Magnetic , thermal and spectral properties of Ni ( II ) 2 , 3-, 3 , 5-and 2 , 6-dimethoxybenzoates

Complexes of Ni(II) 2,3-, 3,5- and 2,6-dimethoxybenzoates have been synthesized, theirphysico-chemical properties have been compared and the influence of the position of –OCH3 sub-stituent on their properties investigated. The analysed compounds are crystalline, hydrated salts withgreen colour. The carboxylate ions show a bidentate chelating or bridging coordination modes. Thethermal stabilities of Ni(II) dimethoxybenzoates were investigated in air in the range of 293-1173 K.The complexes decompose in three steps, yelding the NiO as the final product of decomposition. Theirsolubilities in water at 293 K are in the order of 10 -2 -10 -4 mol⋅dm -3 . The magnetic susceptibilities forthe analysed dimethoxybenzoates of Ni(II) were measured over the range of 76-303 K and the mag-netic moments were calculated. The results reveal that the complexes are the high-spin ones and theligands form the weak electrostatic field in the octahedral coordination sphere of the central Ni(II) ion.The various position –OCH3 groups in benzene ring cause the different steric, mesomeric and induc-tive effects on the electron density in benzene ring.

At present we decided to synthesize the complexes of Ni(II) ions with 2,3-, 3,5-and 2,6dimethoxybenzoic acid anions, as solids, to examine some of their physico-chemical properties and to compare them.In our previous papers [9][10][11][12] we characterized these complexes by elemental analysis, IR spectral data , thermogravimetric studies and X-ray diffraction measurements but now taking into account the presence and positions of two methoxy-groups in benzene ring we decided to compare the properties of Ni(II) complexes in order to investigate the influence of substituent positions on their properties.quantities of 0.1 M ammonium 2,3-, 3,5-and 2,6dimethoxybenzoates (pH≈ 5) to warm 0.1 M aqueous solutions containing the nitrates (V) of Ni(II) ions and crystallizing at 293 K.The solids were filtered off, washed several times with hot water and methanol to remove the ammonium ions and dried at 303 K.
Elemental analysis for C, H was performed using a Perkin-Elmer CHN 2400 analyser.The contents of Ni 2+ ions were established gravimetrically, and by ASA method with the use of ASA 880 spectrophotometer (Varian).
The FTIR spectra of the complexes were recorded in the range of 4000 -400 cm -1 using an FTIR 1725X Perkin -Elmer spectrometer.The samples for the FTIR spectroscopy were prepared as KBr discs.Some of the results are presented in Table 1 and Fig. 1.
The X-ray diffraction patterns were taken on a HZG-4 (Carl Zeiss, Jena) diffractometer using Ni filtered CuK α radiation.The measurements were made within the range of 2Θ=4-80°by means of the Debye-Scherrer-Hull method.The relationships between I/I 0 and 2Θ for these complexes are presented in Fig. 2.
The thermal stability and decomposition of the complexes were studied in air using a Q-1500 D derivatograph with a Derill converter, which simultaneously records TG, DTG and DTA curves.The measurements were made at a heating rate of 10 K⋅min -1 .The 100 mg samples were heated in platinum crucibles in static air to 1173 K with a TG sensitivity of 100 mg (i.e., the whole scale of the balance was equal to 100 mg).The DTG and DTA sensitivities were regulated by the Derill computer programme.The paper speed was 2.5 mm⋅min -1 and Al 2 O 3 was used as a standard.The decomposition products were calculated from the TG curves and verified by powder diffraction analysis.
Magnetic susceptibilities of polycrystalline samples of 2,3-, 3,5-and 2,6-dimethoxybenzoates of Ni(II) were investigated at 76-303 K.The measurements were carried out using the Gouy method.Weight changes were obtained from Cahn RM-2 electrobalance.The calibrant employed was Hg[Co(SCN) 4 ] for which the magnetic susceptibility was asummed to be 1.644⋅10 -5 cm 3 ⋅g - 1 .Correction for diamagnetism of the calibrant atoms was calculated by the use of Pascal's constants [13,14].Magnetic moments were calculated from Eq.( 1) The solubilities of 2,3-, 3,5-and 2,6dimethoxybenzoates of Ni(II) in water were measured at 293 K. Saturated solution of the  obtained compounds were prepared under isothermal conditions.The contents of Ni(II) were determined by using ASA 880 spectrophotometer (Varian).The values of solubilities are presented in Table 1.
The X-ray diffraction patterns of the 2,3-, 3,5-and 2,6-dimethoxybenzoates of Ni(II) were recorded.The analysis of the diffractograms suggest that the complexes are polycrystalline compounds with various degrees of crystallinity and different structures [22].Their structures have not been determined as attempts to obtain single crystals failed.

