Pressure-induced postsynthetic cluster anion substitution in a MIL-53 topology scandium metal-organic framework

Postsynthetic modification of metal-organic frameworks (MOFs) has proven to be a hugely powerful tool to tune physical properties and introduce functionality, by exploiting reactive sites on both the MOF linkers and their inorganic secondary building units (SBUs), and so has facilitated a wide range of applications. Studies into the reactivity of MOF SBUs have focussed solely on removal of neutral coordinating solvents, or direct exchange of linkers such as carboxylates, despite the prevalence of ancillary charge-balancing oxide and hydroxide ligands found in many SBUs. Herein, we show that the μ2-OH ligands in the MIL-53 topology Sc MOF, GUF-1, are labile, and can be substituted for μ2-OCH3 units through reaction with pore-bound methanol molecules in a very rare example of pressure-induced postsynthetic modification. Using comprehensive solid-state NMR spectroscopic analysis, we show an order of magnitude increase in this cluster anion substitution process after exposing bulk samples suspended in methanol to a pressure of 0.8 GPa in a large volume press. Additionally, single crystals compressed in diamond anvil cells with methanol as the pressure-transmitting medium have enabled full structural characterisation of the process across a range of pressures, leading to a quantitative single-crystal to single-crystal conversion at 4.98 GPa. This unexpected SBU reactivity - in this case chemisorption of methanol - has implications across a range of MOF chemistry, from activation of small molecules for heterogeneous catalysis to chemical stability, and we expect cluster anion substitution to be developed into a highly convenient novel method for modifying the internal pore surface and chemistry of a range of porous materials.

Use of a miniature diamond-anvil cell in a joint X-ray and neutron high-pressure study on copper sulfate pentahydrate

Single-crystal X-ray and neutron diffraction data are usually collected using separate samples. This is a disadvantage when the sample is studied at high pressure because it is very difficult to achieve exactly the same pressure in two separate experiments, especially if the neutron data are collected using Laue methods where precise absolute values of the unit-cell dimensions cannot be measured to check how close the pressures are. In this study, diffraction data have been collected under the same conditions on the same sample of copper(II) sulfate pentahydrate, using a conventional laboratory diffractometer and source for the X-ray measurements and the Koala single-crystal Laue diffractometer at the ANSTO facility for the neutron measurements. The sample, of dimensions 0.40 × 0.22 × 0.20 mm3 and held at a pressure of 0.71 GPa, was contained in a miniature Merrill-Bassett diamond-anvil cell. The highly penetrating diffracted neutron beams passing through the metal body of the miniature cell as well as through the diamonds yielded data suitable for structure refinement, and compensated for the low completeness of the X-ray measurements, which was only 24% on account of the triclinic symmetry of the sample and the shading of reciprocal space by the cell. The two data-sets were combined in a single 'XN' structure refinement in which all atoms, including H atoms, were refined with anisotropic displacement parameters. The precision of the structural parameters was improved by a factor of up to 50% in the XN refinement compared with refinements using the X-ray or neutron data separately.

Suppression of isotopic polymorphism

Crystallisation at pressure overcomes the effect of isotopic polymorphism in the methylpyridine pentachlorophenol co-crystal. Though the hydrogenated Cc polymorph can only be obtained at pressure, it is stable on recovery to ambient conditions.

Effect of Chirality on the Compression of 2-(2-Oxo-1-pyrrolidinyl)butyramide: A Tale of Two Crystals

Understanding polymorphism in chiral systems for drug manufacturing is essential to avoid undesired therapeutic effects. Generally, polymorphism is studied through changes in temperature and solution concentration. A less common approach is the application of pressure. The goal of this work is to investigate the effect of pressure on levetiracetam (pure enantiomer) and etiracetam (racemic compound). Anisotropic compressions of levetiracetam and etiracetam are observed to 5.26 and 6.29 GPa, respectively. The most compressible direction for both was identified to be perpendicular to the layers of the structure. Raman spectroscopy and an analysis of intermolecular interactions suggest subtle phase transitions in levetiracetam (∼2 GPa) and etiracetam (∼1.5 GPa). The stability of etiracetam increases with respect to levetiracetam on compression; hence, the chiral resolution of this system is unfavorable using pressure. This work contributes to the ongoing efforts in understanding the stability of chiral systems.

