Controlling the Uptake and Release of Semiochemicals in Channel-Type Metal-Organic Frameworks Through Pore Expansion.

Semiochemicals can be used to manipulate insect behaviour for sustainable pest management strategies, but their high volatility is a major issue for their practical implementation. Inclusion of these molecules within porous materials is a potential solution to this issue, as it can allow for a slower and more controlled release. In this work, we demonstrate that a series of Zr(IV) and Al(III) metal-organic frameworks (MOFs) with channel-type pores enable controlled release of three semiochemicals over 100 days by pore size design, with the uptake and rate of release highly dependent on the pore size. Insight from grand canonical Monte Carlo simulations indicates that this is due to weaker MOF-guest interactions per guest molecule as the pore size increases. These MOFs are all stable post-release and can be reloaded to show near-identical re-release profiles. These results provide valuable insight on the diffusion behaviour of volatile guests in MOFs, and for the further development of porous materials for sustainable agriculture applications.

Identifying pathways to metal-organic framework collapse during solvent activation with molecular simulations.

Metal-organic framework (MOF) materials are a vast family of nanoporous solids with potential applications ranging from drug delivery to environmental remediation. Application of MOFs in these scenarios is hindered, however, by difficulties in MOF 'activation' after initial synthesis - removal of the synthesis solvent from the pores to make the pore space accessible - often leading to framework collapse if improperly performed. While experimental studies have correlated collapse to specific solvent properties and conditions, the mechanism of activation-collapse is currently unknown. Developing this understanding would enable researchers to create better activation protocols for MOFs, accelerating discovery and process intensification. To achieve this goal, we simulated solvent removal using grand-canonical Monte Carlo and free energy perturbation methods. By framing activation as a fluid desorption problem, we investigated activation processes in the isoreticular metal organic framework (IRMOF) family of MOFs for different solvents. We identified two pathways for solvent activation - the solvent either desorbs uniformly from each individual pore or forms coexisting phases during desorption. These mesophases in turn lead to large capillary stresses within the framework, corroborating experimental hypotheses for the cause of activation-collapse. Finally, we found that the activation energy of solvent removal increased with pore size and connectivity due to the increased stability of solvent mesophases, matching experimental findings. Using these simulations, it is possible to screen MOF activation procedures, enabling rapid identification of ideal solvents and conditions and thus enabling faster development of MOFs for practical applications.

Cisplatin uptake and release in pH sensitive zeolitic imidazole frameworks.

Cancer remains hard to treat, partially due to the non-specificity of chemotherapeutics. Metal-organic frameworks (MOFs) are promising carriers for targeted chemotherapy, yet, to date, there have been few detailed studies to systematically enhance drug loading while maintaining controlled release. In this work, we investigate which molecular simulation methods best capture the experimental uptake and release of cisplatin from UiO-66 and UiO-66(NH2). We then screen a series of biocompatible, pH-sensitive zeolitic imidazolate frameworks (ZIFs) for their ability to retain cisplatin in healthy parts of the patient and release it in the vicinity of a tumor. Pure-component GCMC simulations show that the maximum cisplatin loading depends on the pore volume. To achieve this maximum loading in the presence of water, either the pore size needs to be large enough to occupy both cisplatin and its solvation shell or the MOF-cisplatin interaction must be more favorable than the cisplatin-shell interaction. Both solvated and non-solvated simulations show that cisplatin release rates can be controlled by either decreasing the pore limiting diameters or by manipulating framework-cisplatin interaction energies to create strong, dispersed adsorption sites. The latter method is preferable if cisplatin loading is performed from solution into a pre-synthesized framework as weak interaction energies and small pore window diameters will hinder cisplatin uptake. Here, ZIF-82 is most promising. If it is possible to load cisplatin during crystallization, ZIF-11 would outcompete the other MOFs screened as cisplatin cannot pass through its pore windows; therefore, release rates would be purely driven by the pH triggered framework degradation.

Inclusion and release of ant alarm pheromones from metal-organic frameworks.

Zinc(ii) and zirconium(iv) metal-organic frameworks show uptake and slow release of the ant alarm pheromones 3-octanone and 4-methyl-3-heptanone. Inclusion of N-propyl groups on the MOFs allows for enhanced uptake and release over several months. In preliminary field trials, leaf cutting ants show normal behavioural responses to the released pheromones.

Early stages of phase selection in MOF formation observed in molecular Monte Carlo simulations.

