Aromatic Carboxylic Acids Synthesis, Characterization And Properties Of Rare Earth Complexes

The rare earth elements, based on the special electronic configuration, have many unique physical and chemical properties. The lanthanide coordination polymers create better conditions for the development of new functional materials because of their magnetic, optical performance and intriguing topologies structures. Especially, the lanthanide coordination polymers with aromatic carboxylic acid have deserved a special attention in the fields of inorganic chemistry and applied chemistry. In this paper, the study is carried out according to the molecular design and assembly method, using the benzoic acid as the main ligands and dimethyl sulfoxide (DMSO) or N, N-dimethylformamide (DMF) as the secondary ligands, and six complexes are synthesized by solution evaporation and hydrothermal methods. These complexes are determined by Fourier transfer infrared spectroscopy (FT-IR), element analysis (EA), and thermal gravimetric analysis (TG). The crystal structures are characterized by single-crystal X-ray diffraction (SCXRD). The luminescence and magnetic properties of these complexes are investigated by fluorescence excitation and emission spectroscopy (FES) and magnetic property measurement system (MPMS).[Nd(DNBA)3(DMSO)2]2 and [Sm(DNBA)3(DMSO)2]2 are prepared by solution evaporation. Nd(NO3)3·6H2O and 3,5-dinitrobenzoic acid (DNBA) are dissolved in ethanol-water solution, precipitates of Nd3+compound are produced when the pH value is adjusted to 5.2. Precipitates are dissolved in DMSO and allowed to crystallize at room temperature. After three days, [Nd(DNBA)3(DMSO)2]2 is collected. [Sm(DNBA)3(DMSO)2]2 is synthesized by a method similar to that of [Nd(DNBA)3(DMSO)2]2 except that Sm(NO3)3·6H2O is used instead of Nd(NO3)3·6H2O. After five days, [Sm(DNBA)3(DMSO)2]2 is collected. SCXRD analysis reveals that the carboxylate group of the 3,5-dinitrobenzoic acid connects two neighboring Nd(Ⅲ) ions in [Nd(DNBA)3(DMSO)2]2 or Sm(Ⅲ) ions in [Sm(DNBA)3(DMSO)2]2 by a bidentate bridging mode, and the coordination number of Nd(Ⅲ) ion or Sm(Ⅲ) ions are eight.Nd(m-TA)3 is prepared by hydrothermal method. Nd(NO3)3·6H2O and m-toluic acid (m-TA) are dissolved in ethanol, then the pH value is adjusted to 5.0. The liquid is heated at 120℃for three days, and Nd(m-TA)3 is collected. SCXRD analysis reveals that coordination number of Nd(Ⅲ) ion is nine. All the carboxylate groups of the m-toluic acid connect Nd(Ⅲ) ions by a chelating-bridging bidentate mode.[Ce0.5Sm0.5(o-NBA)3(DMF)2]2 is prepared by solution evaporation. Sm(NO3)3·6H2O, Ce(NO3)3·6H2O and o-nitrobenzoic acid (o-NBA) are dissolved in ethanol-water solution, precipitates of Ln3+ compound are produced when the pH value is adjusted to 5.5. Precipitates are dissolved in DMF and allowed to crystallize at room temperature. After three days, [Ce0.5Sm0.5(o-NBA)3(DMF)2]2 is collected. SCXRD analysis indicates that coordination number of Ln(Ⅲ) ion is eight. Hydrogen bonds andπ…πstacking assemble [Ce0.5Sm0.5(o-NBA)3(DMF)2]2 into a three-dimensional network.Nd2Ce(DNBA)9(DMF)6 is prepared by hydrothermal method. Ce(NO3)3·6H2O, Nd(NO3)3·6H2O,3, 5-dinitrobenzoic acid and N,N-dimethylformamide are dissolved in ethanol, then the pH value is adjusted to 4.5. The liquid is heated at 120℃for four days, and Nd2Ce(DNBA)9(DMF)6 is collected. SCXRD analysis reveals that