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

Abstract

Four new isostructural lanthanide-based three-dimensional (3D) coordination polymers (CPs), {[Ln₄(OH)₄(L)₂(H₂O)₈]·4.6H₂O·1.4CH₃CN}_n (Ln³⁺ = Gd³⁺ (1), Dy³⁺ (2), Ho³⁺ (3) and Er³⁺ (4)), have been constructed using a sulfonate-carboxylate-based ligand (Na₂H₂L = disodium-2,2'-disulfonate-4,4'-oxydibenzoic acid) and the corresponding lanthanide metal(iii) nitrates. All the CPs 1-4 contain [Ln₄(μ₃-OH)₄]⁸⁺ cubane-like cores interconnected through L^4- 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_m) = 34.6 J kg^-1 K^-1 for ΔH = 7 T at 3 K. CP 2 shows field-induced slow magnetic relaxation properties with energy barrier (U_eff/k_B) = 30.40 K and relaxation time (τ₀) = 2.47 × 10^-7 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^-6, 2.96 × 10^-6, 4.56 × 10^-3 and 6.59 × 10^-3 S cm^-1 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^-3 S cm^-1vs. 10^-6 S cm^-1), and the conductivity increases periodically following the decreasing order of ionic radius (Gd³⁺ > Dy³⁺ > Ho³⁺ > Er³⁺). This demonstrates the effective employment of the lanthanide contraction strategy to tune proton conductivity while preserving proton-conducting pathways.