Comparison of some properties of 2 , 3 – and 3 , 4 – dimethoxybenzoates of Cu ( II ) , Co ( II ) and Nd ( III )

The physicochemical properties of 2,4-, and 3,4dimethoxybenzoates of Cu(II), Co(II) and Nd(III) were studied and compared to observe the -OCH3 substituent positions in benzene ring on the character of complexes. The analysed compounds are crystalline hydrated or anhydrous salts with colours depending on the kind of central ions: blue for Cu(II), pink for Co(II) and violet for Nd(III) complexes. The carboxylate groups bind as monodentate, bidentate bridging or chelating and even tridentate ligands. Their thermal stabilities were studied in air at 293-1173K. When heated the hydrated complexes release the water molecules and form anhydrous compounds which are then decomposed to the oxides of respective metals. Their magnetic moment values were determined in the range of 76303K. The results reveal the compounds of Nd(III) and Co(II) to be the high-spin and that of Cu(II) forms dimer. The various positions of -OCH3 groups in benzene ring influence some of physicochemical properties of analysed compounds.


Introduction
2,4-and 3,4-Dimethoxybenzoates of Cu(II), Co(II) and Nd(III) were synthesized, their properties investigated and presented in our previously published papers [1,2].We undertook the above topic of our paper for studying the influences of the -OCH 3 group positions in benzene ring on the complex properties.That presention has been still a continuation of our experiments on the carboxylates with various dand 4f -metal ions.These type of compounds have the significant positions in inorganic and bioinorganic chemistry and the again many metal cations in a great number of various biological processes especially six-membered ring system are com-pounds of several vitamins and drugs [3,4].Moreover, carboxylates of d and 4f ion elements depending on their magnetic properties, as magnets, may be used in the modern branches of techniques and technology as electronic materials, and they have applications as precursors in superconducting ceramic and magnetic material productions.Multifunctional rigid type of carboxylate ligands as tethers for constructions of solid phases were also exploited with diverse and unusual topologies [5].
2,4-and 3,4-Dimethoxybenzoic acids are used in various branches of techniques and technology as electric materials, and for magnetic material productions [6][7][8][9][10][11].In medicine and in pharmacy they are applied for the preparation of antibiotics and various dyes.From the survey of literature it follows that there are papers on the complexes of some of rare earth elements with 2,4-and 3,4-dimethoxybenzoic acid anions and on their properties [12][13][14][15][16][17].
There is no information about the comparison of properties of solid of 2,4-and 3,4dimethoxybenzoates of Cu(II), Co(II) and Nd(III).Therefore, we decided to investigate the influence of the positions of methoxy-groups in the benzene ring on such their properties having been presented in our previous papers [1,2] as: thermal stability in air, spectral characterization, X-ray powder investigations and magnetic behaviour in the range of 76 -303K.

Experimental
The preparation of complexes were given in our previously published papers [1,2].
The carbon and hydrogen analysis was performed using a CHN 2400 Perkin-Elmer analyzer.The contents of M(II) and M(III) metals were calculated from TG curves and for comparison also established gravimetrically [1,2].
The IR spectra of complexes were recorded over the range of 400-4000 cm -1 using M-80 spectrophotometer (Carl -Zeiss, Jena).Samples for IR spectra measurements were performed as KBr discs.
The thermal stability and decomposition of Co(II), Cu(II) and Nd(III) complexes were studied in air using a Q-1500D derivatograph with Derill converter recording TG, DTG and DTA curves.The details of measurements are presented in our papers previously published [1,2].
From the comparison of ∆ν COO -values for 2,4-and 3,4-dimethoxybenzoates it appears that they are changed in the following sequence: According to the spectroscopic criteria [1,2,21,22] the carboxylate ions appear to be monodentate, bidentate bridging or chelating and tridentate groups.All details connected with the estimations of their dentates are presented in our previously published papers [1,2].