Table 4. Magnetic moment values of the complexes with the central ion of Ni(II) in octahedral coordination
The thermal stability of Ni(II) 2,3-, 3,5-and 2,6-dimethoxybenzoates was studied in air.All the details concerning their thermal decomposition were described in our previous papers [8][9][10][11][12].Accordingly, in this paper only some selected results obtained for their thermal stability in air are presented (Table 2).The 2,3-, 3,5-and 2,6dimethoxybenzoates of Ni(II) are stable up to 331-335 K.The hydrated complexes lose water molecules in one or two steps and form the anhydrous compounds.The dehydration processes are associated with an endothermic effect on the DTA curves.The anhydrous Ni(II) dimethoxybenzoates decompose to NiO, (with intermediate formation of Ni in the case of 2,3-and 3,5-dimethoxybenzoates), which is the final product of complex decompositions.The thermal stability of hydrated dimethoxybenzoates increases in the following order: 2,3-< 3,5-< 2,6-, while that of the anhydrous ones in the sequence: 2,6-< 2,3-< 3,5-.From the comparison of the decomposition way results it appears that the various position of -OCH 3 substituents in benzene ring influences the decomposition process being connected with various participations of the inductive, and mesomeric effects of methoxy-groups in the electron density of the system.
The ways of the thermal decomposition of Ni(II) dimethoxybenzoates are as follows: Considering the temperature at which the dehydration processes occur and the ways in which they proceed, it is possible to assume that the water molecules are in the outer or inner coordination spheres of the complexes [23][24][25].
The solubilities of analysed Ni(II) 2,3-, 3,5-and 2,6-dimethoxybenzoates in water at room temperature were determined (Table 1).They are of the order of 10 -4 -10 -2 mol⋅dm -3 .2,6-Dimethoxybenzoate of Ni(II) is the best soluble salt while that of 2,3-dimethoxybenzoate the least one.The values of solubilities increase in the order: 2,3-< 3,5-< 2,6-.The changes in the values presented above are connected with the various influences of inductive, mesomeric and steric effects of methoxy-groups in the electron density of the system depending on their position in benzene ring.The inductive effects of each methoxy-groups cause the delocalization of the electrons in the molecule and the change of its energy state brought about the conjugation of electrons.It leads to the stabilization of the system [26][27][28].
Magnetic susceptibility of the analysed compounds was measured in the range of 76-303 K (Table 3).Their values decrease with rising temperature.The effective magnetic moment values of the 2,3-, 3,5-and 2,6dimethoxybenzoates of Ni(II) change from 3.28 BM (at 77 K) to 3.30 BM (at 237 K) for 2,3dimethoxybenzoate, from 3.54 BM (at 103 K) to 3.35 BM (at 298 K) for 3,5-dimethoxybenzoate, and from 2.14 BM (at 76 K) to 2.76 BM (at 303 K) for 2,6-dimethoxybenzoate of Ni(II).The complexes show paramagnetic properties and they obey the Curie-Weiss law.The values of the Weiss constant, Θ, for all the complexes were found to be negative, which probably arises from a crystal field splitting of the paramagnetic spin state [9][10][11][12][29][30][31][32].The paramagnetic dependences of values of the magnetic susceptibility as a function of temperatures are given in Table 3.They give informations about the magnetic interaction between paramagnetic centers.As a rule, if the χ M values increase with increasing temperatures, this indicates an antiferromagnetic interaction but when the χ M values decrease with increasing temperature, the magnetic interaction is ferromagnetic.The χ M values for the 2,3-, 3,5-and 2,6dimethoxybenzoates of Ni(II) show a gradual decrease with increasing temperature.This indicates a tendency of ferromagnetic interaction between the metal ions.In the case of the 2,3-, 3,5-and 2,6-dimethoxybenzoates of Ni(II), the effective magnetic moments are equal to 3.28-3.30BM, 3.35-3.54BM and 2.14-2.76BM (Table 3).The magnetic moments measured for the Ni(II) complexes are 3.30, 3.35 and 2.76 BM at room temperature.These values differ from that of the spin-only moment, which amounts to 2.83 BM (Table 4).This difference between the measured and calculated values results from spinorbital coupling [33] in the case of 2,3-and 3,5dimethoxybenzoates of Ni(II), while in the 2,6dimethoxybenzoate of Ni(II) they are connected with a spin-only moment.It indicates that in the solid state the nickel cation exists in an octahedral triplet ground state with molecules of water and bidentate carboxylate groups coordinated to Ni(II) ion.This was confirmed by the IR spectral analysis (Table 1).The ground state configuration of Ni(II) ion in a regular octahedral field is 3 A 2g (t 2g 6 e g 2 ) and it will be paramagnetic with two unpaired electrons.The contribution to the magnetic susceptibility is given by a spin-only term, second order spinorbital coupling and the temperature independent paramagnetism.
The experimental data suggest that the 2,3-, 3,5-and 2,6-dimethoxybenzoates of Ni(II) are high-spin complexes with weak electrostatic ligand fields and octahedral coordination of Ni(II) ions in which there are four oxygen atoms of carboxylate groups and oxygen atoms of water molecules.
Differences in the magnetic properties are obviously evidenced by differences in their molecular and crystal structures [34].The main difference in the structures is connected with the geometry of the octahedral coordination spheres.The ways of coordinations of central ions of Ni(II) in the analysed compounds could be established on the basis of the complete crystal structure determinations of monocrystals but they have not been obtained.In the case of 2,6dimethoxybenzoate of Ni(II) the magnetic moment values change from 2.76 BM at 303 K to 2.14 BM at 76 K.The reason for this could be the second order spin-coupling which splits the 3 A 2g ground state of Ni(II) ion, on perhaps this effect combined with an antiferromagnetic superchange interaction [35][36][37].

Conclusions
From the obtained results it appears that 2,3-, 3,5-and 2,6-dimethoxybenzoates of Ni(II) were synthesized as hydrated complexes with green colours.They are crystalline compounds and on heating in air to 1173 K the complexes decompose in three steps and form NiO, which is the final product of decomposition.The solubility of the complexes in water at 293 K is of the order of 10 -2 -10 -4 mol⋅dm -3 .The values of µ eff calculated for analysed complexes in the range of 76-303 K reveal that the Ni(II) complexes are high-spin with octahedral coordination and weak ligand field.