The Paramagnetic Properties of Breathing Metal–Organic Frameworks: Theoretical Models for Paddle‐Wheel Secondary Building Units of Cobalt, Nickel, and Copper

The metal-organic frameworks (MOFs) that contain paramagnetic metal cations at their secondary building units (SBUs) have potential applications in magnetic materials for single molecule magnets, molecular spintronics, and quantum computing. The quantum chemical investigations were carried out on the systems M₂(O₂CH)₄ and M₂(O₂CH)₄(C₆H₁₂N2)₂, which model paddle-wheel SBUs embedded in breathing paramagnetic MOFs, specifically M₂(bdc)₂(dabco) (dabco = C₆H₁₂N₂, bdc^2- = C₈H₄O₄ ^2-, M²⁺ = Co²⁺, Ni²⁺, Cu²⁺). An analysis of the electronic states and interatomic interactions for these models was performed and compared with experimental data on heat capacity and magnetic susceptibility of M₂(bdc)₂(dabco). It is shown that the antiparallel ordering (broken symmetry) of the spins on the metal atoms is the lowest spin state of the model systems. The coordination of the dabco molecules makes the high-spin state the second most stable spin state, with a relatively small energy separation from the broken symmetry solution.