Synthesis, characterization and thermal studies on solid state 3-methoxybenzoate of lighter trivalent lanthanides

Solid-state Ln -3-MeO-Bz compounds, where Ln stands for lighter trivalent lanthanides (La – Sm) and 3-methoxybenzoate, have been synthesized. Thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray powder diffractometry, infrared spectroscopy, and complexometry were used to characterize and to study the thermal behaviour of these compounds. The results led to information concerning the composition, dehydration, polymorphic transformation, thermal behaviour and thermal decomposition of the synthesized compounds.


Introduction
Benzoic acid and some of their derivatives have been used as conservant, catalyst precursors, in pharmaceutical industries, beyond other applications.The literature shows that the compounds of rare earth acid d-block elements with benzoic acid and some of its derivatives have been investigated in aqueous and in the solid state.
In the solid state, the works reported the thermal and spectral behaviour on solid compounds of benzoates and its derivatives with rare earth elements [6][7][8][9][10][11][12]; the vibrational and electronic spectroscopy study of lanthanides and effect of sodium on the aromatic system of benzoic acid [13][14]; the reaction of divalent cooper, cobalt and nickel with 3-hydroxy-4-methoxy and 3methoxy-4-hydroxybenzoic acid and a structure for these compounds has been proposed on the basis of spectroscopy and thermogravimetric data [15].The thermal decomposition of thorium salts of benzoic acid 4-methoxybenzoic acids in air atmosphere [16] and thermal behaviour of solid state 4-methoxy; 3-methoxy; 2-methoxybenzoates and benzoate of some bivalent transition metal ions have also been described [17][18][19][20].
Lanthanide chlorides were prepared from the corresponding metal oxides (except for cerium) by treatment with hydrochloric acid solution.The resulting solutions were evaporated to near dryness, the residues were again dissolved in distilled water, transferred to a volumetric flask and diluted in order to obtain ca.0.1 mol L -1 solutions, whose pH were adjusted to 5.5 by adding diluted sodium hydroxide or hydrochloric acid solutions.Cerium (III) was used as its nitrate and ca 0.1 mol L -
The solid state compounds were prepared by adding slowly, under continuous stirring, the solution of the ligand to the respective metal chloride or nitrate solutions, until total precipitation of the metal ions.The precipitates were washed with distilled water until elimination of the chloride (or nitrate) ions, filtered through and dried on Whatman nº 42 filter paper, and kept in a desiccator over anhydrous calcium chloride.
In the solid state compounds, ligand and metal ion contents were determined from TG curves.The metal ions were also determined by complexometric titrations with standard EDTA solution, using xylenol orange as indicador [21][22].
X-ray powder patterns were obtained by using a SIEMENS D-5000 X-ray diffractometer employing Cu Ka radiation (l = 1.541Å) and setting of 40 kV and 20 mA.
Infrared spectra for Na-3-MeO-Bz as well as for its metal ion compounds were run on a Nicolet model Impact 400 FT-IR Instrument, within the 4000-400 cm -1 range.The solid samples were pressed into KBr pallets.

