Crystal structures and thermodynamic properties of lanthanide complexes with 2-chloro-4,5-difluorobenzoate and 1,10-phenanthroline

A series of lanthanide complexes with the 2-chloro-4,5-difluorobenzoate (2-cl-4,5-dfba) and 1,10-phenanthroline (phen), have been synthesized with the formulae of [La(2-cl-4,5-dfba)3phen]n·nH2O (1), [Nd(2-cl-4,5-dfba)3phenH2O]2 (2), [Ln(2-cl-4,5-dfba)3phen]2 (Ln = Eu (3), Ho (4)). The complexes are characterized by elemental analysis, infrared and fluorescent spectra and X-ray single-crystal diffraction. The structures of the four complexes are very different. Complex 1 is an infinite 1D chain polymeric structure formed by the asymmetric units with the mirror growth pattern. Each La3+ ion is coordinated to four bridging carboxylic groups, two tridentate chelating–bridging carboxylic groups, simultaneous with one phen molecule, giving the coordination number of nine. In the molecular structures of complexes 2 and 3, two Ln3+ ions are linked by four carboxyl groups, forming two binuclear molecules. In addition, each Nd3+ ion in complex 2 is bonded to one H2O molecule and one carboxyl group by monodentate mode, one phen molecule by bidentate chelating, and each Eu3+ ion is also chelated to one phen molecule and one carboxyl group in complex 3. And in complex 4, the Ho3+ ion yields a eight-coordinated distorted square anti-prism coordination geometry. The three-dimensional IR accumulation spectra of gaseous products for complexes 1 to 4 are analyzed and further authenticated the thermal decomposition processes with TG-DTG curves. The heat capacities of complexes 2 to 4 are measured and fitted to a polynomial equation by the least squares method on the basis of the reduced temperature x (x = [T−(Tmax + Tmin)/2]/[(Tmax − Tmin)/2]). Then the smoothed molar heat capacities and thermodynamic functions of complexes 2 to 4 are calculated. The fluorescence intensity of complex 3 is markedly improved as well.

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► We have synthesized firstly and characterized four new lanthanide complexes.
► The structure of complex 1 is a one-dimensional chain structure.
► The heat capacities of the complexes were measured in the range of 258.15 K to 493.15 K by means of DSC.
► The thermodynamic parameters were also obtained on the base of the fitted polynomials.