Temperature- and vapor-induced reversible single-crystal-to-single-crystal transformations of three 2D/3D GdIII–organic frameworks exhibiting significant magnetocaloric effects

Abstract

By using Gd₂O₃, propanedioic acid (H₂pda) and oxalic acid (H₂ox), a new Gd-based metal–organic framework (MOF) [Gd(pda)(ox)_0.5(H₂O)₂]_n (1) has been successfully constructed and structurally characterized. Interestingly, temperature- and vapor-induced reversible single-crystal-to-single-crystal transformations occurred and two new MOFs, namely [Gd(pda)(ox)_0.5(H₂O)]_n (1a) and [Gd(pda)(ox)_0.5]_n (1b), have been obtained. Complex 1 displays a two-dimensional (2D) layer structure composed of zigzag [Gd(pda)]_n chains and it could also be made up of numerous Gd₆ macrocycles. Thermal dehydration leads to more complicated three-dimensional (3D) ‘pillar-layer’ structures (1a and 1b) with the same coordination mode of pda²⁻ anions. Magnetic studies suggest the presence of ferromagnetic couplings between the intrachain or intralayer Gd^III ions and large magnetocaloric effects (MCEs) with −ΔS^max_m = 45.0 J kg⁻¹ K⁻¹ (1), 46.1 J kg⁻¹ K⁻¹ (1a) and 46.8 J kg⁻¹ K⁻¹ (1b) under a 7 T applied field. Therefore, the interest of ‘robust magnetocaloric MOFs’ is now extended to compounds showing weak ferromagnetic couplings and hence having better magnetocaloric performances for small field changes.