Synthesis and Characterization of Cu–Ni Mixed Metal Paddlewheels Occurring in the Metal–Organic Framework DUT-8(Ni_0.98Cu_0.02) for Monitoring Open-Closed-Pore Phase Transitions by X‑Band Continuous Wave Electron Paramagnetic Resonance Spectroscopy

A Cu²⁺-doped metal–organic framework (DUT-8­(Ni_0.98Cu_0.02), M₂(NDC)₂­DABCO, M = Ni, Cu, NDC = 2,6-napththalene dicarboxylate, DABCO = 1,4-diazabicyclo[2.2.2]­octane, DUT = Dresden University of Technology) was synthesized in the form of large (>1 μm) and small crystals (<1 μm) to analyze their switchability by X-band continuous wave (cw) electron paramagnetic resonance (EPR) spectroscopy. The large crystals are flexible and in a porous open pore (<b>op</b>) phase after solvation in <i>N</i>,<i>N</i>-dimethylformamide (DMF), but in the activated solvent-free form, a nonporous closed pore (<b>cp</b>) phase forms. EPR measurements of the rigid Ni-free DUT-8­(Cu) show a characteristic electron spin <i>S</i> = 1 room temperature signal of the antiferromagnetically coupled Cu²⁺-Cu²⁺ paddlewheel building unit of this metal–organic framework. None of the mixed metal DUT-8­(Ni_0.98Cu_0.02) materials showed comparable signals, indicating the absence of dimeric Cu²⁺-Cu²⁺ paddlewheel units in the materials. Instead, characteristic electron spin <i>S</i> = 3/2 signals are detected for all DUT-8­(Ni_0.98Cu_0.02) samples at temperatures <i>T</i> < 77 K, which can be assigned to ferromagnetically coupled mixed metal Ni²⁺-Cu²⁺ paddlewheel units. Those signals differ characteristically for the <b>op</b> and <b>cp</b> phase and enable monitoring the reversible <b>op</b>–<b>cp</b> transition during the de-/adsorption of DMF.