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