A Multifunctional Lanthanide Carbonate Cluster Based Metal-Organic Framework Exhibits High Proton Transport and Magnetic Entropy Change.

A novel multifunctional, three-dimensional (3D) lanthanide carbonate cluster based metal-organic framework (MOF) with the general formula {[Gd2(CO3)(ox)2(H2O)2]·3H2O} n (1) has been synthesized via self-assembly of gadolinium (Gd) carbonate and oxalate under hydrothermal conditions. Single-crystal X-ray diffraction reveals that the compound 1 consists of the Gd carbonate cluster with oxalic acid ligands, which form a 3D framework structure with an ordered one-dimensional (1D) pore channel along the a-axis. The coordination water molecules of Gd3+ ions point to the interior of the pore and form a 1D hydrogen bond pathway with oxygen atoms in adjacent oxalic acid that is stable at high temperature (up to 150 °C). The compound 1 features multiple hydrogen-bonding walls and good thermal stabilities, and shows the highest proton conductivity of 1.98 × 10-3 S cm-1 at T = 150 °C and in room air without additional humidity. Magnetic investigations of compound 1 demonstrate that weak antiferromagnetic couplings between adjacent Gd3+ ions bring about large cryogenic magnetocaloric effects. Remarkably, the maximum entropy change (-Δ Sm) of compound 1 reaches 58.5 J kg-1 K-1 at 2 K for a moderate field change (Δ H = 7 T). Moreover, the isomorphous MOFs: {[Ln2(CO3)(ox)2(H2O)2]·3H2O} n (Ln3+ = Ce3+(2), Pr3+(3), Nd3+(4), Tb3+(5)) also are structurally and functionally characterized, and compounds 2-5 exhibit proton conductivity above 10-3 S cm-1 in room air and without additional humidity.