Metal-organic frameworks (MOF) comprising metal nodes bridged by organic linkers show great promise because of their guest-specific gas sorption, separation, drug-delivery, and catalytic properties. The selection of metal node, organic linker, and synthesis conditions in principle offers engineered control over both structure and function. For MOFs to realise their potential and to become more than just promising materials, a degree of predictability in the synthesis and a better understanding of the self-assembly or initial growth processes is of paramount importance. Using cobalt succinate, a MOF that exhibits a variety of phases depending on synthesis temperature and ligand to metal ratio, as proof of concept, we present a molecular Monte Carlo approach that allows us to simulate the early stage of MOF assembly. We introduce a new Contact Cluster Monte Carlo (CCMC) algorithm which uses a system of overlapping "virtual sites" to represent the coordination environment of the cobalt and both metal-metal and metal-ligand associations. Our simulations capture the experimentally observed synthesis phase distinction in cobalt succinate at 348 K. To the best of our knowledge this is the first case in which the formation of different MOF phases as a function of composition is captured by unbiased molecular simulations. The CCMC algorithm is equally applicable to any system in which short-range attractive interactions are a dominant feature, including hydrogen-bonding networks, metal-ligand coordination networks, or the assembly of particles with "sticky" patches, such as colloidal systems or the formation of protein complexes.

Conformational isomerism controls collective flexibility in metal-organic framework DUT-8(Ni).

Metal-organic frameworks (MOFs) are coordination networks with organic ligands containing potential voids. Some MOFs show pronounced structural flexibility that may result in closing and re-opening these pores. Here, we show that collective flexibility in a MOF-DUT-8(Ni) - is controlled by conformational isomerism. DUT-8(Ni), a pillared-layer MOF with Ni2 paddle-wheels, dabco pillars and naphthalene dicarboxylate (ndc) linkers, can crystallize in many conformational isomers that depend on the orientation of the non-linear ndc linkers with respect to each other. While the open form is compatible with several of these conformations, only one of them, with alternating linker orientations, is stable as the closed form. We show, by means of first principles calculations, that in the stable closed form, the appreciable lattice strain is compensated by London-dispersion forces between the ndc linkers that arrange with maximum overlap in a stacking order similar to the stacking in graphite. We substantiate these results by well-tempered metadynamics calculations on the DFT-based Born-Oppenheimer potential energy surface, by refined X-ray diffraction data and by nitrogen adsorption data obtained by experiment and grand-canonical Monte-Carlo simulations based on the DFT-optimized and PXRD-derived geometries. While the reported origin of flexibility cannot be generalized to all flexible MOFs, it offers a rational design concept of folding mechanisms in switchable MOFs by exploitation of the stabilization effect of linker stacking in the closed form.

Tuning the Mechanical Response of Metal-Organic Frameworks by Defect Engineering.

The incorporation of defects into crystalline materials provides an important tool to fine-tune properties throughout various fields of materials science. We performed high-pressure powder X-ray diffraction experiments, varying pressures from ambient to 0.4 GPa in 0.025 GPa increments to probe the response of defective UiO-66 to hydrostatic pressure for the first time. We observe an onset of amorphization in defective UiO-66 samples around 0.2 GPa and decreasing bulk modulus as a function of defects. Intriguingly, the observed bulk moduli of defective UiO-66(Zr) samples do not correlate with defect concentration, highlighting the complexity of how defects are spatially incorporated into the framework. Our results demonstrate the large impact of point defects on the structural stability of metal-organic frameworks (MOFs) and pave the way for experiment-guided computational studies on defect engineered MOFs.

Understanding the adsorption process in ZIF-8 using high pressure crystallography and computational modelling.

Some porous crystalline solids change their structure upon guest inclusion. Unlocking the potential of these solids for a wide variety of applications requires full characterisation of the response to adsorption and the underlying framework-guest interactions. Here, we introduce an approach to understanding gas uptake in porous metal-organic frameworks (MOFs) by loading liquefied gases at GPa pressures inside the Zn-based framework ZIF-8. An integrated experimental and computational study using high-pressure crystallography, grand canonical Monte Carlo (GCMC) and periodic DFT simulations has revealed six symmetry-independent adsorption sites within the framework and a transition to a high-pressure phase. The cryogenic high-pressure loading method offers a different approach to obtaining atomistic detail on guest molecules. The GCMC simulations provide information on interaction energies of the adsorption sites allowing to classify the sites by energy. DFT calculations reveal the energy barrier of the transition to the high-pressure phase. This combination of techniques provides a holistic approach to understanding both structural and energetic changes upon adsorption in MOFs.

Modulator-Controlled Synthesis of Microporous STA-26, an Interpenetrated 8,3-Connected Zirconium MOF with the the-i Topology, and its Reversible Lattice Shift.

