to central metal. The second carboxylic group does not coordinate with central ions.