Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect

Two lanthanide-containing structurally analogous porous coordination polymers (PCPs) have been isolated with the general molecular formula [Ln₂(L₁)₂(H₂O)₄(ox)]_n.4nH₂O (where L₁ = fumarate, ox = oxalate; Ln = Dy (1), Gd (2)). Thermogravimetric analysis (TGA) and TG-MS measurements performed on 1 and 2 suggest that not only the solvated water molecules in the crystal lattice but also the four coordinated water molecules on the respective lanthanides in 1 and 2 are removed upon activation. Due to the removal of the waters, 1 and 2 lost their crystallinity and became amorphous, as confirmed by powder X-ray diffraction (PXRD). We propose the molecular formula [Ln₂(L₁)₂(ox)]_n for the amorphous phase of 1 and 2 (where Ln = Dy (1′), Gd (2′)) on the basis of XANES, EXAFS, and other experimental investigations. Magnetization relaxation dynamics probed on 1 and 1′ reveal two different relaxation processes with effective energy barriers of 53.5 and 7.0 cm^–1 for 1 and 45.1 and 6.4 cm^–1 for 1′, which have been rationalized by detailed ab initio calculations. For the isotropic lanthanide complexes 2 and 2′, magnetocaloric effect (MCE) efficiency was estimated through detailed magnetization measurements. We have estimated −ΔS_m values of 52.48 and 41.62 J kg^1– K^–1 for 2′ and 2, respectively, which are one of the largest values reported for an extended structure. In addition, a 26% increase in −ΔS_m value in 2′ in comparison to 2 is achieved by simply removing the passively contributing (for MCE) solvated water molecule in the lattice and coordinated water molecules.