3D isomorphous lanthanide coordination polymers displaying magnetic refrigeration, slow magnetic relaxation and tunable proton conduction.

Four new isostructural lanthanide-based three-dimensional (3D) coordination polymers (CPs), {[Ln(OH)(L)(HO)]·4.6HO·1.4CHCN} (Ln = Gd (1), Dy (2), Ho (3) and Er (4)), have been constructed using a sulfonate-carboxylate-based ligand (NaHL = disodium-2,2'-disulfonate-4,4'-oxydibenzoic acid) and the corresponding lanthanide metal(iii) nitrates. All the CPs 1-4 contain [Ln(μ-OH)] cubane-like cores interconnected through L ligands to give rise to 3D coordination frameworks with 1D hydrophilic channels along the crystallographic c direction. From the topological perspective, the underlying 3D nets of the CPs can be classified as a 3,6,6-c net with an undocumented topology. Magnetic studies display that CP 1 exhibits a magnetocaloric effect with a significant magnetic entropy change (-ΔS) = 34.6 J kg K for ΔH = 7 T at 3 K. CP 2 shows field-induced slow magnetic relaxation properties with energy barrier (U/k) = 30.40 K and relaxation time (τ) = 2.47 × 10 s. Theoretical calculations have been performed to corroborate the magnetic exchange coupling constant value for CP 1 and to obtain a deeper understanding of the field-induced slow magnetic relaxation behavior of CP 2. Impedance analyses display high values of proton conductivity which reach 2.02 × 10, 2.96 × 10, 4.56 × 10 and 6.59 × 10 S cm for CPs 1-4, respectively at high temperature (>75 °C) and 95% relative humidity (RH) in the order CP 1 < CP 2 < CP 3 < CP 4. Notably, the proton conductivities for CPs 3 and 4 are a few orders of magnitude higher than those of CPs 1 and 2 (10 S cmvs. 10 S cm), and the conductivity increases periodically following the decreasing order of ionic radius (Gd > Dy > Ho > Er). This demonstrates the effective employment of the lanthanide contraction strategy to tune proton conductivity while preserving proton-conducting pathways.