. orcid.org/0000-0003-3915-8887 et al. (3 more authors) (2017) Solvent-switchable continuous-breathing behaviour in a diamondoid metal-organic framework and its influence on CO2 versus CH4 selectivity. Nature Chemistry.

Metal-organic frameworks (MOFs) are crystalline synthetic porous materials formed by binding organic linkers to metal nodes: they can be either rigid 1,2 or flexible. 3 Zeolites and rigid MOFs have widespread applications in sorption, separation and catalysis that arise from their ability to control the arrangement and chemistry of guests in their pores via the shape and functionality of the internal surface defined by their chemistry and structure. 4,5 Their structures correspond to an energy landscape with a single, albeit highly functional, energy minimum. In contrast, proteins function by navigating between multiple metastable structures using bond rotations of the polypeptide, 6,7 where each structure lies in one of the minima of a conformational energy landscape and can be selected according to the chemistry of the molecules interacting with the protein. These structural changes are realised through the mechanisms of conformational selection (where a higher energy minimum characteristic of the protein is stabilised by small molecule binding), and induced fit (where a small molecule imposes a structure on the protein that is not a minimum in the absence of that molecule). 8 Here we show that rotation about covalent bonds in a peptide linker can change a flexible MOF to afford nine distinct crystal structures, revealing a conformational energy landscape characterised by multiple structural minima. The uptake of small molecule guests by the MOF can be chemically triggered by inducing peptide conformational change. This change transforms the material from a minimum on the landscape that is inactive for guest sorption to an active one. Chemical control of the conformation of a flexible organic linker offers a route to modify the pore geometry and internal surface chemistry and thus the function of open-framework materials. Flexible MOF structures 9,10 can be rearranged in the presence of guests through mechanical mechanisms such as the repositioning of a rigid linker about an inorganic unit 11-13 or the relative displacement of two rigid networks, 14 opening a range of routes to control function 15 that are not accessible to rigid frameworks with their single structural minimum (Figure 1). Similar phenomena have been observed in the host-guest chemistry of interlocked cage molecules. 16-18 Alternatively, rotations about bonds involving sp 3 carbons 19-25 allow MOF to access different structures. For example, low energy conformational changes of dipeptide Gly-X linkers produce open and closed forms of Zn(Gly-X)2 frameworks. 26,27 The greater chemical diversity and more complex conformational space of higher order oligopeptides offer MOF with multiple open structures (Figure 1). This could allow interaction with molecules in the pores to select a specific structure for a defined function from the resulting energy landscape. That structure would be accessed through the single bond rotation pathway used by proteins (Figure 1). The tripeptide glycine-glycine-L-histidine (GGH) affords a three-dimensional chiral MOF Zn...

Choice of solvent is used to direct the formation of either hydrogen bonds or halogen bonds in competitive self-assembly.

Metal-organic frameworks (MOFs) are a class of porous crystalline materials of modular design. One of the primary applications of these materials is in the adsorption and separation of gases, with potential benefits to the energy, transport and medical sectors. In situ crystallography of MOFs under gas atmospheres has enabled the behaviour of the frameworks under gas loading to be investigated and has established the precise location of adsorbed gas molecules in a significant number of MOFs. This article reviews progress in such crystallographic studies, which has taken place over the past decade, but has its origins in earlier studies of zeolites, clathrates etc. The review considers studies by single-crystal or powder diffraction using either X-rays or neutrons. Features of MOFs that strongly affect gas sorption behaviour are discussed in the context of in situ crystallographic studies, specifically framework flexibility, and the presence of (organic) functional groups and unsaturated (open) metal sites within pores that can form specific interactions with gas molecules.

