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) Zn1.9Cu0.1(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.