Lanthanide-Aromatic Iminodiacetate Frameworks with Helical Tubes: Structure, Properties, and Low-Temperature Heat Capacity.

A series of lanthanide coordination polymers [LnL(HO)] [Ln = Pr (), Nd (), Sm (), Eu (), and Gd (), HL = -(4-carboxy-benzyl)iminodiacetic acid] was hydrothermally prepared and structurally characterized. All the five compounds have been confirmed as 3D Ln-CPs with one-dimensional helical tunnels composed of four helical chains, although there are different coordination geometries around Ln. Enantiomeric helixes in , and absolute left-handed and right-handed helical chains in and , respectively, lead to different tunnel spaces. Their conformations can also be featured by different space groups and unit cell dimensions. Photoluminescence measurement on and show characteristic emission peaks of Sm and Eu ions, respectively. The low-temperature heat capacity of has been investigated in the temperature range of 1.9-300 K. Their heat capacity values are nearly equal below 10 K and display a crossover with the value order ( > ( ≈ ( > ( above 10 K. The measured heat capacities have been fitted, and the corresponding thermodynamic functions were consequently calculated based on the fitting parameters. The standard molar entropies at 298.15 K have been determined to be (415.71 ± 4.16), (451.32 ± 4.51), (308.53 ± 3.09), and (407.62 ± 4.08) J·mol·K for , , , and , respectively.