To understand the structural and compositional factors controlling lithium transport in sulfides, we explored the Li5AlS4–Li4GeS4 phase field for new materials. Both parent compounds are defined structurally by a hexagonal close packed sulfide lattice, where distinct arrangements of tetrahedral metal sites give Li5AlS4 a layered structure and Li4GeS4 a three-dimensional structure related to γ-Li3PO4. The combination of the two distinct structural motifs is expected to lead to new structural chemistry. We identified the new crystalline phase Li4.4Al0.4Ge0.6S4, and investigated the structure and Li+ ion dynamics of the family of structurally related materials Li4.4 M 0.4 M′0.6S4 (M = Al3+, Ga3+ and M′ = Ge4+, Sn4+). We used neutron diffraction to solve the full structures of the Al-homologues, which adopt a layered close-packed structure with a new arrangement of tetrahedral (M/M′) sites and a novel combination of ordered and disordered lithium vacancies. AC impedance spectroscopy revealed lithium conductivities in the range of 3(2) × 10–6 to 4.3(3) × 10–5 S cm–1 at room temperature with activation energies between 0.43(1) and 0.38(1) eV. Electrochemical performance was tested in a plating and stripping experiment against Li metal electrodes and showed good stability of the Li4.4Al0.4Ge0.6S4 phase over 200 h. A combination of variable temperature 7Li solid state nuclear magnetic resonance spectroscopy and ab initio molecular dynamics calculations on selected phases showed that two-dimensional diffusion with a low energy barrier of 0.17 eV is responsible for long-range lithium transport, with diffusion pathways mediated by the disordered vacancies while the ordered vacancies do not contribute to the conductivity. This new structural family of sulfide Li+ ion conductors offers insight into the role of disordered vacancies on Li+ ion conductivity mechanisms in hexagonally close packed sulfides that can inform future materials design.

Cocrystal formation is considered as one of the most effective solid-state methods to alter the physicochemical properties of active pharmaceutical ingredients (APIs). In silico methods for cocrystal prediction are mostly based on structural and energetic considerations. We have developed a computational method that ranks the probability of cocrystal formation of APIs with large databases of crystal coformers (CCFs). This approach is based on using molecular electrostatic potential surfaces to assess molecular complementarity between two cocrystal components. The screening tool was applied to two low solubility drugs, namely griseofulvin and spironolactone. Promising coformer candidates were selected from a database of 310 pharmaceutically acceptable CCFs, and experimental screening was carried out. Novel solid forms were obtained by liquid-assisted grinding and were characterised by XRPD, DSC, TGA and IR. One new cocrystal of griseofulvin and two new cocrystals of spironolactone were identified, and the crystal structures were determined from the XRPD patterns. For these systems, phenols tend to act as successful H-bond donors in forming cocrystals, while carboxylic acids only give rise to physical mixtures of the two components.

Flexible metal–organic frameworks (MOFs) undergo structural transformations in response to physical and chemical stimuli. This is hard to control because of feedback between guest uptake and host structure change. We report a family of flexible MOFs based on derivatized amino acid linkers. Their porosity consists of a one-dimensional channel connected to three peripheral pockets. This network structure amplifies small local changes in linker conformation, which are strongly coupled to the guest packing in and the shape of the peripheral pockets, to afford large changes in the global pore geometry that can, for example, segment the pore into four isolated components. The synergy among pore volume, guest packing, and linker conformation that characterizes this family of structures can be determined by the amino acid side chain, because it is repositioned by linker torsion. The resulting control optimizes noncovalent interactions to differentiate the uptake and structure response of host–guest pairs with similar chemistries.

Post-synthetic modification (PSM) of the interpenetrated diamondoid metal-organic framework (Me 2 NH 2 )[In-(BDC-NH 2 ) 2 ](BDC-NH 2 = aminobenzenedicarboxylate) SHF-61 proceeds quantitatively in a single-crystal-to-singlecrystal manner to yield the acetamide derivative (Me 2 NH 2 )-[In(BDC-NHC(O)Me) 2 ] SHF-62. Continuous breathing behaviour during activation/desolvation is retained upon PSM, but pore closing now leads to ring-flipping to avert steric clash of amide methyl groups of the modified ligands. This triggers a reduction in the amplitude of the breathing deformation in the two dimensions associated with pore diameter, but a large increase in the third dimension associated with pore length. The MOF is thereby converted from predominantly 2D breathing (in SHF-61) to a distinctly 3D breathing motion (in SHF-62) indicating a decoupling of the pore-width and pore-length breathing motions. These breathing motions have been mapped by a series of single-crystal diffraction studies.