Ross J. Marshall

3D Printed High‐Throughput Hydrothermal Reactionware for Discovery, Optimization, and Scale‐Up

3D printing techniques allow the laboratory-scale design and production of reactionware tailored to specific experimental requirements. To increase the range and versatility of reactionware devices, sealed, monolithic reactors suitable for use in hydrothermal synthesis have been digitally designed and realized. The fabrication process allows the introduction of reaction mixtures directly into the reactors during the production, and also enables the manufacture of devices of varying scales and geometries unavailable in traditional equipment. The utility of these devices is shown by the use of 3D printed, high-throughput array reactors to discover two new coordination polymers, optimize the synthesis of one of these, and scale-up its synthesis using larger reactors produced on the same 3D printer. Reactors were also used to produce phase-pure samples of coordination polymers MIL-96 and HKUST-1, in yields comparable to synthesis in traditional apparatus.

Single-Crystal to Single-Crystal Mechanical Contraction of Metal–Organic Frameworks through Stereoselective Postsynthetic Bromination

The properties of metal-organic frameworks (MOFs) can be tuned by postsynthetic modification (PSM) to introduce specific functionalities after their synthesis. Typically, PSM is carried out on pendant functional groups or through metal/ligand exchange, preserving the structure of the MOF. We report herein the bromination of integral alkyne units in a pair of Zr(4+) and Hf(4+) MOFs, which proceeds stereoselectively in a single-crystal to single-crystal manner. The chemical and mechanical changes in the MOFs are extensively characterized, including the crystal structures of the postsynthetically brominated materials, which show a mechanical contraction of up to 3.7% in volume. The combination of stability and chemical reactivity in these MOFs leads to the possibility of tuning mechanical properties by chemical transformation while also opening up new routes to internal pore functionalization.

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

Abstract 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.

Amino acids as highly efficient modulators for single crystals of zirconium and hafnium metal–organic frameworks

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.

Drug delivery and controlled release from biocompatible metal–organic frameworks using mechanical amorphization

We have used a family of Zr-based metal-organic frameworks (MOFs) with different functionalized (bromo, nitro and amino) and extended linkers for drug delivery. We loaded the materials with the fluorescent model molecule calcein and the anticancer drug α-cyano-4-hydroxycinnamic acid (α-CHC), and consequently performed a mechanical amorphization process to attempt to control the delivery of guest molecules. Our analysis revealed that the loading values of both molecules were higher for the MOFs containing unfunctionalized linkers. Confocal microscopy showed that all the materials were able to penetrate into cells, and the therapeutic effect of α-CHC on HeLa cells was enhanced when loaded (20 wt%) into the MOF with the longest linker. On one hand, calcein release required up to 3 days from the crystalline form for all the materials. On the other hand, the amorphous counterparts containing the bromo and nitro functional groups released only a fraction of the total loaded amount, and in the case of the amino-MOF a slow and progressive release was successfully achieved for 15 days. In the case of the materials loaded with α-CHC, no difference was observed between the crystalline and amorphous form of the materials. These results highlight the necessity of a balance between the pore size of the materials and the size of the guest molecules to accomplish a successful and efficient sustained release using this mechanical ball-milling process. Additionally, the endocytic pathway used by cells to internalize these MOFs may lead to diverse final cellular locations and consequently, different therapeutic effects. Understanding these cellular mechanisms will drive the design of more effective MOFs for drug delivery applications.

Structure-directing factors when introducing hydrogen bond functionality to metal–organic frameworks

The introduction of H-bond donor/acceptor functionality into metal–organic frameworks (MOFs) can have a beneficial effect on their molecular recognition, uptake selectivity and catalytic properties. The changes in ligand geometry induced by incorporation of functional groups may also affect the topology and composition of the resultant MOFs. Herein, we present a comprehensive study of functional group incorporation into MOFs, linked by either Zn2+ paddlewheel units or monomeric Zn2+ corners, which exhibit pcu and dia topology, respectively. Crystallographic analysis shows that amide groups can be easily incorporated into isoreticular pcu pillared-MOFs, whilst integration of urea units results in materials with dia topology. Molecular simulations allow the examination of hypothetical structures with differing constitutions and topologies, and highlight the influence of the urea units in generating the experimentally observed topologies. Noncovalent interactions between independent nets may be significant structure-directing influences, a finding which has great implications for the design of MOFs containing more complex functional groups.

