Crystal structures, luminescence, and thermal properties of lanthanide complexes with 2,3,4-trimethoxybenzoic acid and 1,10-phenanthroline

Highlights: ► We have synthesized firstly and characterized three new lanthanide complexes [Ln(2,3,4-tmoba)3phen]2. ► The complex 2 was found existing two solid–solid phase transition endothermic peaks. ► The heat capacities of the complexes were measured in the range of (256.15 to 476.15) K by means of DSC. ► The thermodynamic parameters were also obtained on the base of the fitted polynomials. - Abstract: The complexes of [Ln(2,3,4-tmoba)3phen]2 (Ln = Dy (1), Eu (2), Tb (3); 2,3,4-tmoba = 2,3,4-trimethoxybenzoate; phen = 1,10-phenanthroline) were synthesized and characterized by a series of techniques including the elemental analysis, IR and fluorescent spectra and TG/DSC-FTIR technology. The crystal structures were determined by X-ray crystallography. Each complex include two Ln3+ ions, six 2,3,4-tmoBA and two phen molecules forming a binuclear structure, giving the coordination number of nine. The three-dimensional IR accumulation spectra of gaseous products for the complexes 1 to 3 are analyzed and the thermal decomposition processes are further authenticated. Through means of differential scanning calorimeter (DSC), two solid-solid phase transition endothermic peaks were found in the complex 2, which was different from the complexes 1 and 3. The heat capacities of these complexes were measured and fitted to a polynomial equation with the least squares method for each complex on the basis of the reduce temperature x (x = [T − (Tmax + Tmin)/2]/[(Tmax − Tmin)/2]) over the range from (256.15 to 476.15) K. Subsequently, the smoothed molar heat capacities and thermodynamic functions (HT−H298.15K), (ST−S298.15K), and (GT−G298.15K) of the complexes 1 to 3 were calculated based on the fitted polynomial of the heat capacities. The fluorescent intensity of the complexes 2 and 3 are markedly improved as well.