Spectral and Thermal Behaviours of Rare Earth Element Complexes with 3,5-Dimethoxybenzoic Acid

3, 5-dimethoxybenzoic acid is a predicted metabolite generated by BioTransformer that is produced by the metabolism of 3-(3, 5-dimethoxyphenyl) propanoic acid [1]. The compounds of 3,5-dimethoxybenzoic acid with various cations are little known. Papers exist on its complexes with cations such as: Cu(II) and Ag(I), Zn(II) and Pb(II). The compounds were obtained as solids or were investigated in solution.1,2 The 3,5-dimethoxybenzoate of Cu(II) was isolated in the solid state and its thermal stability was studied1 while those of Ag(I), Zn(II) and Pb(II) were only investigated in solution. 

The physico-chemical properties of 2,3-dimethoxybenzoic acid were also studied and the parameters of its unit cell in the solid state were determined. There is no information about the solid-state properties of complexes of 3,5-dimethoxybenzoic acid with rare earth elements. Therefore, it was decided to synthesize them in the solid state and to examine some of their properties, such as thermal stability in air and nitrogen, solubility in water at room temperature, IR spectral characteristics and crystalline form in order to determine whether they are crystalline or amorphous compounds. The thermal stability investigations enabled the evaluation of the mechanism of the complex decomposition. The determination of the solubility is valuable because it gives information about the practical use of the acid for the separation of rare-earth elements by extraction or ion-exchange chromatographic methods.

The conditions for the formation of rare earth element 3,5-dimethytoxybenzoates were studied and their quantitative composition and solubilities in water at 293 K were determined. The complexes are anhydrous or hydrated salts and their solubilities are of the orders of 10-5 – 10-4 mol dm-3. Their FTIR, FIR and X-ray spectra were recorded. The compounds were also characterized by thermogravimetric studies in air and nitrogen atmospheres and by magnetic measurements. All complexes are crystalline compounds. The carboxylate group in these complexes is a bidentate, chelating ligand. On heating in air to 1173 K, the 3,5-dimethoxybenzoates of rare earth elements decompose in various ways. The hydrated complexes first dehydrate to form anhydrous salts which then decompose in air to the oxides of the respective metals while in nitrogen to mixtures of carbon and oxides of the respective metals. The complexes are more stable in air than in nitrogen.

Concluding, from the obtained results it appears that the Ln–O bond in rare earth element 3,5-dimethoxybenzoates is mainly electrostatic in nature, since the 4f orbitals of the lanthanide ions are effectively shielded by the 5s25p6 octet. The solubilities of 3,5-dimethoxybenzoates of rare earth elements in water at 293 K were measured (Table I). They are in the order of 10–5 – 10–4 mol dm–3. Samarium 3,5-dimethoxybenzoate is the most soluble salt while that of lutetium the least soluble one. Taking into account the values of the complex solubilities, it is possible to state that 3,5-dimethoxybenzoic acid cannot be used for the separation of rare earth elements by ion exchange chromatography because it forms not readily soluble complexes with these elements [2].

Reference

[1] http://www.hmdb.ca/metabolites/HMDB0127495
[2] FERENC WIESŁAWA, WALKÓW-DZIEWULSKA AGNIESZKA, and CHRUŚCIEL JANUSZ. "Spectral and thermal behaviours of rare earth element complexes with 3,5." Journal of the Serbian Chemical Society 65.11(2000):789-798

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