The analysis of the diffractograms of the complexes suggests them to be polycrystalline compounds with different structures [1,2,23].
The thermal stability of analysed dimethoxybenzoates of Co(II), Cu(II) and Nd(III) was studied in air in the range of 293 -1173K (Table 2).When heated to 1173K the Cu(II) compounds are dehydrated in one step and form anhydrous complexes that next are decomposed to CuO [1,2].The dehydration process is connected with endo-effect while the combustion of the organic ligands with exothermic one.From the values of temperature at which the dehydration process occurs and from the way in which it proceeds, it is possible to assume that the water molecules are in the outer coordination sphere of these complexes [1,2,[24][25][26].
From the comparison of thermal stability values of 2,4-and 3,4-dimethoxybenzoates of Cu(II) it appears that 2,4-dimethoxybenzoate is more thermally stable complex than that of 3,4dimethoxybenzoate.It is connected with the various influence of mesomeric, inductive or steric effects on the electron density in the analysed compound molecule.During heating in air to 1173K the anhydrous cobalt(II) 2,4-dimethoxybenzoates decompose directly to Co 3 O 4 (Fig. 1).The obtained results reveal them to have the similar stability (Table 2).The complexes of Nd(III) with 2,4-and 3,4-dimethoxybenzoic acid anions decompose in two steps.In the range of 495-685K they form the oxycarbonates that next are ultimately decomposed (983-1010K) to Nd 2 O 3 [1,2].The final products of the complex decompositions were identified by Xray powder diffraction measurements.From the Table 2. Temperature ranges of thermal stbility of Nd(III), Co(II), and Cu(II) 2,4-and 3,4-dimethoxybenzoates in air [1,2].
∆T1= temperature range of dehydration process; n= number of crystallization water molecules lost in one endothermic step; ∆T2= temperature range of decomposition of anhydrous complex; ∆T3= temperature range of decomposition of Nd(III) oxycarbonate; IDP= Intermediate decomposition product obtained reasults it appears .that2,4-dimethoxybenzoate of Nd(III) is more thermally stable than 3,4-dimethoxybenzoate of that element (Table 2).In the case of anhydrous compounds of Cu(II) it follows that 2,4-dimethoxybenzoate of Cu(II) is less thermally stable compared to that of 3,4dimethoxybenzoate.
In the series of 2,4-and 3,4-dimethoxybenzoates of Cu(II) and Co(II) it appears that the complexes with Co(II) are more thermally stable than those with Cu(II).
The differences in the presented values show that there is the various influences of inductive, mesomeric and steric effects on the analysed properties of complexes.
The magnetic susceptibility of analysed dimethoxybenzoates was measured over the range of 76-303K [1,2].The complexes of Co(II) and Nd(III) with both ligands obey the Curie-Weiss law, suggesting a weak ferromagnetic interaction.The magnetic moment values experimentally determined for Co(II) and Nd(III) 2,4-and 3,4dimethoxybenzoates change from 4,27µ B to 4,55µ B for 2,4-dimthoxybenzoate of Co(II), from 4,22µ B to 4,61µ B for 3,4-dimethoxybenzoate of Co(II) (Fig. 2), from 2,89µ B to 3,21µ B for 2,4dimthoxybenzoate of Nd(III) and from 2,69µ B to 3,15µ B for 3,4-dimethoxybenzoate of Nd(III) (Fig. 3).These values are very close to the spin only ones for the 2,4-and 3,4-dimethoxybenzoates of Co(II) calculated from the equation µ eff = [4s ( s + 1)] 1/2 in the absence of the magnetic interactions for the present spin system.The theoretical magnetic moment value at 293K for Co(II) ion is equal to 3.88 µ B .The experimental data suggest that the compound of Co(II) seems to be high -spin complexes with probably weak ligand field [1,2 ].The values of µ eff determined for Nd 3+ in the range of 76 -303K are close to those calculated for Nd(III) ion by Hund (3.62µ B ), and van Vleck (3.68 µ B ) [27].This may probably also result from the presence of superexchange magnetic interactions between paramagnetic centers of different molecules of the complexes in the crystal lattice.The 4f electrons causing its paramagnetism are well protected from outside influences and do not participate in the formation of the Nd -O bond.