Stereoselective halogenation of integral unsaturated C-C bonds in chemically and mechanically robust Zr and Hf MOFs

Abstract Metal–organic frameworks (MOFs) containing ZrIV‐based secondary building units (SBUs), as in the UiO‐66 series, are receiving widespread research interest due to their enhanced chemical and mechanical stabilities. We report the synthesis and extensive characterisation, as both bulk microcrystalline and single crystal forms, of extended UiO‐66 (Zr and Hf) series MOFs containing integral unsaturated alkene, alkyne and butadiyne units, which serve as reactive sites for postsynthetic modification (PSM) by halogenation. The water stability of a Zr–stilbene MOF allows the dual insertion of both −OH and −Br groups in a single, aqueous bromohydrination step. Quantitative bromination of alkyne‐ and butadiyne‐containing MOFs is demonstrated to be stereoselective, as a consequence of the linker geometry when bound in the MOFs, while the inherent change in hybridisation and geometry of integral linker atoms is facilitated by the high mechanical stabilities of the MOFs, allowing bromination to be characterised in a single‐crystal to single‐crystal (SCSC) manner. The facile addition of bromine across the unsaturated C−C bonds in the MOFs in solution is extended to irreversible iodine sequestration in the vapour phase. A large‐pore interpenetrated Zr MOF demonstrates an I2 storage capacity of 279 % w/w, through a combination of chemisorption and physisorption, which is comparable to the highest reported capacities of benchmark iodine storage materials for radioactive I2 sequestration. We expect this facile PSM process to not only allow trapping of toxic vapours, but also modulate the mechanical properties of the MOFs.

Tuning the Endocytosis Mechanism of Zr-Based Metal–Organic Frameworks through Linker Functionalization

A critical bottleneck for the use of metal–organic frameworks (MOFs) as drug delivery systems has been allowing them to reach their intracellular targets without being degraded in the acidic environment of the lysosomes. Cells take up particles by endocytosis through multiple biochemical pathways, and the fate of these particles depends on these routes of entry. Here, we show the effect of functional group incorporation into a series of Zr-based MOFs on their endocytosis mechanisms, allowing us to design an efficient drug delivery system. In particular, naphthalene-2,6-dicarboxylic acid and 4,4′-biphenyldicarboxylic acid ligands promote entry through the caveolin-pathway, allowing the particles to avoid lysosomal degradation and be delivered into the cytosol and enhancing their therapeutic activity when loaded with drugs.

Crystallographic investigation into the self-assembly, guest binding, and flexibility of urea functionalised metal-organic frameworks

Abstract Introduction of hydrogen bond functionality into metal-organic frameworks can enhance guest binding and activation, but a combination of linker flexibility and interligand hydrogen bonding often results in the generation of unwanted structures where the functionality is masked. Herein, we describe the self-assembly of three materials, where Cd2+, Ca2+, and Zn2+ are linked by N,Nʹ-bis(4-carboxyphenyl)urea, and examine the effect of the urea units on structure formation, the generation of unusual secondary building units, structural flexibility, and guest binding. The flexibility of the Zn MOF is probed through single-crystal to single-crystal transformations upon exchange of DMF guests for CS2, showing that the lability of the [Zn4O(RCO2)6] cluster towards solvation enables the urea linkers to adopt distorted conformations as the MOF breathes, even facilitating rotation from the trans/trans to the trans/cis conformation without compromising the overall topology. The results have significant implications in the mechanistic understanding of the hydrolytic stability of MOFs, and in preparing heterogeneous organocatalysts.

Controlling interpenetration through linker conformation in the modulated synthesis of Sc metal–organic frameworks

Interpenetration in metal–organic frameworks (MOFs), where multiple nets of metal ions or clusters linked by organic ligands are nested within each others pore spaces, affects important physical properties such as stability and gas uptake, and can be controlled through ligand sterics and modifying synthetic conditions. Herein, we extend the use of coordination modulation – deliberate addition of competing monotopic ligands to syntheses – to prepare Sc MOFs containing related biphenyl-4,4′-dicarboxylate (bpdc) and 2,2′-bipyridine-5,5′-dicarboxylate (bpydc) linkers. The Sc-bpdc MOF adopts a two-fold interpenetrated structure, however, the Sc-bpydc MOF is non-interpenetrated, despite only minor electronic modifications to the ligand. A comprehensive experimental and theoretical examination reveals that ligand twisting (energetically favourable for bpdc but not bpydc) and associated π-stacking interactions are a prerequisite for interpenetration. The more rigid Sc-bpdc is susceptible to modulation, resulting in differences in morphology, thermal stability and the synthesis of a highly defective, acetate-capped mesoporous material, while the large pore volume of Sc-bpydc allows postsynthetic metallation with CuCl2 in a single-crystal to single-crystal manner. Controlling interpenetration through linker conformation could result in design of new materials with desirable properties such as bifunctional solid-state catalysts.