Highly proton-conducting lanthanide metal–organic frameworks featuring highly oxygenated ligands with slow magnetic relaxation or magnetocaloric effect

Abstract

Two isoreticular lanthanide metal–organic frameworks (Ln-MOFs), {[Ln₂(H₂O)₆(H₂dobdc)₆·12H₂O}_n (Ln = Tb³⁺ and Gd³⁺); H₄dobdc = 2,5-di-hydroxy-benzene-1,4-di-carboxylic acid), constructed from a hydroxy-functionalized dicarboxybenzene were synthesized and structurally characterized. Single-crystal X-ray crystallography reveals that the two Ln-MOFs were constructed from Ln³⁺ dimers as secondary building units, which were linked by H₂dobdc²⁻ ligands to form three-dimensional (3D) frameworks featuring pcu topology. Interestingly, 1D triangular porous channels were formed in the frameworks with a high density of coordinated and guest water molecules and hydroxy-containing inwall, which provide a hydrophilic and highly oxygenated environment. Magnetic studies indicated the Tb(III) analogue exhibited slow relaxation of magnetization under an applied static field of 2 kOe while the Gd(III) MOF displayed the magnetocaloric effect (MCE) with a magnetic entropy change of 19.5 J kg⁻¹ K⁻¹. At 55 °C under 98% RH, impedance spectroscopy indicated that the Ln-MOFs exhibit high proton conductivities up to 1.63 × 10⁻² and 2.51 × 10⁻² S cm⁻¹ through the vehicle mechanism, suggesting that the two are highly proton-conducting Ln-MOFs. This work not only demonstrates two rare magnetic-electrical bifunctional Ln-MOFs with high proton-conduction and interesting magnetic properties but also provides potential design guidelines for high-performance Ln-MOF based proton conductors.