RECo-containing metal-organic frameworks based on 3-hydroxyisonicotinic acid: Proton conduction and magnetism.

• Three MOFs based on rare earth and transition metal ions were prepared. • Their structures were characterized by SCXRD, IR, XRD and TGA. • The highest proton conductivity is 3.38×10‒5 S cm‒1 at 338 K and 98% RH. • The entropy change (−Δ S m) of compound 1 is 7.1 J kg−1 K−1 at 7 K and 7 T. Three heterometallic metal-organic frameworks, namely, {RE 2 Co 3 L 6 (H 2 O) 6 } n (RE = Gd (1), Dy (2), Y(3)), have been hydrothermally synthesized based on rare-earth (RE) and transition metal ions and the organic ligand of 3-hydroxyisonicotinic acid (H 2 L). Single-crystal X-ray diffraction reveals that compounds 1 ‒ 3 are isomorphic and features three-dimensional (3D) porous structure. In compound 1 , the Gd3+ ion is located in the C 3 axis, and every two propeller-like {GdCo 3 } units are capped up and down to a {Gd 6 Co 6 } ring through twelve L2‒ ligands, giving rise to the formation of the cavities. Notably, three compounds display high thermal stability determined by thermogravimetric analysis (TGA), and good water stability proved by the well-matched of PXRD curves before and after soaked three samples in water for three days. AC impedance measurements exhibit the proton conductivities of compounds 1 ‒ 3 with powder samples are distinctly temperature and humidity dependent, and the values can reach 3.38×10‒5, 1.91×10‒5, and 2.23×10‒5 S cm‒1 at 98% RH and 338 K, respectively. Moreover, compounds 1 ‒ 3 display antiferromagnetic behaviours, and the magnetocaloric effect (MCE) study indicates that the entropy change (−Δ S m) of compound 1 is 7.1 J kg−1 K−1 at 7 K and 7 T. Compound 2 exhibits the slow magnetic relaxation behaviour. Three MOFs based on rare earth and transition metal ions have been prepared under hydrothermal condition and characterized by single-crystal X-ray diffraction, IR, XRD and TGA. Their proton conductivities can reach 10‒5 S cm‒1 at 338 K and 98% RH. In addition, compounds 1 and 2 displays antiferromagnetic behaviors. The entropy change (−Δ S m) of compound 1 is 7.1 J kg−1 K−1 at 7 K and 7 T. [Display omitted] [ABSTRACT FROM AUTHOR]