Ross S. Forgan

Monofunctionalized Pillar[5]arene as a Host for Alkanediamines

Alkanediamines serve as neutral guests for the recently discovered host pillar[5]arene. The proposed [2]pseudorotaxane nature of the superstructure of the 1:1 host-guest complexes is supported by the template-directed synthesis of a related [2]rotaxane. A synthetic route to monofunctional pillar[5]arenes has also been developed, allowing for the creation of a fluorescent sensor for alkylamine binding. The precursors to this host could act as starting points for a large library of monofunctional pillar[5]arene macrocycles.

Strong and Reversible Binding of Carbon Dioxide in a Green Metal–Organic Framework

The efficient capture and storage of gaseous CO(2) is a pressing environmental problem. Although porous metal-organic frameworks (MOFs) have been shown to be very effective at adsorbing CO(2) selectively by dint of dipole-quadruple interactions and/or ligation to open metal sites, the gas is not usually trapped covalently. Furthermore, the vast majority of these MOFs are fabricated from nonrenewable materials, often in the presence of harmful solvents, most of which are derived from petrochemical sources. Herein we report the highly selective adsorption of CO(2) by CD-MOF-2, a recently described green MOF consisting of the renewable cyclic oligosaccharide γ-cyclodextrin and RbOH, by what is believed to be reversible carbon fixation involving carbonate formation and decomposition at room temperature. The process was monitored by solid-state (13)C NMR spectroscopy as well as colorimetrically after a pH indicator was incorporated into CD-MOF-2 to signal the formation of carbonic acid functions within the nanoporous extended framework.

Metal–Organic Frameworks from Edible Natural Products†

Metal–organic frameworks (MOFs) represent an extensive class of porous crystals in which organic struts link metalcontaining clusters. The success in controlling the functionality and structure of MOFs has led to numerous applications, most notably gas adsorption, storage of clean gas fuels, catalysis, separations, and drug delivery. However, the vast majority of MOFs described to date are composed of organic struts derived from non-renewable petrochemical feedstocks and transition metals. The challenge in preparing MOFs from natural products lies in the inherent asymmetry of the building units, which are not amenable to crystallization in the form of highly porous frameworks. Herein, we report a strategy to overcome this problem using g-cyclodextrin (g-CD), a symmetrical cyclic oligosaccharide that is mass-produced enzymatically from starch and comprised of eight asymmetric a-1,4-linked dglucopyranosyl residues. These g-CD building units are then linked by potassium ions, in aqueous media at ambient temperature and pressure, to form a body-centered cubic structure, termed CD-MOF-1, which has the empirical formula [(C48H80O40)(KOH)2]n. CD-MOFs can be prepared entirely from edible ingredients: combining food-grade g-CD with salt substitute (KCl) or potassium benzoate (food additive E212) in bottled water and Everclear grain spirit (EtOH) yields porous frameworks which constitute edible MOFs. While there have been a few reports of MOFs assembled from amino acids, nucleobases, peptides, magnesium formates, and metal glutarates, examples of these materials are not common despite the rapidly growing desire to fabricate MOFs from naturally available building blocks. We suspect that the key to our success in assembling CD-MOFs lies in the symmetric arrangement (C8) within the g-CD torus of eight asymmetric (C1) a-1,4-linked d-glucopyranosyl residues and the ready availability of g-CD as a chiral molecular building block (Figure 1). CD-MOF-1 was prepared by combining 1.0 equiv of g-CD with 8.0 equiv of KOH in aqueous solution, followed by vapor diffusion of MeOH into the solution during 2–7 days, resulting in colorless, cubic, single crystals, suitable for X-ray crystallography, in approximately 70% yield. Other CD-MOFs were readily obtained using salts of Na, Rb, and Cs, giving rise to an extensive new family of porous materials. A complete list of metal salts employed to form CD-MOFs and the full synthesis of CDMOFs are provided in Section S2 of the Supporting Information. The X-ray crystal structure of CD-MOF-1 reveals that eight-coordinate K ions not only assist in the assembly of (gCD)6 cubes (Figure 2a,b), wherein six g-CD units occupy the faces of a cube, but they also serve to link these cubes together in a three-dimensional array which extends throughout the crystal (Figure 2c). The (g-CD)6 repeating motifs adopt a body-centered cubic packing arrangement wherein each symmetrically equivalent K ion links two contiguous g-CD

