Structural Dynamics and Adsorption Properties of the Breathing Microporous Aliphatic Metal-Organic Framework

A mixed-ligand metal-organic framework [Zn₂(chdc)₂(dabco)]·2NMP (chdc^2- = trans-1,4-cyclohexanedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane; NMP = N-methylpyrrolidone) was synthesized under solvothermal conditions. This coordination compound demonstrates a guest-driven framework breathing due to a conformational change between e,e-chdc and a,a-chdc forms of the linkers with a reversible restoration of crystallinity. Both the local and longer-range coordination environment of the metal centers were extensively studied by electron paramagnetic resonance on a Cu(II)-doped compound. This approach allowed the detailed investigation of the ligand structural conformations and the framework structural dynamics, supported by an X-ray diffraction method. Carbon dioxide and methane adsorption measurements as well as vapor sorption of benzene and cyclohexane at 298 K of the activated compound were studied. While adsorption of small gas molecules, such as CO₂, CH₄, and N₂, is moderate and does not induce the phase transition, the multistepped character of C₆H₆ and C₆H₁₂ adsorption isotherms characterize the breathing nature of [Zn₂(chdc)₂(dabco)]. The uptake of benzene from the vapor phase reaches 125 mL·g^-1 at 298 K, which surpasses most of benzene uptake values reported for microporous metal-organic frameworks.