A Series of Lanthanide-Based Metal–Organic Frameworks Derived from Furan-2,5-dicarboxylate and Glutarate: Structure-Corroborated Density Functional Theory Study, Magnetocaloric Effect, Slow Relaxation of Magnetization, and Luminescent Properties

Herein, through a dual-ligand strategy, we report eight isorecticular lanthanide­(III) furan-2,5-dicarboxylic acid metal–organic frameworks (Ln-MOFs) with the general formula {[Ln­(2,5-FDA)_0.5(Glu)­(H₂O)₂]·xH₂O}_n [Ln = Sm (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7), and Yb (8); 2,5-FDA^2– = furan-2,5-dicarboxylate and Glu^2– = glutarate; x = 0.5 for 1, 2, and 4 and x = 0 for 3 and 5–8], synthesized under solvothermal conditions by using an N,N′-dimethylformamide/H₂O mixed solvent system. Crystallographic data reveal that all eight Ln-MOFs 1–8 crystallize in the orthorhombic Pnma space group. All of the MOFs are isostructural as well as isomorphous with distorted monocapped square-antiprismatic geometry around the Ln1 metal center. In Ln-MOFs 1–8, the 2,5-FDA^2– and Glu^2– ligands exhibit μ₂-κ⁴,η¹:η¹:η¹:η¹ and μ₃-κ⁵,η²:η¹:η¹:η¹ coordination modes, respectively. Topologically, assembled Ln-MOFs 1–8 consist of the 2D cem topological type. The designed Ln-MOFs 1–8 are further explored for structure-corroborated density functional theory study. Meanwhile, room temperature photoluminescence properties of Ln-MOFs 2 and 4 and magnetic properties of Ln-MOFs 3 and 5 have been explored in detail. A highly intense, ligand-sensitized, Ln³⁺ f–f photoluminescence emission is exhibited by Ln-MOFs 2 [Eu³⁺ (red emission)] and 4 [Tb³⁺ (green emission)]. Magnetic studies suggest weak antiferro- and ferromagnetic interactions between adjacent Gd^III ions in Ln-MOF 3, thereby displaying a large magnetocaloric effect. The magnetic data measured at T = 2 K and ΔH = 30 kOe depict that the −ΔS_m value per unit mass reaches 32.1 J kg^–1 K^–1, which is larger than most of the Gd^III-based complexes reported. The alternating-current susceptibility measurements on Ln-MOF 5 revealed that out-of-phase signals are frequency- and temperature-dependent under both 0 and 2 kOe direct-current fields, thereby suggesting a typical slow magnetic relaxation behavior with two relaxation processes. This is further supported by the Cole–Cole plots at 2.4–6 K.