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

Three
lanthanide coordination polymers (Ln-CPs) formulated as [Ln­(azdc)­(HCOO)]n [Ln = GdIII (1),
TbIII (2), DyIII (3); H2azdc = 4,4′-azobenzoic acid] have been successfully
obtained by the solvothermal reaction of LnIII 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 GdIII ions that are replaced by HCOO– groups
in 1, resulting in the distinct structures and properties
of the final products. Adjacent LnIII ions in 1–3 are connected by HCOO– groups
through bridging and chelating modes to give 2D layers, which are
further linked by azdc2– ligands to produce 3D frameworks.
Magnetic results declare that antiferromagnetic couplings exist in 1, although two different magnetic interactions among adjacent
GdIII 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.