Rationally Designing Metal-Organic Frameworks Based on [Ln₂] Magnetic Building Blocks Utilizing 2-Hydroxyisophthalate...

By utilizing the 2-hydroxyisophthalic acid (H₃ipO) ligand, 2D metal-organic frameworks (MOFs) featuring rare O_phenol-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 Ln^III 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 -ΔS_m 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 (U_eff) 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 Dy^III 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.