Modulation of Co3-Based Secondary Building Units in Metal–Organic Frameworks via Pyridine-Derived Ligands: Crystal Structures, Magnetic Properties, and Electronic Spin States

Two trinuclear cobalt (Co3)-based metal–organic frameworks, Co3(CHDC)3(py)4 (2; CHDC = trans-1,4-cyclohexanedicarboxylate, py = pyridine) and Co3(CHDC)3(mpy)2· 2DMF (3; mpy = 4-methylpyridine, DMF = N,N-dimethylformamide), were successfully prepared via the solvothermal reactions of Co(NO3)2·6H2O, trans-1,4-cyclohexanedicarboxylic acid, and py/mpy in DMF solution. Single crystal X-ray diffraction analyses revealed that the Co3-secondary building units (SBUs) in 2 and 3 adopt Cooctahedral···Cooctahedral···Cooctahedral and Cotetrahedral···Cooctahedral···Cotetrahedral coordination environments, respectively, and are connected by six CHDC linkers to form two-dimensional sheet structures with a triangular lattice. The structural differences of these Co3-SBUs led to clear differences in the magnetic properties and electronic spin states of 2 and 3; temperature-dependent magnetic susceptibility measurements revealed that 2 and 3 exhibited antiferromagnetic and ferromagnetic interactions, respectively, within the Co3-SBUs. These experimental magnetic results are consistent with the density-functional theory calculations of the model structures of Co3-SBUs, which indicate that the most stable spin states are S = 3/2 for 2 and S = 9/2 for 3.