Rationally Designing Metal-Organic Frameworks Based on [Ln2] Magnetic Building Blocks Utilizing 2-Hydroxyisophthalate and Fine-Tuning the Magnetic Properties of Dy Analogues by Terminal Coordinated Solvents.

By utilizing the 2-hydroxyisophthalic acid (H3ipO) ligand, 2D metal-organic frameworks (MOFs) featuring rare Ophenol-bridged [Ln2]-magnetic building blocks (MBBs), [Ln2(ipO)2(DMF)(H2O)] [Ln = Gd (1), Dy (2); DMF = N,N-dimethylformamide], were rationally designed and synthesized. When the reaction solvents that behave as terminal ligands were changed, the coordination geometries of LnIII ions and the arrangement fashion of [Ln2]-MBBs for these MOFs were modified accordingly. Another type of 2D MOF of [Ln2(ipO)2(H2O)4]·2H2O [Ln = Gd (3), Dy (4)] was thus obtained. MOFs 1 and 3 exhibited favorable magnetocaloric effect, whose maximum -ΔSm values reach 30.0 and 31.7 J kg-1 K-1, respectively. None of the single-molecule-magnet (SMM) behavior was observed in 2. However, from 2 to 4, the change of the terminal coordinated solvents brought obvious improvement of the magnetic properties. MOF 4 showed interesting relaxation behavior, in which dual relaxation was only visible under weak direct-current fields, and its highest effective energy barrier (Ueff) reached up to 243 K. Ab initio calculations revealed the tuning mechanism of the terminal coordinated solvents. Their change optimized the arrangements of the magnetic axis of the DyIII centers in both each MBB and the whole framework, thus improving the magnetic anisotropy and magnetic interactions of the system. Significantly, within the [Dy2]-MBBs of 4, the angle made by the individual magnetic axis and Dy···Dy' line is nearly 0°. This case favoring a high SMM performance not only was scarcely achieved in discrete {Ln2}-SMMs with numerous members but also has never been observed in any MBB-based MOFs as far as we know.