Synthesis, Structures And Thermodynamic Properties Of 5-Nitroisophthalic Acid Lanthanide Coordination Polymers

The structures of lanthanide carboxylate complexes exhibit good regularity with increasing atomic number based on the electron configuration of lanthanides, while the thermodynamic properties of the complexes also present a regular change with increasing atomic number. It is of signifance for oriental regulating and preparation of the complex to explore the investigation of the thermodynamics and summarize the relationship of the thermodynamics with the structures for the complexes. In this dissertation,5-nitroisophthalic acid was selected as the ligand to interact with lanthanide ions. The regularity of crystal structures and thermodynamics of the lanthanide coordination polymers and their relationship were investigated. The contents and conclusion were described as follows:(1) A series of lanthanide coordination polymers (1)～(15) were synthesized by reaction of 5-nitroisophthalic acid with lanthanide ions in aqueous solution and characterized by X-ray single crystal diffraction, chemical analysis, element analysis, and infrared spectrum. Their structures were determined and the coordination modes were compared. The results showed that the complexes could be divided into five groups according to the structure and composition: [RE4(5-N02-bdc)6(H20)10]·7H20 (RE=La, Ce); [RE3(5-NO2bdc)4(5-NO2-Hbdc)(H2O)7]·nH2O (RE=Pr, Nd, Sm, Eu); [RE2(5-NO2-bdc)3(H2O)5]·5H2O (RE=Gd, Tb); [RE2(5-NO2-Hbdc)2 (5-NO2bdc)2(H2O)8]-4H2O (RE=Dy, Ho, Y) and [RE(5-NO2-bdc)4(H2O)6][RE2(5-NO2-bdc)2 (H2O)8]·nH2O (RE=Er, Tm, Yb, Lu). The complexes in the same series are isomorphism with quintuple effect.(2) The enthalpy of solution for the lanthanides nitrate hydrate and the enthalpy changes of the liquid phase formation reaction for the complexes were measured by a microcalorimetry. A series of rational thermochemical cycles were designed for the reaction process. The enthalpy changes of formation reactions of the solid complexes, as well as the standard molar enthalpy, were calculated through the thermochemical cycles. The plots of the enthalpy changes, the standard molar enthalpy of the liquid and solid phase for formation complexes against the lanthanide atomic numbers revealed quintuple effect, which was consistent with that of the crystal structure of the series complexes. The yttrium complex was attributed to the fourth group, which is same as dysprosium and holmium complexes.(3) Low-temperature heat capacities of the complexes were determined by a precision automated adiabatic calorimeter from 78 to 398 K. The heat capacity curves showed that no any changes, such as phase change, association, and thermal decomposition occurred during measurement. This result revealed that the compound is stable from 78 K to 398 K. Having Fitted the experimental points by means of the least-squares method, a polynomial equation of the experimental molar heat capacities (Cp,m) versus reduced temperature could be obtained. The smoothed molar heat capacities of the complexes and the thermodynamic functions with 5K temperature intervals were calculated by the fitted polynomial equation. The plots of the experimental molar heat capacities (Cp,m) at 298.15 K, the data of thermodynamic functions (HT-H298.15k) and (ST-S398.15k) at 300K, against the lanthanide atom numbers exhibited quintuple effect, which is consistent with that of the crystal structure of the series complexes. The yttrium complex attributed to the forth group, which is same as dysprosium and holmium complexes.