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

A mixed-ligand metal-organic framework [Zn2(chdc)2(dabco)]·2NMP (chdc2- = 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.