Probing Local Structural Changes at Cu²⁺ in a Flexible Mixed-Metal Metal-Organic Framework by in Situ Electron Paramagnetic Resonance during CO₂ Ad- and Desorption

The flexible mixed-metal pillared-layered metal-organic framework (MOF) Zn_1.9Cu_0.1(BME-bdc)₂(dabco) (BME-bdc^2– = 2,5-bis­(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = diazabicyclo[2.2.2]­octane) with Cu²⁺ dopants as electron paramagnetic resonance (EPR) active probes was synthesized, and the guest-induced phase transition was studied by a newly developed in situ gas sorption EPR setup. Within the structure, the Cu²⁺ ions do not build any Cu²⁺–Cu²⁺ paddlewheels, and only mixed-metal Zn²⁺–Cu²⁺ paddlewheels were identified by EPR. Two distinct Zn²⁺–Cu²⁺ species (A and B) occur in the structure: Species A occurs in regions with low Cu²⁺ concentrations and has no adjacent mixed-metal paddlewheel units. Species B is present in domains with higher local copper concentrations, where adjacent mixed-metal paddlewheel units alter the EPR signal. The parent monometallic Zn MOF is known to undergo phase transitions from a narrow pore (np) to a large pore (lp) phase due to host-guest interactions with CO₂. In situ EPR measurements during CO₂ ad- and desorption at T = 195 K reveal the reversible transition from an np phase at low pressures to an lp phase at high pressures. The EPR experiments suggest an increase of the Cu²⁺ equatorial ligand field as well as a decrease of the rhombic distortion of the mixed-metal paddlewheel unit upon the CO₂ adsorption triggered phase transition. The transition shows a large hysteresis typical for flexible materials, and the corresponding transition pressures are almost identical to those reported for the parent monometallic compound by gas adsorption isotherms. This result indicates that doping the monometallic material with a small amount of a Cu²⁺ probe does not change its responsiveness significantly and helps gather information on the local structure during the phase transition. The results demonstrate that the newly built in situ EPR setup is sensitive to small structural changes in the Cu²⁺ coordination environment induced by guest molecules which can hardly be resolved by other experimental methods.