The magnetic susceptibility values of 2,4and 3,4-dimethoxybenzoates of Cu(II) incrase with rising temperature suggesting a weak antiferromagnetic interaction.The magnetic moment experimentally determined change from 0,49µ B (at 76K) to 1,17µ B (at 303K) for 3,4-dimethoxy- benzoates of Cu(II) and 0,64µ B (at 76K) to 1,57µ B (at 303K) for 2,4-dimethoxybenzoates of Cu(II) (Fig. 4).These values are lower than the d 9 spin-only magnetic moment ∆ eff =1,73µ B .Such dependence is a typical behaviour for copper dimer [1,2,27,28].Magnetic susceptibility measurements revealed the χ=f(T) relation course to be typical for copper(II) carboxylates, where the paramagnetic centers are antiferromagnetically coupled.Magnetic susceptibility is the highest at room temperature and decreases with the temperature lowering.This is related to the occupation of the triplet and singlet states.Occupation of the energetically lower singlet state increases with the temperature lowering [1,2,27,[28][29][30].
From the comparison of the magnetic susceptibility and magnetic moment values for these two analysed series of dimethoxybenzoates of Cu(II), Co(II) and Nd(III) it follows that the various positions of -OCH 3 substituents in benzene ring do not influence their magnetic properties but effect the magnetic moment values.All complexes are paramagnetic compounds.The values of the and 1:3 for Nd(III).The general formula for 2,4dimethoxybenzoates of Cu(II) and Co(II) is M(C 9 H 9 O 4 ) 2 • nH 2 O, where M(II) = Cu, Co, and n=1 for Cu(II), and n=0 for Co(II).For 3,4dimethoxybenzoates of Co(II) and Cu(II) the general formula is M(C 9 H 9 O 4 ) 2 • nH 2 O, where M(II)=Cu, Co, and n=2 for Cu(II), and n=0 for Co(II).2,4-and 3,4-Dimethoxybenzoates of Nd(III) form anhydrous salts Nd(C 9 H 9 O 4 ) 3 .The colours of complexes are following: for Copink, Cu -blue, Nd -violet [1,2,19,20].
Co(II) > Cu(II) > Nd(III), which indicates that in Nd(III) complexes the M-O bonds are more covalent than those in Cu(II) and Co(II) compounds.These values are greater for 3,4-dimethoxybenzoates than those for 2,4-dimethoxybenzoates which may show the stronger covalent bonds in 2,4-dimethoxybenzoates than in 3,4-dimethoxybenzoates of those elements.The values of frequencies of M-O vibration bands for Co(II) and Nd(III) 2,4-and 3,4dimethoxybenzoates are changed in sequence: 2,4-< 3,4-, while for Cu(II) complexes they incrase in the order: 3,4-< 2,4-.These relations may indicate that the stronger M-O bonds are in the case of Co(II) and Nd(III) complexes whereas reciprocal relation is observed for Cu(II) compounds.The IR spectra recorded for complexes show that for 2,4-and 3,4-dimethoxybenzoates of Cu(II), Co(II) and Nd(III) there is only one band of asymmetrical COO -group vibrations but in the case of symmetrical COO -group vibration bands only one is observed for 2,4-and 3,4-dimethoxybenzoates of Cu(II) and 2,4-dimethoxybenzoates of Co(II) and Nd(III).In the IR spectra of 3,4dimethoxybenzoates of Co(II) and Nd(III) there are two bands of symmetrical COO -group vibrations, which indicates the various dentates of carboxylate groups in the analysed complexes.

Figura 4 .
Figura 4. Relationship between m vs T for complexes of Cu(II).