Results and discussion
The analytical and thermoanalytical (TG) results are shown in Table 1.These results establish the stoichiometry of these compounds, which are in agreement with the general formula, Ln (3-MeO-Bz) 3 .nH 2 O, where Ln represents lighter trivalent lanthanides (La-Sm), 3-MeO-Bz is 3methoxybenzoate and n = 2 (La, Pr, Nd, Sm) , 2.5 (Ce).
The DSC curves were obtained with a thermal analysis system model DSCQ10 from TA Instruments.The purge gas was an air flow of 50 mL min -1 .A heating rate was 20 ºC min -1 was adopted with samples weighing about 5 mg.Aluminium crucibles with perforated cover were used for recording the DSC curves.
The TG curves were obtained by using a Mettler TA-4000 thermal analysis system, with air flowing at 100 mL min -1 , a heating rate of 20 ºC min - 1 and with samples weighing about 7 mg.An alumina crucible was used to obtain the TG curve.Infrared spectroscopic data on 3methoxybenzoate sodium salt and its compounds with ligther trivalent lanthanides are shown in Table 2.The investigation was focused mainly within the 1700-1400 cm -1 range, because this region is potencially most informative to assign coordination sites.In the sodium 3methoxybenzoate, strong bands located at 1568 cm -1 and 1400 cm -1 are attributed to the antisymmetrical and symmetrical frequencies of the carboxylate groups, respectively [23,24].In the compounds considered in this work, analysis of the frequencies of the n as and n sym (COO -) bands shows that the lanthanides are linked to the carboxylate group by a bidentate bond with an incomplete equalization of bond lengths in the carboxylate anion; this is in agreement with the literature [5,[11][12][13].The TG curves of the compounds are shown in Fig. 2.These curves show mass in two (Ce), three (Pr, Sm) or four (La, Nd) steps.In all the TG curves, the first mass loss between 50 and 100 ºC is attributed to dehydration, which occurs in a single step.Once dehydrated, the anhydrous compounds are stable up to 270 ºC (Ce), 330 ºC (La, Nd, Sm) and 355 ºC (Pr) and above these temperatures the TG curves show that the thermal decomposition is characteristic for each compound.
For the lanthanum compound, Fig. 2 (a) the first mass loss is due to dehydration, with loss of 2 H 2 O (calcd.= 5.74 %, TG = 5.93 %).The thermal decomposition of the anhydrous compound occurs in three steps between 330 and 750 ºC, with the two first ones a fast processes and losses of 29.46 and 28.90 %, respectively.In the last step the mass loss occurs through a slow process with formation of lanthanum oxide, La 2 O 3 as final residue.
For the cerium compound, Fig. 2 (b) the first mass loss is due to dehydration with loss of 2.5 H 2 O (calcd.= 7.05 %, TG = 6.71 %).The thermal decomposition of the anhydrous compound occurs in a single step between 270 and 570 ºC, with loss of 66.25 %.The lower thermal stability of the cerium compound, as compared with the other compounds is undoubtedly related to the exothermic oxidation reaction that results in the formation of cerium (IV) oxide.
In the praseodymium compound, Fig. 2  (c) the first mass loss is due to the dehydration with loss of 2 H 2 O (calcd.= 5.72 %, TG = 6.12 %).The thermal decomposition of the anhydrous compound occurs in two steps between 355 ºC and 720 ºC.The mass loss up to 440 ºC occurs through a fast process followed by a slow one with losses of 29.36 and 37.66 %, respectively and with formation of praseodymium oxide, Pr 6 O 11 as final residue.
Temperature / (ºC ) In the neodymium compound, Fig. 2 (d) the first mass loss is attributed to dehydration with loss of 2 H 2 (calcd.= 5.69 %, TG = 5.95 %).The thermal decomposition of the anhydrous compound occurs in three steps between 330 and 655 ºC.The first mass loss up to 440 ºC, also occurs through a fast process, followed by two slow processes with losses of 60.54, 6.00 and 1.09 %, respectively resulting in the formation of neodymium oxide, Nd 2 O 3 , as final residue.
For the samarium compound, Fig. 2 (e) the first mass loss is due to dehydration with loss of 2 H 2 O (calcd.= 5.63 %, TG = 5.17 %).The thermal decomposition of the anhydrous compound occurs in two steps between 330 and 620 ºC.The first mass loss up to 435 ºC occurs through a fast process followed by a slow process with losses of 61.70 and 5.37 %, respectively, leading to formation of samarium oxide, Sm 2 O 3 as final residue.
The DSC curves of the compounds are shown in Fig. 3.These curves show endothermic and exothermic events that are in agreement with the mass losses observed in the TG curves and endothermic peaks due to crystalline phase transition.The endothermic peak at 115 ºC (La), 117 ºC (Ce), 107 ºC (Pr), 114 ºC (Nd) and 106 ºC (Sm) is due to dehydration.The dehydration enthalpies found for the lanthanum to samarium compounds were: 112.2, 105.4,106.8, 95.5 and 93.3 kJ mol -1 .broad exothermic event observed for all compounds, except the cerium, between 325 and temperature higher than 600 ºC without the appearance of definitive peaks are due to the thermal decomposition of the anhydrous compounds, where the oxidation of the organic matter takes place in consecutive and/or overlapping steps.For the cerium compound the exothermic event between 225 and temperature higher than 600 ºC, the oxidation of the organic matter takes place in a single step.

Conclusion
From analytical and thermoanalytical (TG) results a general formula could be established for these compounds in the solid state.
The X-ray powder patterns pointed out that the synthesized compounds have a crystalline structure without evidence for formation of isomorphous compounds.