Lanthanide Coordination Polymers with 4,4'-Azobenzoic Acid: Enhanced Stability and Magnetocaloric Effect by Removing Guest Solvents.

ABSTRACT

Three lanthanide coordination polymers (Ln-CPs) formulated as [Ln(azdc)(HCOO)]n [Ln = Gd(III) (1), Tb(III) (2), Dy(III) (3); H2azdc = 4,4'-azobenzoic acid] have been successfully obtained by the solvothermal reaction of Ln(III) ions with H2azdc ligands in the mixed solvent N,N-dimethylformamide (DMF)/H2O. Compared with our previous work on Ln-CPs with H2azdc ligands, [Gd2(azdc)3(DMA)2]n·2nDMA (1'; DMA = dimethylacetamide), in which the DMA molecules coordinate to Gd(III) ions that are replaced by HCOO(-) groups in 1, resulting in the distinct structures and properties of the final products. Adjacent Ln(III) ions in 1-3 are connected by HCOO(-) groups through bridging and chelating modes to give 2D layers, which are further linked by azdc(2-) ligands to produce 3D frameworks. Magnetic results declare that antiferromagnetic couplings exist in 1, although two different magnetic interactions among adjacent Gd(III) ions derived from antiferromagnetic interactions of the smaller Gd-O-Gd angles (Gd···Gd distances) and weak ferromagnetic interactions of the larger Gd-O-Gd angles (Gd···Gd distances) coexist in 1. Furthermore, the magnetocaloric effect (MCE) value of 1 is 1.5 times as large as that of 1'. More importantly, 1 exhibits excellent stabilities toward air, thermal, solvent, and acid/alkaline conditions. The results manifest that the crystalline structure of 1 can be stable at at least 425 °C supported by the in situ variable-temperature powder X-ray diffraction patterns and thermogravimetric analyses, in air for at least 3 months, and in common solvents for more than 1 week, as well as in aqueous solutions ranging from pH = 2 to 12 for more than 1 week.