A fully interpenetrated 8,3-connected zirconium MOF with the the-i topology type, STA-26 (St Andrews porous material-26), has been prepared using the 4,4',4"-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoate (TMTB) tritopic linker with formic acid as a modulating agent. In the as-prepared form STA-26 possesses Im3‾ m symmetry compared with the Pm3‾ m symmetry of the non-interpenetrated analogue, NU-1200, prepared using benzoic acid as a modulator. Upon removal of residual solvent there is a shift between the interpenetrating lattices and a resultant symmetry change to Cmcm which is fully reversible. This is observed by X-ray diffraction and 13 C MAS NMR is also found to be remarkably sensitive to the structural transition. Furthermore, heating STA-26(Zr) in vacuum dehydroxylates the Zr6 nodes leaving coordinatively unsaturated Zr4+ sites, as shown by IR spectroscopy using CO and CD3 CN as probe molecules. Nitrogen adsorption at 77 K together with grand canonical Monte Carlo simulations confirms a microporous, fully interpenetrated, structure with pore volume 0.53 cm3 g-1 while CO2 adsorption at 196 K reaches 300 cm3 STP g-1 at 1 bar. While the pore volume is smaller than that of its non-interpenetrated mesoporous analogue, interpenetration makes the structure more stable to moisture adsorption and introduces shape selectivity in adsorption.

Tuning the Swing Effect by Chemical Functionalization of Zeolitic Imidazolate Frameworks.

Many zeolitic imidazolate frameworks (ZIFs) are promising candidates for use in separation technologies. Comprising large cavities interconnected by small windows they can be used, at least in principle, as molecular sieves where molecules smaller than the window size are able to diffuse into the material while larger molecules are rejected. However, "swing effect" or "gate opening" phenomena resulting in an enlargement of the windows have proven to be detrimental. Here, we present the first systematic experimental and computational study of the effect of chemical functionalization of the imidazole linker on the framework dynamics. Using high-pressure (HP) single-crystal X-ray diffraction, density functional theory, and grand canonical Monte Carlo simulations, we show that in the isostructural ZIF-8, ZIF-90, and ZIF-65 functional groups of increasing polarity (-CH3, -CHO, and -NO2) on the imidazole linkers provide control over the degree of rotation and thus the critical window diameter. On application of pressure, the substituted imidazolate rings rotate, resulting in an increase in both pore volume and content. Our results show that the interplay between the guest molecules and the chemical function of the imidazole linker is essential for directing the swing effect in ZIF frameworks and therefore the adsorption performance.

Ultra-large supramolecular coordination cages composed of endohedral Archimedean and Platonic bodies.

Pioneered by Lehn, Cram, Peterson and Breslow, supramolecular chemistry concepts have evolved providing fundamental knowledge of the relationships between the structures and reactivities of organized molecules. A particular fascinating class of metallo-supramolecular molecules are hollow coordination cages that provide cavities of molecular dimensions promoting applications in diverse areas including catalysis, enzyme mimetics and material science. Here we report the synthesis of coordination cages with exceptional cross-sectional diameters that are composed of multiple sub-cages providing numerous distinctive binding sites through labile coordination solvent molecules. The building principles, involving Archimedean and Platonic bodies, renders these supramolecular keplerates as a class of cages whose composition and topological aspects compare to characteristics of edge-transitive {Cu2} MOFs with A3X4 stoichiometry. The nature of the cavities in these double-shell metal-organic polyhedra and their inner/outer binding sites provide perspectives for post-synthetic functionalizations, separations and catalysis. Transmission electron microscopy studies demonstrate that single molecules are experimentally accessible.

A Computational and Experimental Approach Linking Disorder, High-Pressure Behavior, and Mechanical Properties in UiO Frameworks.

Whilst many metal-organic frameworks possess the chemical stability needed to be used as functional materials, they often lack the physical strength required for industrial applications. Herein, we have investigated the mechanical properties of two UiO-topology Zr-MOFs, the planar UiO-67 ([Zr6O4(OH)4 (bpdc)6], bpdc: 4,4'-biphenyl dicarboxylate) and UiO-abdc ([Zr6O4(OH)4 (abdc)6], abdc: 4,4'-azobenzene dicarboxylate) by single-crystal nanoindentation, high-pressure X-ray diffraction, density functional theory calculations, and first-principles molecular dynamics. On increasing pressure, both UiO-67 and UiO-abdc were found to be incompressible when filled with methanol molecules within a diamond anvil cell. Stabilization in both cases is attributed to dynamical linker disorder. The diazo-linker of UiO-abdc possesses local site disorder, which, in conjunction with its longer nature, also decreases the capacity of the framework to compress and stabilizes it against direct compression, compared to UiO-67, characterized by a large elastic modulus. The use of non-linear linkers in the synthesis of UiO-MOFs therefore creates MOFs that have more rigid mechanical properties over a larger pressure range.

Stabilization of scandium terephthalate MOFs against reversible amorphization and structural phase transition by guest uptake at extreme pressure.

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.

Polymorphism of metal-organic frameworks: direct comparison of structures and theoretical N2-uptake of topological pto- and tbo-isomers.

