Trinuclear Cobalt(II)-Based Metal–Organic Framework Showing Solvent-Induced Switching to Magnetic Ordering

A new self-assembled two-dimensional trinuclear cobalt-based metal–organic framework (MOF), [Co₃(CHDC)₃(DMF)_2.5(H₂O)_0.5]·(DMF)_1.5 (1C), which was constructed from two types of trinuclear cobalt secondary building units (Co₃–SBUs) with trans-1,4-cyclohexanedicarboxylate, was synthesized by a solvothermal reaction and structurally characterized by synchrotron single crystal X-ray diffraction analyses. 1C undergoes a phase transition to an amorphous MOF (1A) upon heating at 160 °C, due to the desorption of DMF/H₂O molecules. Conversely, 1A can revert back to crystalline 1C through reflux in aqueous DMF solutions, demonstrating a reversible phase transition behavior. Magnetic susceptibility measurements of 1C revealed that the ferromagnetic interactions are operative between Co ions within the Co₃–SBUs (J = +2 ∼ +5 cm^–1), which is consistent with DFT calculations indicating that the ground state of the Co₃–SBUs is S = 9/2. In contrast, 1A show the antiferromagnetic interactions; χ_MT decreased continuously upon cooling due to the orbital contribution. That is, 1C acts as a magnetic sponge, in which magnetic ordering switches in response to desorption and adsorption of DMF/H₂O molecules.