Proton-Conducting Magnetic Coordination Polymers

Three isostructural lanthanide‐based two‐ dimensional coordination polymers (CPs) {[Ln2(L)3(H2O)2]n⋅2n CH3OH)⋅2n H2O} (Ln=Gd3+ (1), Tb3+ (2), Dy3+ (3); H2L=cyclobutane‐1,1‐dicarboxylic acid) were synthesized by using a low molecular weight dicarboxylate ligand and characterized. Single‐crystal structure analysis showed that in complexes 1–3 lanthanide centers are connected by μ3‐bridging cyclobutanedicarboxylate ligands along the c axis to form a rod‐shaped infinite 1D coordination chain, which is further linked with nearby chains by μ4‐connected cyclobutanedicarboxylate ligands to form 2D CPs in the bc plane. Viewing the packing of the complexes down the b axis reveals that the lattice methanol molecules are located in the interlayer space between the adjacent 2D layers and form H‐bonds with lattice and coordinated water molecules to form 1D chains. Magnetic properties of complexes 1–3 were thoroughly investigated. Complex 1 exhibits dominant ferromagnetic interaction between two nearby gadolinium centers and also acts as a cryogenic magnetic refrigerant having a significant magnetic entropy change of −ΔSm=32.8 J kg−1 K−1 for ΔH=7 T at 4 K (calculated from isothermal magnetization data). Complex 3 shows slow relaxation of magnetization below 10 K. Impedance analysis revealed that the complexes show humidity‐dependent proton conductivity (σ=1.5×10−5 S cm−1 for 1, σ=2.07×10−4 S cm−1 for 2, and σ=1.1×10−3 S cm−1 for 3) at elevated temperature (>75 °C). They retain the conductivity for up to 10 h at high temperature and high humidity. Furthermore, the proton conductivity results were correlated with the number of water molecules from the water‐vapor adsorption measurements. Water‐vapor adsorption studies showed hysteretic and two‐step water vapor adsorption (182000 μL g−1 for 1, 184000 μL g−1 for 2, and 1874000 μL g−1 for 3) in the experimental pressure range. Simulation of water‐vapor adsorption by the Monte Carlo method (for 1) confirmed the high density of adsorbed water molecules, preferentially in the interlayer space between the 2D layers. Conducting magnets: A rare coexistence of magnetic refrigeration, slow magnetic relaxation, and proton conduction in a series of magnetically coupled lanthanide‐based 2D coordination polymers {[Ln2(L)3(H2O)2]n⋅2n CH3OH)⋅ 2n H2O} (Ln=Gd3+, Tb3+, Dy3+; H2L=cyclobutane‐1,1‐dicarboxylic acid) was explored (see figure).