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

A mixed-ligand metal-organic framework [Zn-2(chdc)(2)(dabco)]center dot 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 N-2, is moderate and does not induce the phase transition, the multistepped character of C6H6 and C6H12 adsorption isotherms characterize the breathing nature of [Zn-2(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.