Gareth O. Lloyd

Metal- and Anion-Binding Supramolecular Gels

1. Importance and Scope of Supramolecular Gels 1960 2. Gel Characterization 1963 2.1. General Considerations 1963 2.2. Rheology 1964 2.3. Models for Gel Rheology 1965 3. Concepts of Gel Formation 1966 4. Metals in Gels 1967 4.1. Metals as Structural Components in Gels 1967 4.2. Discrete Metal Complex Gelators 1973 5. Metal Ion Binding by Gels and Gelators 1978 5.1. Coordination Polymers as Metallogels 1979 5.2. Tuning Gel Properties by Ion Binding 1986 5.3. Salts as Gels and Tuning Agents 1993 6. Anion Binding by Gels and Gelators 1994 7. Gels as Templates 2000 8. Conclusions 2001 9. Acknowledgments 2001 10. References 2001

Anion-tuning of supramolecular gel properties

The study of supramolecular gels has developed into a well-recognised field of materials science, pertaining to the general area of soft matter. The use of small molecules that aggregate through supramolecular interactions (such as hydrogen bonds, π–π interactions, coordination bonds and van der Waals interactions) has given materials scientists an alternative to polymeric compounds for the development of practical gels. There have been further attempts to functionalize, activate or control the physical properties of such gels by means of the reversibility of the interactions between the component molecules. Tuning of these characteristics has been accomplished by using mechanical, thermal, electrochemical, electromagnetic and chemical stimuli. The use of anions as a chemical stimulus has been a recent development and is the subject of this Perspective. Small anions can be used to modulate the physical properties of supramolecular gels by interacting with the low-molecular-weight gelators from which such materials are composed. A better understanding of this anion-tuning effect will aid in the rational design of responsive gels that may prove useful for a number of practical applications.

A Cocrystal Strategy to Tune the Luminescent Properties of Stilbene-Type Organic Solid-State Materials†

The one- and two-photon luminescence of stilbene-type solid-state materials can be tuned and controlled from blue to yellow color by a supramolecular cocrystal method.

Anion-switchable supramolecular gels for controlling pharmaceutical crystal growth

We describe the use of low-molecular-weight supramolecular gels as media for the growth of molecular crystals. Growth of a range of crystals of organic compounds, including pharmaceuticals, was achieved in bis(urea) gels. Low-molecular-weight supramolecular gelators allow access to an unlimited range of solvent systems, in contrast to conventional aqueous gels such as gelatin and agarose. A detailed study of carbamazepine crystal growth in four different bis(urea) gelators, including a metallogelator, is reported. The crystallization of a range of other drug substances, namely sparfloxacin, piroxicam, theophylline, caffeine, ibuprofen, acetaminophen (paracetamol), sulindac and indomethacin, was also achieved in supramolecular gel media without co-crystal formation. In many cases, crystals can be conveniently recovered from the gels by using supramolecular anion-triggered gel dissolution; however, crystals of substances that themselves bind to anions are dissolved by them. Overall, supramolecular gel-phase crystallization offers an extremely versatile new tool in pharmaceutical polymorph screening. Supramolecular gels based on small-molecule gelators have been shown to be effective media for the growth of organic crystals, including pharmaceutical compounds. Moreover, the gel-to-sol transition can be triggered by molecular recognition with anions, thereby enabling facile recovery of the crystals.

Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers

Herein we report the photocontrol of cucurbit[8]uril (CB[8])-mediated supramolecular polymerization of azobenzene-containing monomers. The CB[8] polymers were characterized both in solution and in the solid state. These host-guest complexes can be reversibly switched between highly thermostable photostationary states. Moreover, a remarkable stabilization of Z-azobenzene was achieved by CB[8] complexation, allowing for structural characterization in the solid state.

Gelation is crucially dependent on functional group orientation and may be tuned by anion binding

The gelation ability of a series of chiral bis(urea) gels alternates between even and odd chain length and for the even numbered spacers the rheological characteristics can be tuned by the addition of anions according to the anion binding constant.

On crystal versus fiber Formation in dipeptide hydrogelator systems

Naphthalene dipeptides have been shown to be useful low-molecular-weight gelators. Here we have used a library to explore the relationship between the dipeptide sequence and the hydrogelation efficiency. A number of the naphthalene dipeptides are crystallizable from water, enabling us to investigate the comparison between the gel/fiber phase and the crystal phase. We succeeded in crystallizing one example directly from the gel phase. Using X-ray crystallography, molecular modeling, and X-ray fiber diffraction, we show that the molecular packing of this crystal structure differs from the structure of the gel/fiber phase. Although the crystal structures may provide important insights into stabilizing interactions, our analysis indicates a rearrangement of structural packing within the fibers. These observations are consistent with the fibrillar interactions and interatomic separations promoting 1D assembly whereas in the crystals the peptides are aligned along multiple axes, allowing 3D growth. This observation has an impact on the use of crystal structures to determine supramolecular synthons for gelators.

Monofunctionalised cucurbit[6]uril synthesis using imidazolium host–guest complexation

Monohydroxylated cucurbit[6]uril was prepared for the first time through the controlled oxidation of CB[6] in the presence of a tailor-made bisimidazolium guest, as verified by (1)H NMR, ESI-MS and X-ray crystallography. Further chemical modification of monohydroxylated CB[6] was also readily achieved.

Metal-induced gelation in dipyridyl ureas

A series of pyridyl-appended bis(urea) ligands form supramolecular gels in the presence of metal ions (metallogels), particularly copper(II) and silver(I). The gels have been characterised by rheometry and SEM, and the effect of the metal ions on gel strength and morphology examined. The metal-induced gelation is linked to the competition between urea–urea and urea–pyridyl hydrogen bonding interactions. Crystals grown from these gels reveal a wealth of structural information about these systems that can be related to gel structure using powder X-ray diffraction data of their xerogels.

A “Compartmental” Borromean Weave Coordination Polymer Exhibiting Saturated Hydrogen Bonding to Anions and Water Cluster Inclusion

been several recent reports of Borromean coordination polymers, with previously unrecognised examples being correctly classified by Carlucci et al. while Stoddart and coworkers have synthesized a discrete Borromeate complex. Much of the interest in coordination polymer chemistry stems from the creation of robust porous frameworks capable of containing guest molecules. Topological connectivity such as interpenetration and the formation of Borromean linkages potentially represents a way of increasing the mechanical strength of these materials since the rings cannot be separated without breaking generally strong chemical bonds. While Borromean linkages or interpenetration can reduce the available free space within a coordination polymer or metal–organic framework (MOF), it increases the surface area of the framework thereby promoting the surface adsorption of gases, for example. Considerable effort has been expended to prevent interpenetration in MOFs, with the use of the bulky secondary building unit (SBU) concept borrowed from zeolite chemistry being particularly effective. The tendency to fill space either by guest inclusion or interpenetration/interlinking arises from the nondirectional stabilizing influence of van der Waals forces. We have been working on a concept of “compartmentalized” coordination polymers in which van der Waals interactions between components of the coordination polymer framework are replaced by a cascade of more specific, directional interactions that limit the ways in which the coordination polymer strands can interact. More generally, we have shown that such a strategy can be used to engineer the formation of discrete water clusters. In the particular case of coordination polymers, we aim to engineer the creation of materials with a high surface area that contain void space for guest inclusion. We now report such a compartmental coordination polymer in which a Borromean structure forms. The rings are held strictly in register by coordination interactions, argentophilic interactions, and saturated hydrogen bonding to nitrate counterions. We (and others) have designed ligands of type L (Scheme 1) to contain well-defined, divergent cation-binding,