Dense inorganic-organic sodium-gadolinium sulfite-oxalate networks constructed from short bridging ligands: synthesis, structures and magnetic refrigeration behaviour.

Two new inorganic-organic hybrid, three-dimensional heterometallic gadolinium-sulfite-oxalate coordination polymers, [NaGd(SO3)(C2O4)(H2O)2]∞ (1) and [Na3Gd(SO3)2(C2O4)(H2O)]∞ (2), featuring densely packed network architectures, have been successfully synthesized under mild hydrothermal conditions. The structures were comprehensively characterized by single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). To maximize the metal-to-ligand ratio and enhance magnetic density, short bridging sulfite (SO32-) and oxalate (C2O42-) linkers were strategically incorporated, enabling the formation of compact frameworks suitable for magnetic refrigeration applications. Magnetic measurements reveal weak antiferromagnetic interactions between Gd(III) centers in both compounds, which contribute to pronounced magnetocaloric effects. Compound 1 exhibits a maximum magnetic entropy change (ΔSm) of -41.58 J kg-1 K-1, while compound 2 shows -32.84 J kg-1 K-1 under an applied field 7 T at 3 K. These results highlight the potential of densely packed Gd-based coordination polymers as efficient low-temperature magnetic cooling materials.