Solvent- and Pressure-Induced Phase Changes in Two 3D Copper Glutarate-Based Metal-Organic Frameworks via Glutarate...

Two isoreticular three-dimensional copper(II) glutarate-based pillared-layered metal-organic frameworks (MOFs) with flexible pillars, [Cu₂(glu)₂(bpa)] and [Cu₂(glu)₂(bpp)] (bpa = 1,2-bis(4-pyridyl)ethane; bpp = 1,3-bis(4-pyridyl)propane), undergo spontaneous phase changes upon solvent loss at room temperature. Using single-crystal X-ray diffraction analysis (SCXRD), we show that the phase changes result in new narrow-channel forms that experience a large reduction in solvent-accessible volume. Moreover, the [Cu₂(glu)₂(bpa)] MOF displays a stepped sorption isotherm for the uptake of CO₂ at room temperature. This is indicative of reversion of the framework to the wide-channel form under CO₂ pressure. Supercritical CO₂ was used to isolate the gas-included structures, and by means of SCXRD we were able to determine the positions of the CO₂ molecules in the channels of the frameworks. Finally, we report the use of molecular modeling simulations to elucidate the phase-change mechanism, including the energetic changes involved. Structural limitations in both MOFs allow for only direct gauche-gauche enantiomeric interconversion of the glutarate moieties.