MrPIXEL: automated execution of Pixel calculations via the Mercury interface

A program is described that enables the energetics of crystal packing to be analysed quickly and easily through a user interface, using Pixel, Python scripts and the widely known visualizer Mercury.

The interpretation of crystal structures in terms of intermolecular interaction energies enables phase stability and polymorphism to be rationalized in terms of quantitative thermodynamic models, while also providing insight into the origin of physical and chemical properties including solubility, compressibility and host–guest formation. The Pixel method is a semi-empirical procedure for the calculation of intermolecular interactions and lattice energies based only on crystal structure information. Molecules are represented as blocks of undistorted ab initio molecular electron and nuclear densities subdivided into small volume elements called pixels. Electrostatic, polarization, dispersion and Pauli repulsion terms are calculated between pairs of pixels and nuclei in different molecules, with the accumulated sum equating to the intermolecular interaction energy, which is broken down into physically meaningful component terms. The MrPIXEL procedure enables Pixel calculations to be carried out with minimal user intervention from the graphical interface of Mercury, which is part of the software distributed with the Cambridge Structural Database (CSD). Following initial setup of a crystallographic model, one module assigns atom types and writes necessary input files. A second module then submits the required electron-density calculation either locally or to a remote server, downloads the results, and submits the Pixel calculation itself. Full lattice energy calculations can be performed for structures with up to two molecules in the crystallographic asymmetric unit. For more complex cases, only molecule–molecule energies are calculated. The program makes use of the CSD Python API, which is also distributed with the CSD.

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study

Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure solid forms were observed. The transition between each of the forms requires a substantial rearrangement of the molecules and we observe that the kinetic barrier to the conversion can aid retention of phases beyond their region of stability. Form II of caprolactam shows a small pressure region of stability between 0.5 GPa and 0.9 GPa with Form III being stable from 0.9 GPa to 5.4 GPa. The two high-pressure forms have a catemeric hydrogen-bonding pattern compared with the dimer interaction observed in ambient pressure Form I. The interaction between the chains has a marked effect on the directions of maximal compressibility in the structure. Neither of the high-pressure forms can be recovered to ambient pressure and there is no evidence of any polymerization occurring.

The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile

The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interact through CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase up to 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic b axis these data characterise the compressibility of the I···N interaction relative to the inter-chain interactions, and indicate that the halogen bond is the most robust intermolecular interaction in the structure, shortening from 3.168(4) at ambient pressure to 2.840(1) Å at 5.0 GPa. The π∙∙∙π contacts are most sensitive to pressure, and in one case the perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) Å. Packing energy calculations (PIXEL) indicate that the π∙∙∙π interactions have been distorted into a destabilising region of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic ( P 1 ¯ ) phase at 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkens on account of formation of microscopic cracks. The resistance drops by 10% and the optical transmittance drops by almost two orders of magnitude. The I···N bond increases in length to 2.928(10) Å and become less linear [<C-I∙∙∙N = 166.2(5)°]; the energy stabilises by 2.5 kJ mol^-1 and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm^-1. The driving force of the transition is shown to be relief of strain built-up in the π∙∙∙π interactions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa.

Antisolvent addition at extreme conditions

Antisolvent addition at high pressure (0.8 GPa) allows crystallization and recovery to ambient pressures of metastable form II paracetamol.

Discovery and recovery of delta p-aminobenzoic acid

A new high-pressure recoverable form has been observed in the model system, p-aminobenzoic acid.

Investigation of Methacrylic Acid at High Pressure Using Neutron Diffraction

This article shows that pressure can be a low-intensity route to the synthesis of polymethacrylic acid. The exploration of perdeuterated methacrylic acid at high pressure using neutron diffraction reveals that methacrylic acid exhibits two polymorphic phase transformations at relatively low pressures. The first is observed at 0.39 GPa, where both phases were observed simultaneously and confirm our previous observations. This transition is followed by a second transition at 1.2 GPa to a new polymorph that is characterized for the first time. On increasing pressure, the diffraction pattern of phase III deteriorates significantly. On decompression phase III persists to 0.54 GPa before transformation to the ambient pressure phase. There is significant loss of signal after decompression, signifying that there has been a loss of material through polymerization. The orientation of the molecules in phase III provides insight into the possible polymerization reaction.