Synthesis and Characterization of Cu-Ni Mixed Metal Paddlewheels Occurring in the Metal-Organic Framework DUT-8(Ni0.98Cu0.02) for Monitoring Open-Closed-Pore Phase Transitions by X-Band Continuous Wave Electron Paramagnetic Resonance Spectroscopy.

A Cu2+-doped metal-organic framework (DUT-8(Ni0.98Cu0.02), 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 (op) phase after solvation in N, N-dimethylformamide (DMF), but in the activated solvent-free form, a nonporous closed pore (cp) phase forms. EPR measurements of the rigid Ni-free DUT-8(Cu) show a characteristic electron spin S = 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(Ni0.98Cu0.02) materials showed comparable signals, indicating the absence of dimeric Cu2+-Cu2+ paddlewheel units in the materials. Instead, characteristic electron spin S = 3/2 signals are detected for all DUT-8(Ni0.98Cu0.02) samples at temperatures T < 77 K, which can be assigned to ferromagnetically coupled mixed metal Ni2+-Cu2+ paddlewheel units. Those signals differ characteristically for the op and cp phase and enable monitoring the reversible op-cp transition during the de-/adsorption of DMF.