Claire Louise Hobday

Metal–organic frameworks (MOFs) of the type MIL-53(M)-fum, where M3+ cations are connected by fumarate (fum) linkers in the canonical MIL-53 topology, have been investigated as possible adsorbents due to their structural rigidity, high porosity, and biologically endogenous organic linker. Examples reported to date are limited to those comprised of p-block metal ions (e.g., Al3+, Ga3+, and In3+); d-block metal ions (e.g., Fe3+ and Sc3+) tend to form MIL-88A(M) materials with the fumarate linker. Herein, we describe the solvent-induced self-assembly of MIL-53(Sc)-fum, by immersing MIL-88A(Sc) in water or aqueous solutions of N,N-dimethylformamide (DMF). Single-crystal X-ray diffraction confirms the solvent-selective formation of two different conformational isomers of MIL-53(Sc)-fum in this unusual MOF-to-MOF synthesis, and both are found to exhibit structural breathing, in contrast to the predominantly rigid p-block metal homologues. Furthermore, we show that the extent of flexibility can be tuned by postsynthetic cluster anion substitution. Refluxing MIL-53(Sc)-fum samples in methanol results in 40–50% exchange of μ2–OH units for μ2-OCH3 groups, where the steric bulk holds the MOFs in a slightly more open pore configuration, enabling significant N2 adsorption at low pressures in contrast to the fully closed, unfunctionalized precursors. This study provides significant insight into MOF-to-MOF self-assembly, conformational isomerism in MOFs, and the effect of both metal ion and cluster anion substitution on structural flexibility, enabling careful tuning of highly porous MOFs from simple components.

The existence and significant impact of conformational isomerism in the MIL-53-fum family on material properties was demonstrated. Adsorption studies showed improved longevity and refrigeration performance for the uniquely rigid MIL-53(Ga)-fum( c ).

We show that [choline] ₂ CoCl ₄ and [choline] ₂ ZnCl ₄ show competitive reversible adiabatic temperature and entropy changes at low working pressures due to hydrogen bonding competition in the disordered orthorhombic and ordered monoclinic phases.

Traditional refrigeration methods based on vapour compression are environmentally damaging, prompting a need for greener alternatives. Hybrid ionic plastic crystals that exhibit order-disorder structural phase transitions around room temperature with colossal pressure-driven entropy and temperature changes have been identified as viable solid-state refrigerants. The dynamic disorder characteristic of plastic crystals complicates experimental structural probes and requires molecular dynamics simulations to gain insight into the disorder. Thus far however, the dynamics of the disordered phase are seldom unambiguously characterised and rarely linked to how they drive the phase transition. In this work we investigate novel choline-based plastic crystals, [choline]2CoCl4 and [choline]2ZnCl4, and through comprehensive characterisation via variable pressure-temperature diffraction and ab initio molecular dynamics simulations we show that the microscopic mechanism for the colossal entropy changes and the disorder observed can be attributed to hydrogen bonding competition in the disordered orthorhombic (P mcn) and ordered monoclinic (P 21/c) phases. Our study illustrates the potential of choline-based plastic crystals as viable candidates for barocaloric refrigeration and shows that the combination of theoretical and experimental approaches presented here can lead to an unprecedented insight into the structural dynamics which drive colossal entropy changes and competitive reversible adiabatic temperatures at low working pressures.

By virtue of their open network structures and low densities, metal–organic frameworks (MOFs) are soft materials that exhibit elastic instabilities at low applied stresses. The conventional strategy for improving elastic stability is to increase the connectivity of the underlying MOF network, which necessarily increases the material density and reduces the porosity. Here we demonstrate an alternative paradigm, whereby elastic stability is enhanced in a MOF with an aperiodic network topology. We use a combination of variable-pressure single-crystal X-ray diffraction measurements and coarse-grained lattice-dynamical calculations to interrogate the high-pressure behaviour of the topologically aperiodic system TRUMOF-1, which we compare against that of its ordered congener MOF-5. We show that the topology of the former quenches the elastic instability responsible for pressure-induced framework collapse in the latter, much as irregularity in the shapes and sizes of stones acts to prevent cooperative mechanical failure in drystone walls. Our results establish aperiodicity as a counter-intuitive design motif in engineering the mechanical properties of framework structures that is relevant to MOFs and larger-scale architectures alike.

Communications Chemistry is delighted to introduce a Collection of research works focused on the modelling and advanced characterization of framework materials. Here, the Guest Editors outline the themes within and look towards the future of the field.

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.

Data-Driven Chemistry is a course aimed at undergraduate students in chemistry with no prior knowledge of programming and programmatic data analysis. It is designed as a 10-week-long course, 1 introducing Python programming and its usage in data analysis typically required for a chemistry degree. The course consists of 10 units designed to be used in a blended learning environment of live coding and explanations, followed by a set of in-course tasks to be solved individually or through pair programming.

The high‐pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF‐62 family with the chemical composition M(im)_2−x(bim)_xis presented (M²⁺=Zn²⁺, Co²⁺; im⁻=imidazolate; bim⁻=benzimidazolate, 0.02≤x≤0.37). High‐pressure powder X‐ray diffraction shows that the materials contract reversibly from an open pore (op) to a closed pore (cp) phase under a hydrostatic pressure of up to 4000 bar. Sequentially increasing the bim⁻fraction (x) reinforces the framework, leading to an increased threshold pressure for theop‐to‐cpphase transition, while the total volume contraction across the transition decreases. Most importantly, the typical discontinuousop‐to‐cptransition (first order) changes to an unusual continuous transition (second order) forx≥0.35. This allows finetuning of the void volume and the pore size of the material continuously by adjusting the pressure, thus opening new possibilities for MOFs in pressure‐switchable devices, membranes, and actuators.