Over the last 10 years or so, the interest and number of high-pressure studies has increased substantially. One area of growth within this niche field is in the study of metal-organic frameworks (MOFs or coordination polymers). Here we present a review on the subject, where we look at the structural effects of both non-porous and porous MOFs, and discuss their mechanical and chemical response to elevated pressures.

Pressures up to 0.8 GPa have been used to squeeze a range of sterically 'oversized' C5-C8 alkane guest molecules into the cavities of a small-pore Sc-based metal-organic framework. Guest inclusion causes a pronounced reorientation of the aromatic rings of one third of the terephthalate linkers, which act as 'torsion springs', resulting in a fully reversible change in the local pore structure. The study demonstrates how pressure-induced guest uptake can be used to investigate framework flexibility relevant to 'breathing' behavior and to understand the uptake of guest molecules in MOFs relevant to hydrocarbon separation.

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally nonporous materials. Here we report a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC = 1,4-benzenedicarboxylate) and its nitro-functionalized derivative (Sc2(NO2-BDC)3) and compare it to direct compression behavior in a nonpenetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations, and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against nonpenetrative hydrostatic fluids at high pressure, this study demonstrates a novel high-pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favorable adsorption sites.

Here we report four post-synthetic modifications, including the first ever example of a high pressure-induced post-synthetic modification, of a porous copper-based metal-organic framework. Ligand exchange with a water ligand at the axial metal site occurs with methanol, acetonitrile, methylamine and ethylamine within a single-crystal and without the need to expose a free metal site prior to modification, resulting in significant changes in the pore size, shape and functionality. Pressure experiments carried out using isopropylalcohol and acetaldehyde, however, results in no ligand exchange. By using these solvents as hydrostatic media for high-pressure single-crystal X-ray diffraction experiments, we have investigated the effect of ligand exchange on the stability and compressibility of the framework and demonstrate that post-synthetic ligand exchange is very sensitive to both the molecular size and functionality of the exchanged ligand. We also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using high pressures which results in a pressure-induced chemical decomposition of the Cu-framework.

This work details a crystal engineering strategy to obtain a novel solid form of the liquid drug molecule propofol using isonicotinamide as a cocrystal former. Knowledge of intermolecular hydrogen bonded supramolecular synthons has been exploited to select a potential cocrystal former based on the likely growth unit formed. The structure of the cocrystal, solved using single-crystal X-ray diffraction, is reported, confirming the molecular packing and key intermolecular interactions adopted in the novel solid form. The potential to enhance a drug's properties is demonstrated by an increased melting point compared to the native drug form, such that the liquid drug becomes a stable solid at room temperature. Unusually, the propofol/isonicotinamide complex has three structurally similar, temperature-dependent polymorphs, and the crystal structure of each form is reported herein.