Peter J. Cragg

Cooperative anion binding and electrochemical sensing by modular podands

A series of podands based on two or three hydrogen bonding “arms” situated in mutually ortho, meta, or para relationships about an aryl core have been prepared, and their affinities for simple inorganic anions were measured. Of the two-arm hosts the meta compound and to a lesser extent the ortho host exhibit a cooperative anion binding effect. The two arms function essentially independently in the para derivative. The mutually meta three-arm host shows dramatically enhanced cooperative binding. Conformational changes within the meta two-arm host result in significantly enhanced electrochemical anion sensing compared with the more conformationally rigid three-arm host.

A pyrene-based fluorescent sensor for Zn2+ ions: a molecular ‘butterfly’

A simple pyrene-based triazole receptor has been synthesised and shown to self-assemble in the presence of ZnCl(2) in an exclusively 2:1 ratio, whereas a mixture of 2:1 and 1:1 ratios are observed for other Zn(2+) salts. The pyrene units are syn in orientation; this is supported by a strong excimer signal observed at 410 nm in the presence of ZnCl(2) in acetonitrile. DFT calculations and 2D NMR support the proposed structure.

Fifty years of oxacalix[3]arenes: A review

Summary Hexahomotrioxacalix[3]arenes, commonly called oxacalix[3]arenes, were first reported in 1962. Since then, their chemistry has been expanded to include numerous derivatives and complexes. This review describes the syntheses of the parent compounds, their derivatives, and their complexation behaviour towards cations. Extraction data are presented, as are crystal structures of the macrocycles and their complexes with guest species. Applications in fields as diverse as ion selective electrode modifiers, fluorescence sensors, fullerene separations and biomimetic chemistry are described.

Transmembrane ion transport by calixarenes and their derivatives

Regulation of transmembrane ion transport is a vital aspect of bioinorganic chemistry. To understand how this highly selective process occurs, how it can become impaired and how impairment may be treated, model compounds are useful tools. Several systems are presently being explored but one of the most widely applicable combines a rigid macrocycle, capable of size-based ion recognition, with membrane-spanning substituents that allow the target ions to traverse a phospholipid bilayer. The calixarene class of macrocycles is ideally suited to this task. This article sets out the biological background to transmembrane ion transport, the methods available to study the phenomenon, examples of model compounds, and proposes areas of further study.

An activated coumarin-enamine Michael acceptor for CN−

Two coumarin-enamine chemodosimeters have been synthesized in three steps. They have been shown to selectively detect the cyanide ion with a fluorescent response (t1/2 = 20 s) and a limit of detection approximately 4.2 ppb in DMSO. The X-ray crystal structure of the thermodynamically stable E-(keto) enamine isomer of one compound was obtained and exhibited offset π-stacking.

Crown ether lanthanide complexes as building blocks for luminescent ternary complexes

Lanthanide complexes of the macrocyclic ligands 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N′-diacetic acid (H2(dacda)) and 1,4,7,10-tetraoxa-13-azacyclopentadecane-13-acetic acid (H(macma)) have been isolated and characterised. The photophysical properties of the red and green emissive, Eu(III) and Tb(III), respectively, complexes have been elucidated. Upon addition of mono- or bi-dentate aromatic carboxylates or dibenzoylmethide in solutions of the Eu(III) macrocycles triggering of the lanthanide emission is observed. The formation of the ternary complex between the lanthanide macrocycle and the sensitiser is controlled by the available coordinaton sites of the lanthanide macrocycle prior to the binding event.

C-Donor lariat ether ‘scorpionates’

Structural studies on a range of Ag+ and Pb2+ complexes of N-allyl and N-butenyl lariat ethers are reported. In the case of smaller ligands with allyl side arms (L = N-allylaza-15-crown-5 and N-azaallyl-18-crown-6), coordination polymers of type [Ag(L)]+∞ exhibiting Ag+⋯π interactions are observed. Increasing the length of the side arm to N-butenylaza-18-crown-6 gives a monomeric ‘scorpionate’ species in which the Ag+ ion is simultaneously bound to the macrocyclic portion and side arm of a single ligand. NMR titration results indicate that the Ag+⋯π interactions persist in solution. In the case of Pb2+ complexes no side arm involvement is observed in any case. The novel protonated complex (H3O)(N-allylaza-18-crown-6)[Cu2I3] is also reported.

An artificial sodium ion channel from calix[4]arene in the 1,3-alternate conformation

The synthesis of a calix[4]arene with pendent polyether substituents is described. The compound was shown by NMR to bind Na+ when in the 1,3-alternate conformation and computational simulations suggest metal binding within the calixarene annulus. 1H NMR indicated that the complexed cation attracts water and the formation of a stable dihydrate was also indicated by in silico methods. Lipid bilayer experiments confirmed that the 1,3-alt calixarene functions as an artificial transmembrane ion in the presence of Na+ but not K+.

Artificial Transmembrane Channels for Sodium and Potassium

Transport of alkali metals, particularly sodium and potassium, across cell membranes is an essential function performed by special proteins that enable cells to regulate inter- and extracellular ion concentrations with exceptional selectivity. The importance of these channel-forming proteins has led to researchers emulating of their structural features: an ion-specific filter and conduction at rates up to 108 ions per second. Synthetic helical and cyclic polypeptides form channels, however, the specificity of ion transport is often low. Ion-specific macrocycles have been used as filters from which membrane-spanning derivatives have been prepared. Success has been limited as many compounds act as ion carriers rather than forming transmembrane channels. Surfactant compounds also allow ions to cross membranes but any specificity is serendipitous. Overall it seems possible to mimic either ion specificity or efficient transmembrane ion transport. The goal for the future will be to combine both characteristics in one artificial system.

Binding Properties and Molecular Mechanics Calculations of p-tert-Butylhexahomotrioxacalix[3]arene Tri(diethyl)amide with Alkali, Alkaline Earth, Transition and Heavy Metal Cations

The binding of alkali, alkaline earth, transition (Mn2 + , Fe2 + , Co2 + , Ni2 + , Cu2 + and Zn2 + ) and heavy metal (Ag+, Cd2 + , Hg2 + and Pb2 + ) cations by cone p-tert-butylhexahomotrioxacalix[3]arene tri(diethyl)amide (1b) has been assessed by extraction studies of metal picrates from water to dichloromethane and proton NMR titration experiments. Molecular modelling techniques were also employed to complement the NMR data. The results are compared to those obtained with the dihomooxacalix[4]arene (2b) and calix[4]arene (3b) derivative analogues. Triamide 1b displays a high extracting ability towards Na+, Ba2 + , Ni2 + , Co2 + , Ag+ and mainly Pb2 + (80% E). However, these values are lower than those obtained with tetraamides 2b and 3b, presumably due to the higher conformational flexibility of 1b. 1H NMR titrations indicate the formation of 1:1 complexes between 1b and all the cations studied, and also that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms.