Synthesis, Crystal Structures, Thermal Decomposition Kinetics Of Rare Earths Aromatic Carboxylic Acid Complexes

In this paper, eighteen rare earth complexes with aromatic carboxylic acids and 1,10-phenanthroline(or 2,2'-bipyridine) are synthesized. And the properties have been characterized by elemental analysis, molar conductivity, infrared spectra, ultraviolet spectra, fluorescence spectra and TG-DTG techniques.(1) Six rare earth complexes with 4-ethylbenzoic acid and phenanthroline [Ln(4-EBA)₃(phen)]₂ (Ln=Nd(1) , Sm(2), Eu(3), Tb(4), Dy(5) and Ho(6); 4-EBA= 4-ethylbenzoate; phen=1,10-phenanthroline), have been synthesized. Although each complex has the identical ligands, there are three different structures in the six complexes. Complexes 1-4 are isostructural. Two Ln³⁺ ions are connected together by four bridging 4-eba ligands. The Ln³⁺ ions have a coordination number of nine with a distorted monocapped square-antiprism. In complex 5, two eight-coordinated Dy³⁺ ions are held together only by two bidentate-bridging 4-eba ligands, and the coordination geometry of Dy³⁺ is a trigondodecahedron. The structure of complex 6 is different from complexes 1-5. The coordination number of Ho³⁺ ion is eight and the coordination polyhedron is a distorted square-antiprism. Thermal analysis of the six complexes is discussed by TG-DTG and IR techniques. The activation energy E of the first decomposition stage for complexes 1-6 are calculated by intergral isoconversional non-linear(NL-INT)and Ozawa iteration methods, respectively. The fluorescent properties of complexes 3 and 4 are also studied.(2) Five rare earth complexes with 3,4-dimethylbenzoic acid and phen, [Ln(3,4-DMBA)₃phen]₂ (Ln=Nd(1), Sm(2), Tb(3), Dy(4) and Ho(5), 3,4-DMBA= 3,4-dimethylbenzoate, phen=1,10-phenanthroline) have been synthesized and the structures of the complexes 3 and 4 have been determined by single-crystal X-ray diffraction. The results show that complexes 3 and 4 are isostructural. Each Ln³⁺ ion is coordinated by six oxygen atoms from four 3,4-DMBA groups and two nitrogens from one phen molecule, giving a coordination number of eight. The coordination polyhedron of each central Ln³⁺ ion is a distorted square-antiprism. Thermal analysis of the six complexes are discussed by TG-DTG and IR techniques. The mechanism function of the first decomposition stage for the complexes 1-4 are investigated by using double equal-double step method. The kinetic parameters (activation energy E and pre-exponential factor A) and the thermodynamic parameters (ΔH^≠,ΔG^≠andΔS^≠) of the four complexes are calculated. The fluorescent properties of complex 3 is also studied.(3) Five rare earth complexes with 2-ethoxylbenzoic acid and phen, [Ln(2-EOBA)₃phen]₂ (Ln=Sm(1), Eu(2), Tb(3), Dy(4) and Ho(5), 2-EOBA= 2-ethoxylbenzoate, phen=1,10-phenanthroline) have been synthesized. Thermal analysis of the five complexes are discussed by TG-DTG and IR techniques. The kinetic parameters(activation energy E and pre-exponential factor A) are calculated by Kissinger method and the Arrhenius equations have been obtained. The fluorescent properties of complexes 2 and 3 are also studied.(4) Two neodymium complexes with benzoic acid (or 2,4-dichlorobenzoic acid) and bipy (or phen), [Nd(BA)₃bipy]₂ (BA=benzoate; bipy=2,2'-bipyridine) and [Nd(2,4-DClBA)₃phen]₂(2,4-DClBA=2,4-dichlorobenzoate; phen=1,10-phenanthroline) have been synthesized. Results of the single-crystal X-ray diffraction show that [Nd(BA)₃bipy]₂ is monoclinic with space group P2(1)/n. The coordination number of Nd³⁺ ion is eight and the coordination polyhedron is trigondodecahedron. The complex of [Nd(2,4-DClBA)₃phen]₂ crystallize in triclinic with space group P-1 and the coordination number of Nd³⁺ ion is nine. The coordination environment of the Nd³⁺ ion is a distorted monocapped square-antiprism. Thermal analysis of the two complexes are discussed by TG-DTG and IR techniques. The mechanism function of the first decomposition stage for [Nd(BA)₃bipy]₂ is investigated by using double equal-double step method. The kinetic parameters (activation energy E and pre-exponential factor A) and the thermodynamic parameters (ΔH^≠,ΔG^≠andΔS^≠) of [Nd(BA)₃bipy]₂ are also calculated.