The syntheses, calculated surface areas and N2 uptakes of two highly augmented {Cu}2 'paddlewheel'-based MOFs provide a direct comparison of tbo and pto framework polymorphs with identical composition.

Hetero-epitaxial approach by using labile coordination sites to prepare catenated metal-organic frameworks with high surface areas.

A solid-state approach that takes advantage of the ordered 3D arrangement of active secondary building units allows the preparation of new interlocked MOFs that grow hetero-epitaxially on the crystal faces of a precursor phase that acts as a "topological blueprint". The synthetic strategy is exemplified by using rigid acetylene-based ligands to produce highly augmented Cu(II) acetate-based MOFs.

Elucidating the breathing of the metal-organic framework MIL-53(Sc) with ab initio molecular dynamics simulations and in situ X-ray powder diffraction experiments.

Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal-organic framework (MOF) MIL-53(Sc) in response to changes in temperature over the range 100-623 K and adsorption of CO2 at 0-0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory accurately simulated the experimentally observed closure of MIL-53(Sc) upon solvent removal and the transition of the empty MOF from the closed-pore phase to the very-narrow-pore phase (symmetry change from P2(1)/c to C2/c) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations to mimic the CO2 adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc) framework to evolve freely in response to CO2 loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO2-containing intermediate and large-pore phases observed by experimental synchrotron X-ray diffraction studies with increasing CO2 pressure; this would not have been possible for the intermediate structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the intermediate structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53(Sc) upon temperature variation and CO2 adsorption. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids.

Flexibility and swing effect on the adsorption of energy-related gases on ZIF-8: combined experimental and simulation study.

ZIF-8, a prototypical zeolitic porous coordination polymer, prepared via the self-assembly of tetrahedral atoms (e.g. Zn and Co) and organic imidazolate linkers, presents large cavities which are interconnected by narrow windows that allow, in principle, molecular sieving. However, ZIF-8 shows flexibility due to the swing of the imidazolate linkers, which results in the adsorption of molecules which are too large to fit through the narrow window. In this work, we assess the impact of this flexibility, previously only observed for nitrogen, and the level of agreement between the experimental and simulated isotherms of different energy-related gases on ZIF-8 (CO(2), CH(4) and alkanes). We combine experimental gas adsorption with GCMC simulations, using generic and adjusted force fields and DFT calculations with the Grimme dispersion correction. By solely adapting the UFF force field to reduce the Lennard-Jones parameter ε, we achieve excellent agreement between the simulated and experimental results not only for ZIF-8 but also for ZIF-20, where the transferability of the adapted force field is successfully tested. Regarding ZIF-8, we show that two different structural configurations are needed to properly describe the adsorption performance of this material, demonstrating that ZIF-8 is undergoing a structural change during gas adsorption. DFT calculations with the Grimme dispersion correction are consistent with the GCMC and experimental observations, illustrating the thermodynamics of the CH(4) adsorption sites and confirming the existence of a new adsorption site with a high binding energy within the 4-ring window of ZIF-8.

Calix[4]arene-based metal-organic frameworks: towards hierarchically porous materials.

An upper rim-functionalised calix[4]arene dicarboxylic acid (H(2)caldc) has been used to prepare four metal-organic frameworks, three of which have been structurally characterised and shown to form two- or three-dimensional network structures. Simulations suggest that such networks are likely to display interesting selectivity to guest molecules.

Hydrogen thermal desorption spectra: insights from molecular simulation.

Thermal desorption spectra of a number of metal-organic frameworks were studied using grand canonical Monte Carlo simulation. Our simulation results are in qualitative agreement with experimental results but also show that great care must be taken when choosing the force field to describe the hydrogen/framework interaction. As the simulations additionally yield the positions and potential energies of the adsorbed molecules it is straightforward to assign the peaks and features in the thermal desorption spectra to specific adsorption sites. We show that the location of the peaks is directly related to the hydrogen-framework interaction which is a complex function of the chemical and topological environment of the pore space, the pore size and the presence of specific interaction sites such as open metal sites. Finally, we demonstrate that an IRMOF-8 sample used to obtain an experimental thermal desorption spectrum must have indeed been catenated as previously suspected. Overall, molecular simulation is a useful tool to complement the interpretation of experimental thermal desorption spectra.

A novel structural form of MIL-53 observed for the scandium analogue and its response to temperature variation and CO2 adsorption.

The scandium analogue of the flexible terephthalate MIL-53 yields a novel closed pore structure upon removal of guest molecules which has unusual thermal behaviour and stepwise opening during CO(2) adsorption. By contrast, the nitro-functionalised MIL-53(Sc) cannot fully close and the structure possesses permanent porosity for CO(2).