Study of the thermal behavior, evolved gas analysis and temperature diffraction patterns of new light lanthanide complexes with niflumic acid

Abstract

Seven new complexes of niflumic acid with light lanthanide ions (La-Gd except Pm) were obtained in solid state. With the application of chemical, elemental and thermal analysis, the minimal formulas of the complexes were established, which were different despite the fact that the syntheses of the complexes were carried out under the same conditions. The complex with La(III) ions is mononuclear and its stoichiometry is represented by the formula La(nif)3∙2.5 H2O. The remaining complexes are binuclear and their stoichiometry is expressed by the formula Ln2(nif)4(OH)2∙nH2O (where: Ln is Ce or Pr, n is equal to 4 or 5, respectively) or Ln2(nif)3(OH)3∙4 H2O (where: Ln = Nd, Sm, Eu or Gd). On the basis of the FTIR spectra, it was stated that the carboxylate groups of the ligand can coordinate metal ions in a bidentate chelating or tridentate chelating-bridging mode. XRPD analysis of the obtained compounds showed that they are amorphous. Based on the TG/DTG–DSC analysis, it was shown that the thermal decomposition of the complexes is a multistage process, depending on the metal ion and the used atmosphere. It was found that the stability of the dehydrated compounds in air atmosphere increases in the order: La < Pr < Nd < Sm < Gd = Eu < Ce, while in nitrogen atmosphere, the stability of these complexes is as follows: La < Nd < Ce = Pr < Eu < Sm < Gd. Using coupled TG-FTIR-MS techniques, the decomposition pathways of the synthesized compounds were examined and the gaseous products released during thermal oxidation and pyrolysis of the complexes were identified. The EGA analysis showed that the gaseous products contained mainly H2O, CO2, COF2, fragments from the partial decomposition of the organic ligands (both in air and nitrogen atmosphere), NO (in air) and CO (in N2). Moreover, based on the temperature diffraction patterns, it was found that the thermal degradation of most of the studied complexes, both in air and nitrogen atmosphere, did not end at 1000 °C. It has been experimentally proven that during thermal decomposition of niflumic acid complex with Nd(III) ions in an oxidizing atmosphere at approximately 440 °C NdF3 is formed as one of a solid intermediate product. At the temperature of 570 °C, the tetragonal form of this compound appears additionally in the solid residue. At 800 °C, the solid residue is a mixture of Nd2O3 and NdOF, while the final product obtained at 1000 °C is Nd2O3 and NdOF. Under pyrolysis conditions, only at a temperature of 800 °C it was possible to identify solid intermediate products and in the case of the niflumic acid complex with Nd(III) ions complex it was a mixture of NdOF and NdF3. NOF has been identified as one of the solid products of thermal degradation.