Philip A. Gale

Anion Recognition and Sensing: The State of the Art and Future Perspectives

Anion recognition chemistry has grown from its beginnings in the late 1960s with positively charged ammonium cryptand receptors for halide binding to, at the end of the millennium, a plethora of charged and neutral, cyclic and acyclic, inorganic and organic supramolecular host systems for the selective complexation, detection, and separation of anionic guest species. Solvation effects and pH values have been shown to play crucial roles in the overall anion recognition process. More recent developments include exciting advances in anion-templated syntheses and directed self-assembly, ion-pair recognition, and the function of anions in supramolecular catalysis.

Anion receptor chemistry: highlights from 2007

This critical review includes advances in anion complexation in the year 2007. The review covers anion receptors that employ amides and thioamides, pyrroles and indoles, ureas and thioureas, guanidinium, ammonium, and imidazolium groups and receptors containing hydroxyl groups. In addition, receptors containing metal ions or Lewis acids are discussed along with anion-pi interactions and the membrane transport of anionic species by synthetic transporters and channels (204 references).

Pyrrolic and polypyrrolic anion binding agents

This review traces the emergence of pyrrole-based receptors for anion recognition. It outlines how serendipitous findings that the diprotonated form of sapphyrin, a pentapyrrolic expanded porphyrin, formed a centrally-bound complex with fluoride anion made over a decade ago spawned studies of this and other expanded porphyrins as receptors, carriers, and sensors of anions. Further evolutions of the field, including in particular the finding that neutral, non-aromatic oligopyrrole macrocycles, such as the calixpyrroles and calixphyrins, can act as cheap, and easy-to-prepare anion receptors will also be highlighted, as will recent work with acyclic systems, including dipyrrolylquinoxalines (DPQs) and simple derivatives of pyrrole itself.

Anion and ion-pair receptor chemistry: highlights from 2000 and 2001

This review article highlights advances made in abiotic anion coordination chemistry in 2000 and 2001. The structure of this review is that similar to the previous reviews in this series that covered 1997, 1998 and 1999 [P.A. Gale, Coord. Chem. Rev. 199 (2000) 18 1; P.A. Gale, Coord. Chem. Rev. 213 (2001) 79]. The review also includes examples of ion-pair receptors. The first section examines anion receptors that do not contain metal ions. This is followed by a review of metal containing anion receptors in which the metal can function as: (i) a coordination site for the anion; (ii) an agent withdrawing electron density from the receptor; (iii) an organisational element in the receptor; (iv) a sensor; and (v) a co-bound guest in ion-pair receptor. Examples of the role of anions in directing the self-assembly of complex molecular architectures are presented in the final section.

Anion receptor chemistry: highlights from 1999

Abstract This review article highlights advances made in abiotic anion coordination chemistry in 1999. The structure of this review is similar to the previous review in this series that covered 1997 and 1998. The first section examines anion receptors that do not contain metal ions. This is followed by a review of metal containing anion receptors in which the metal can function as: (i) a coordination site for the anion; (ii) an organisational element in an anion receptor; and (iii) a sensor. Examples of the role of anions in directing the self-assembly of complex molecular architectures are presented in the final section.

Anion coordination and anion-directed assembly: highlights from 1997 and 1998

Abstract This review article highlights advances made in anion coordination chemistry in 1997 and 1998. The first section of the review examines anion receptors that do not contain metal ions. This is followed by a review of metal containing anion receptors in which the metal can function as (i) a coordination site for the anion; (ii) a non-coordinating reporter group that signals the presence of the anion by a perturbation of its physical properties; (iii) an element of a receptor designed to withdraw electron density from a π-electron system and so increase the affinity of a hydrophobic receptor for anions or (iv) part of a self-assembled array that is binding an anionic guest. The role of anions in directing the self-assembly of complex molecular architectures will also be examined.