coordination number of Ln(Ⅲ) ion is eight, and carboxylate group of the 3,5-dinitrobenzoic acid connects two neighboring Ln(Ⅲ) ions by a bidentate bridging mode.LaCe(DNBA)6(DMSO)4 is prepared by hydrothermal method. La(NO3)3·6H2O, Ce(NO3)3·6H2O,3, 5-dinitrobenzoic acid and dimethyl sulfoxide are dissolved in ethanol, then the pH value is adjusted to 4.7. The liquid is heated at 120℃for four days, and LaCe(DNBA)6(DMSO)4 is collected. SCXRD analysis shows that coordination number of La(Ⅲ) ion is eight, thus Ce(Ⅲ) ion is nine. The nitro groups of the 3,5-dinitrobenzoic acid connect Ce(Ⅲ) ions in a chelating-bridging bidentate mode, and connects La(Ⅲ) ions by a bidentate bridging mode. Hydrogen bonds andπ…πstacking assemble LaCe(DNBA)6(DMSO)4 into a three-dimensional network.Therefore, for the synthesized Nd(m-TA)3 by hydrothermal method, adding triethylamine as template agent and changing nitro to methyl group in ligands can obtain Nd(m-TA)3 in which coordination number of Nd(Ⅲ) ion is 9. However, the coordination number of Nd(Ⅲ) ion is 8 in [Nd(DNBA)3(DMSO)2]2 and Nd2Ce(DNBA)9(DMF)6. LaCe(DNBA)6(DMSO)4 is prepared by hydrothermal method, adding triethylamine as template agent and changing N,. N-dimethylformamide to dimethyl sulfoxide as the secondary ligands, in which the nitro groups of the 3,5-dinitrobenzoic acid instead of the carboxylate group connect Ce(Ⅲ) ions in a chelating-bridging bidentate mode, and connect La(Ⅲ) ions in a bidentate bridging mode.The fluorescence properties of [Nd(DNBA)3(DMSO)2]2, [Sm(DNBA)3(DMSO)2]2, Nd(m-TA)3, [Ceo.5Smo.5(o-NBA)3(DMF)2]2 and Nd2Ce(DNBA)9(DMF)6 are characterized. The magnetic properties of [Nd(DNBA)3(DMSO)2]2, Nd(m-TA)3, Nd2Ce(DNBA)9(DMF)6 and LaCe(DNBA)6(DMSO)4 are studied. Fluorescence analysis shows that the observed luminescences of [Nd(DNBA)3(DMSO)2]2 and Nd(m-TA)3 are mainly attributed to the coordinated ligand, however, luminescences of [Sm(DNBA)3(DMSO)2]2 and [Ce0.5Sm0.5(o-NBA)3(DMF)2]2 are attributed to the characteristic emission of Sm(Ⅲ) ion. The fluorescence emission intensity of [Ce0.5Sm0.5(o-NBA)3(DMF)2]2 is much stronger than the fluorescence emission intensity of [Sm(DNBA)3(DMSO)2]2, indicating that addition of Ce(Ⅲ) ion can enhance the Sm(Ⅲ) ion fluorescence intensity because of the co-luminescence effect of Ce(Ⅲ) and Sm(Ⅲ) ions. Luminescences of Nd2Ce(DNBA)9(DMF)6 is attributed to the characteristic emission of Ce(Ⅲ) ion. Magnetic analysis shows that the effective magnetic momentμeff of complex [Nd(DNBA)3(DMSO)2]2 is 3.96μB at room temperature, somewhat higher than that of a Nd(Ⅲ) ion in the ground state 4I9/2,3.62μB at room temperature. The effective magnetic momentμeff of complex Nd(m-TA)3 is 3.51μB, a little less than 3.62μB. The effective magnetic momentμeff of complex [Nd(DNBA)3(DMSO)2]2 is higher than that of Nd(m-TA)3, which may due to the reason that the spin coupling of Nd(Ⅲ) ions are smaller because of long distance between Nd(Ⅲ) ions in the mononuclear complex Nd(m-TA)3. The plots ofχM-1(χM is molar magnetic susceptibility) vs temperature of complexes [Nd(DNBA)3(DMSO)2]2, Nd(m-TA)3, Nd2Ce(DNBA)9(DMF)6 and LaCe(DNBA)6(DMSO)4 are close to a straight line, which consistent with Curie-Weiss law. According to Curie-Weiss equationχM=C/(T-θ), the Weiss constants are both negative, indicating that [Nd(DNBA)3(DMSO)2]2, Nd(m-TA)3, Nd2Ce(DNBA)9(DMF)6 and LaCe(DNBA)6(DMSO)4 exit anti-ferromagnetic interactions.