Nanoporous carbohydrate metal-organic frameworks

The binding of alkali and alkaline earth metal cations by macrocyclic and diazamacrobicyclic polyethers, composed of ordered arrays of hard oxygen (and nitrogen) donor atoms, underpinned the development of host-guest supramolecular chemistry in the 1970s and 1980s. The arrangement of -OCCO- and -OCCN- chelating units in these preorganized receptors, including, but not limited to, crown ethers and cryptands, is responsible for the very high binding constants observed for their complexes with Group IA and IIA cations. The cyclodextrins (CDs), cyclic oligosaccharides derived microbiologically from starch, also display this -OCCO- bidentate motif on both their primary and secondary faces. The self-assembly, in aqueous alcohol, of infinite networks of extended structures, which have been termed CD-MOFs, wherein γ-cyclodextrin (γ-CD) is linked by coordination to Group IA and IIA metal cations to form metal-organic frameworks (MOFs), is reported. CD-MOF-1 and CD-MOF-2, prepared on the gram-scale from KOH and RbOH, respectively, form body-centered cubic arrangements of (γ-CD)(6) cubes linked by eight-coordinate alkali metal cations. These cubic CD-MOFs are (i) stable to the removal of solvents, (ii) permanently porous, with surface areas of ~1200 m(2) g(-1), and (iii) capable of storing gases and small molecules within their pores. The fact that the -OCCO- moieties of γ-CD are not prearranged in a manner conducive to encapsulating single metal cations has led to our isolating other infinite frameworks, with different topologies, from salts of Na(+), Cs(+), and Sr(2+). This lack of preorganization is expressed emphatically in the case of Cs(+), where two polymorphs assemble under identical conditions. CD-MOF-3 has the cubic topology observed for CD-MOFs 1 and 2, while CD-MOF-4 displays a channel structure wherein γ-CD tori are perfectly stacked in one dimension in a manner reminiscent of the structures of some γ-CD solvates, but with added crystal stability imparted by metal-ion coordination. These new MOFs demonstrate that the CDs can indeed function as ligands for alkali and alkaline earth metal cations in a manner similar to that found with crown ethers. These inexpensive, green, nanoporous materials exhibit absorption properties which make them realistic candidates for commercial development, not least of all because edible derivatives, fit for human consumption, can be prepared entirely from food-grade ingredients.

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.

The Dynamic Chemistry of Molecular Borromean Rings and Solomon Knots

The dynamic solution equilibria between molecular Borromean rings (BRs) and Solomon knots (SKs), assembled from transition metal-templated macrocycles, consisting of exo-bidentate bipyridyl and endo-tridentate diiminopyridyl ligands, have been examined with respect to the choice of the metal template and reaction conditions employed in the synthesis of the metalated BRs, otherwise known as Borromeates. Three new Borromeates, their syntheses templated by Cu(II), Co(II), and Mn(II), have been characterized extensively (two by X-ray crystallography) to the extent that the metal centers in the assemblies have been shown to be distanced sufficiently from each other not to communicate. The solid-state structure of the Co(II)-Borromeate reveals that six MeOH molecules, arranged in a [O--H...O] hydrogen bonded, chair-like conformation, are located within its oxophilic central cavity. When a mixture of Cu(II) and Zn(II) is used as the source of templation, there exists a dynamic equilibrium, in MeOH at room temperature, between a mixed-metal BR and a SK, from which the latter has been fractionally crystallized. By employing appropriate synthetic protocols with Zn(II) or Cd(II) as the template, significant amounts of SKs are formed alongside BRs. Modified crystallization conditions resulted in the isolation of both an all-zinc BR and an all-zinc SK, crystals of which can be separated manually, leading to the full characterization of the all-zinc SK by (1)H NMR spectroscopy and X-ray crystallography. This doubly interlocked [2]catenate has been identified retrospectively in recorded spectra, where it was attributed previously to a Borromeate with a Zn(II) cation coordinated to the oxophilic interior walls of the ensemble. Interestingly, these Zn(II)-templated assemblies do not interconvert in MeOH at room temperature, indicating the significant influence of both the metal template and solvent on the solution equilibria. It would also appear that d(10) metal ions favor SK formation-no evidence of Cu(II)-, Co(II)-, or Mn(II)-templated SKs has been found, yet a 1:0.9 ratio of BR:SK has been identified by (1)H NMR spectroscopy when Cd(II) is used as the template.

Imprinting Chemical and Responsive Micropatterns into Metal–Organic Frameworks

Wet stamping allows metal–organic framework (MOF) crystals to be imprinted with micropatterns of various organic chemicals. Printing the MOFs with photochromic molecules and pH indicators generates stimuli-responsive micropatterns which change their appearance upon contact with specific chemicals (see picture), thus reporting the environmental “status” of the crystal.

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.

Mechanostereochemistry and the mechanical bond

The chemistry of mechanically interlocked molecules (MIMs), in which two or more covalently linked components are held together by mechanical bonds, has led to the coining of the term mechanostereochemistry to describe a new field of chemistry that embraces many aspects of MIMs, including their syntheses, properties, topologies where relevant and functions where operative. During the rapid development and emergence of the field, the synthesis of MIMs has witnessed the forsaking of the early and grossly inefficient statistical approaches for template-directed protocols, aided and abetted by molecular recognition processes and the tenets of self-assembly. The resounding success of these synthetic protocols, based on templation, has facilitated the design and construction of artificial molecular switches and machines, resulting more and more in the creation of integrated functional systems. This review highlights (i) the range of template-directed synthetic methods being used currently in the preparation of MIMs; (ii) the syntheses of topologically complex knots and links in the form of stable molecular compounds; and (iii) the incorporation of bistable MIMs into many different device settings associated with surfaces, nanoparticles and solid-state materials in response to the needs of particular applications that are perceived to be fair game for mechanostereochemistry.