Erkennung und Nachweis von Anionen: gegenwärtiger Stand und Perspektiven

Die Chemie der Anionenerkennung hat sich seit ihren Anfangen in den spaten sechziger Jahren des 20. Jahrhunderts stark entwickelt. Anfangs waren nur positiv geladene Ammonium-Cryptanden als Rezeptoren fur Halogenid-Ionen bekannt. Nun, am Ende des Millenniums, sind eine Vielzahl geladener und neutraler, cyclischer und acyclischer, anorganischer und organischer supramolekularer Wirte fur die selektive Komplexierung, den Nachweis und die Trennung anionischer Gastmolekule verfugbar. Bei der Anionenerkennung spielen Solvatationseffekte und der pH-Wert eine entscheidende Rolle. In jungerer Zeit wurden wichtige Fortschritte bei Synthesen mit anionischen Templaten, der durch Anionen gesteuerten Selbstorganisation, der Ionenpaarerkennung und der Funktion von Anionen bei der supramolekularen Katalyse erzielt.

Changing and challenging times for service crystallography

Crystallography is no longer solely the preserve of the specialist, a situation that has implications for the operation of crystallographic service facilities. This mini-review provides an overview of the challenges in operating a crystallographic facility from the perspective and experience of the UK National Crystallography Service – a modern mid-range facility. Examples of chemical research generating the greatest challenges for the modern crystallography service and the state-of-the-art tools, hardware, facilities and expertise that are required to address them are highlighted. An overview of current research trends in single crystal diffraction research, which will ensure the future development of the technique, is presented. The remit of the service crystallographer is examined, together with proposed examples of best practice.

From Anion Receptors to Transporters

Cystic fibrosis is the most well-known of a variety of diseases termed channelopathies, in which the regulation of ion transport across cell membranes is so disrupted that the threshold of a pathology is passed. The human toll exacted by these diseases has led a number of research groups, including our own, to create compounds that mediate ion transport across lipid bilayers. In this Account, we discuss three classes of synthetic compounds that were refined to bind and transport anions across lipid bilayer membranes. All of the compounds were originally designed as anion receptors, that is, species that would simply create stable complexes with anions, but were then further developed as transporters. By studying structurally simple systems and varying their properties to change the degree of preorganization, the affinity for anions, or the lipophilicity, we have begun to rationalize why particular anion transport mechanisms (cotransport or antiport processes) occur in particular cases. For example, we have studied the chloride transport properties of receptors based on the closely related structures of isophthalamide and pyridine-2,6-dicarboxamide: the central ring in each case was augmented with pendant methylimidazole groups designed to cotransport H(+) and Cl(-). We observed that the more preorganized pyridine-based receptor was the more efficient transporter, a finding replicated with a series of isophthalamides in which one contained hydroxyl groups designed to preorganize the receptor. This latter class of compound, together with the natural product prodigiosin, can transport bicarbonate (as part of a chloride/bicarbonate antiport process) across lipid bilayer membranes. We have also studied the membrane transport properties of calix[4]pyrroles. Although the parent meso-octamethylcalix[4]pyrrole functions solely as a Cs(+)/Cl(-) cotransporter, other compounds with increased anion affinities can function through an antiport process. One example is octafluoro-meso-octamethylcalix[4]pyrrole; with its electron-withdrawing substituents, it can operate through a chloride/bicarbonate antiport process. Moreover, calix[4]pyrroles with additional hydrogen bond donors can operate through a chloride/nitrate antiport process. Thus, increasing the affinity of the receptor in these cases allows the compound to transport an anion in the absence of a cation. Finally, we have studied the transport properties of simple thioureas and shown that these compounds are highly potent chloride/bicarbonate antiport agents that function at low concentrations. In contrast, the urea analogues are inactive. The higher hydrophobicity (reflected in higher values for the logarithm of the water-octanol partition constant, or log P) and lower polar surface areas of the thiourea compounds compared to their urea analogues may provide a clue to the high potency of these compounds. This observation might serve as a basis for designing future small-molecule transporters.

Synthetic indole, carbazole, biindole and indolocarbazole-based receptors : applications in anion complexation and sensing

Indole, biindole, carbazole and indolocarbazole-based receptors are rapidly emerging as an important new class of anion-binding agents. This Feature Article provides a comprehensive overview of the molecular recognition and structural chemistry of these neutral, yet highly effective, anion receptors and sensors.