Synthesis, crystal structures, thermal decomposition mechanism and thermal properties of mononuclear ternry lanthanide complex with 2,4-dichlorobenzoic acid and 2,2′:6′,2″-terpyridine

Abstract

The study of lanthanide complexes combining with aromatic-carboxylic acids has attracted extensive attention, according to their diverse coordination types and fascinating properties. In addition, a wide variety of applications including gas separation and storage, non-linear optics, electroluminescent materials, luminescent bioprobes, and catalysis have been utilized as functional materials. Lanthanide cations have the unique optical, electrical, and magnetic properties. However, the properties can be changed and even improved to be applied to more wider fields through the interactions between lanthanide cations and carboxylic acid ligands. Therefore, we chosen 2,4-dichlorobenzoic acid (2,4-DClHBA) as the main ligands, and 2,2′:6′,2″-terpyridine (terpy) as auxiliary ligands to validly sensitize the lanthanide metal ions to assemble one novel mononuclear lanthanide complex 2,4-DClBA: 2,4-dichlorobenzoate; terpy: 2,2′:6′,2″-terpyridine ([Ho(2,4-DClBA)3(terpy)(H2O)]·H2O). We dissolve 2,4-DClHBA (0.6 mmol) and terpy (0.2 mmol) in ethanol (95%) and adjust the solution at the pH of 5–7 with the prepared NaOH solution (1 mol/L). Add the mixed ligands solution to LnCl3·6H2O (0.2 mmol) aqueous solution under stirring and deposit it for 12 h. The complex was structurally characterized by elemental analysis, single crystal, powder X-ray diffraction, Infrared spectra and Raman spectrometry. The results revealed that the diffraction peak of complex is almost in agreement with simulated data, showing that the structure of the powder of complex is similar with the pure crystal. The detailed IR-R spectra of complex shows the spectrum of complex is different from the two ligands, the stretching vibration peak at 1549 cm - 1 attributed to the C=N bonds in terpy takes place red shift to 1537 cm - 1, The v C=O (1631 cm - 1) of free carboxylic acid ligand completely disappears in the spectra of the complex and appear two another new absorption peak at 1553 and 1437 cm - 1, which are attributed to the asymmetric stretching vibration ( v asCOO - ) and the symmetric stretching vibration ( v sCOO - ) of the carboxylic group, indicating the target product has been produced. Single crystal X-ray diffraction reveals that complex crystallize in the triclinic space group P ī and each Ho3+ ion is nine-coordinated adopting a distorted monocapped square antiprismatic molecular geometry. Mononuclear complex are stitched together via hydrogen bonding and π - π interactions to form the 1D, 2D, 3D supramolecular structures. The thermal decomposition mechanism complex was measured by TG-DTG-DSC/FTIR coupling technique, which shows the uncoordinated water all lose firstly, coordinated water and terpy are resolved into CO2 and other gaseous molecules. At last, the 2.4-DClBA are decomposed into CO2, CO and other gaseous molecules. Based on the above analysis, the general thermal decomposition reaction of the complexes is related to the structure and the decomposition of the framework occurs above 403.15 K, indicating the complex has thermal stability. The average molar heat capacity value of the complex gradually increased with the augment of temperature and its heat capacities fitted to a polynomial equation. Finally, the derived thermodynamic functions ( H T– H 298.15), ( S T– S 298.15) and ( G T– G 298.15) of the complex relative to the standard reference temperature 298.15 K were also obtained. Kinetics of the first decomposition stage for the complex was calculated by integral iso-conversional non-linear (NL-INT) method. It is obviously to observe that the activation energy E of the the complex varies with α respectively, indicating the first thermal decomposition stage for complex is complex reactions.