By decoupling the mechanical behaviour of building units for the first time in a wine-rack framework containing two different strut types, we show that lithium L-tartrate exhibits NLC with a maximum value, Kmax = −21 TPa−1, and an overall NLC capacity, χNLC = 5.1%, that are comparable to the most exceptional materials to date. Furthermore, the contributions from molecular strut compression and angle opening interplay to give rise to so-called “hidden” negative linear compressibility, in which NLC is absent at ambient pressure, switched on at 2 GPa and sustained up to the limit of our experiment, 5.5 GPa. Analysis of the changes in crystal structure using variable-pressure synchrotron X-ray diffraction reveals new chemical and geometrical design rules to assist the discovery of other materials with exciting hidden anomalous mechanical properties.

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.

The synthesis of zirconium and hafnium metal–organic frameworks (MOFs) often relies on coordination modulation – the addition of competing monotopic modulators to reaction mixtures – to reproducibly generate highly crystalline material. Typically, large excesses of monocarboxylic acids such as formic, acetic and benzoic acid are applied, but access to diffraction quality single crystals, particularly of UiO-66 topology MOFs, remains troublesome. Herein, we show that amino acids, in particular L-proline, are highly efficient modulators of Zr and Hf MOFs of the UiO-66 series, with as little as four equivalents affording access to large, diffraction quality single crystals that are free of defects. Five crystal structures are reported, including MOFs which previously could not be characterised in this manner, with molecular dynamics simulations utilised to understand dynamic disorder. Additionally, a series of MOFs are characterised in depth, allowing a comparison of the thermal stabilities and porosities for Zr and Hf analogues. We also show that the protocol can be extended to microwave synthesis, and that modulating ability varies dramatically across a series of amino acids. Access to single crystals has facilitated our own in depth study of the mechanical properties of these MOFs, and we expect that our protocols will enable the discovery of new Zr and Hf MOFs as well as offer new insights into their materials properties.

A novel method for CO2 delivery to a porous material is reported, wherein a perfluorocarbon containing dissolved CO2 has been used as a pressure-transmitting liquid in a high-pressure single-crystal X-ray diffraction experiment. Pressure causes the gas to be squeezed out of the liquid into the host crystal, monitored via a single-crystal to single-crystal phase transition on uptake of CO2.

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.

The hydrostatic compression of glutathione (GSH) has been studied to 5.24 GPa by single crystal X-ray diffraction. Over the course of this pressure range the compound undergoes two phase transitions, the first between 1.65 and 2.27 GPa, yielding GSH-II, and the second between 2.94 and 3.70 GPa, yielding GSH-III. All three polymorphs are orthorhombic, P212121, and feature layers of GSH molecules. These layers are connected together via alternating layers of R33(11) motifs, and OH⋯O and one NH⋯O H-bonding interactions. In the phase-I to II transition voids at the centre of ring motifs begin to close-up, accompanied by two significant changes in the conformation of the GSH molecules. The first is a large re-orientation of the glycine residue quantified by one of the CNCC torsion angles along the backbone of the peptide which increases from ca. −76 to 117°. The second involves a re-orientation of the OH⋯O interaction, which breaks the aforementioned H-bond which acted between layers, to form a much shorter H-bonding interaction in GSH-II within the layers. The conformation of this interaction is rather unfavourable under ambient pressure conditions, however, is stabilised in the solid state structure of GSH-II, shown here by mapping the potential energy surface of the CCOH torsion angle. It is speculated here that the closure of voids within ring motifs, and formation of this shorter OH⋯O interaction are the main driving forces for this transition. On increasing pressure further, formation of GSH-III at 3.70 GPa involves three main structural changes: (i) a further twisting of the glycine residue, quantified by a CNCC torsion angle decrease from ca. 117° to 69°; (ii) a twisting along the glutamic acid residue backbone, quantified by a CCCN torsion angle increase from ca. −79° to −73°; (iii) a change in orientation of the thiol group, quantified by a CCSH torsion angle increase from 83° to 149°. The effect of (i) and (ii) is to increase the effective length of the GSH molecule backbone resulting in an increase in length of the b-axis. This allows the layers to pack more efficiently and further closures of voids within ring motifs can be observed. The conformational change observed in (ii) also allows the formation of a much shorter SH⋯O H-bonding interaction resulting in the conformational change observed in (iii). It is postulated here that both more efficient packing of the layers and formation of a shorter SH⋯O interaction are the driving forces for the phase-II to III transition observed here.

The first ever study on the effects of pressure on a porous dipeptide is presented to 0.2 GPa. On increasing pressure, closing up of the large porous holes, shortening of hydrogen bonds, and a pressure induced ordering of the thermal parameters on the L-valine residue can be observed.

By decoupling the mechanical behaviour of building units for the first time in a wine-rack framework containing two different strut types, we show that lithium L-tartrate exhibits NLC with a maximum value, Kmax = −21 TPa−1, and an overall NLC capacity, χNLC = 5.1%, that are comparable to the most exceptional materials to date. Furthermore, the contributions from molecular strut compression and angle opening interplay to give rise to so-called “hidden” negative linear compressibility, in which NLC is absent at ambient pressure, switched on at 2 GPa and sustained up to the limit of our experiment, 5.5 GPa. Analysis of the changes in crystal structure using variable-pressure synchrotron X-ray diffraction reveals new chemical and geometrical design rules to assist the discovery of other materials with exciting hidden anomalous mechanical properties.

The synthesis of zirconium and hafnium metal–organic frameworks (MOFs) often relies on coordination modulation – the addition of competing monotopic modulators to reaction mixtures – to reproducibly generate highly crystalline material. Typically, large excesses of monocarboxylic acids such as formic, acetic and benzoic acid are applied, but access to diffraction quality single crystals, particularly of UiO-66 topology MOFs, remains troublesome. Herein, we show that amino acids, in particular L-proline, are highly efficient modulators of Zr and Hf MOFs of the UiO-66 series, with as little as four equivalents affording access to large, diffraction quality single crystals that are free of defects. Five crystal structures are reported, including MOFs which previously could not be characterised in this manner, with molecular dynamics simulations utilised to understand dynamic disorder. Additionally, a series of MOFs are characterised in depth, allowing a comparison of the thermal stabilities and porosities for Zr and Hf analogues. We also show that the protocol can be extended to microwave synthesis, and that modulating ability varies dramatically across a series of amino acids. Access to single crystals has facilitated our own in depth study of the mechanical properties of these MOFs, and we expect that our protocols will enable the discovery of new Zr and Hf MOFs as well as offer new insights into their materials properties.