Paul D. Beer

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.

Transition metal and organometallic anion complexation agents

Anions are ubiquitous species, and therefore, their sensing is of considerable interest. Anion receptors containing electrochemically active groups such as ferrocene or cobaltocenium, or optically active groups such as ruthenium(II) bipyridyl derivatives, allow the binding of anions to be detected by a physical response at the metal centre. These systems have been incorporated into various acyclic, macrocyclic and calix[4]arene frameworks, many of which include an amide hydrogen-bonding group. Anions may be recognised in a range of environmental conditions, with some receptors even being active in aqueous solution. The incorporation of new transition metal and organometallic systems has led to the development of several new strategies in anion recognition.

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.

Advances in Anion Supramolecular Chemistry: From Recognition to Chemical Applications

Since the start of this millennium, remarkable progress in the binding and sensing of anions has been taking place, driven in part by discoveries in the use of hydrogen bonding, as well as the previously under-exploited anion-π interactions and halogen bonding. However, anion supramolecular chemistry has developed substantially beyond anion recognition, and now encompasses a diverse range of disciplines. Dramatic advance has been made in the anion-templated synthesis of macrocycles and interlocked molecular architectures, while the study of transmembrane anion transporters has flourished from almost nothing into a rapidly maturing field of research. The supramolecular chemistry of anions has also found real practical use in a variety of applications such as catalysis, ion extraction, and the use of anions as stimuli for responsive chemical systems.

Electrochemical and optical sensing of anions by transition metal based receptors

Abstract This review focuses on the electrochemical and optical sensing of anionic guest species in organic and aqueous media using transition metal signalling units such as cobaltocenium, ferrocene and ruthenium (II) bipyridyl incorporated into various acyclic, macrocyclic and calix[4]arene ligand frameworks.

Anion templated assembly of mechanically interlocked structures

This tutorial review describes the evolution of the field of chemical templation, in particular, emphasising the impact its application has made to the synthesis of mechanically interlocked structures. Recent advances in the use of negatively charged template species for the synthesis of interlocked structures are detailed, with the main focus of this review describing the development of a general anion templation strategy that combines anion recognition with ion-pairing. The versatility of this methodology is demonstrated by the chloride anion templated synthesis of a series of interpenetrated pseudorotaxane, rotaxane and catenane structures. Upon template removal, the mechanically interlocked rotaxanes and catenanes are shown to bind anions within their topologically unique anion binding clefts by virtue of electrostatic and hydrogen bonding interactions, exhibiting a strong selectivity for the chloride halide anion template. The incorporation of the photo-active rhenium(I) bipyridyl signalling group into the rotaxane structural framework highlights the potential of these interlocked systems in future chemical sensor design.

Enhancement of anion recognition exhibited by a halogen-bonding rotaxane host system

We report the first use of solution-phase halogen bonding to control and facilitate the assembly of an interlocked structure through the bromide anion-templated formation of a rotaxane based upon an iodotriazolium axle. The incorporation of a halogen atom into the rotaxane host cavity dramatically improves the anion-recognition capabilities of the interlocked receptor, giving unusual iodide selectivity in a competitive aqueous medium.

Mechanisms of electrochemical recognition of cations, anions and neutral guest species by redox-active receptor molecules

Abstract This short review highlights the mechanisms involved in electrochemically sensing cationic, anionic and neutral guest species by redox-active receptors and is an update to a previously published review article (P.D. Beer, P.A. Gale, Z. Chen, Adv. Phys. Org. Chem. 31 (1998) 1). Mechanisms of redox–complexation coupling are discussed together with recent examples of redox-responsive molecular receptors from the literature that illustrate them.

Calixarene-Based Anion Receptors

The selective complexation of anions by synthetic receptors offers a great challenge to the modern chemist. The last ten years have seen a great expansion in the design of synthetic anion receptors. In particular, this review has shown that calixarenes are excellent platforms for the incorporation of numerous ligands for co-operative binding of anions. Their unique topology offers not only the potential for designed cavities, through selective functionalisation, but also the fine tuning of binding due to the hydrophobic nature of the cavity. The development of calixarene based systems has enabled greater understanding of the complex interactions involved in binding, thus offering the prospect of a number of commercial applications as extractants and sensors.

Core@shell bimetallic nanoparticle synthesis via anion coordination

Core@shell structured bimetallic nanoparticles are currently of immense interest due to their unique electronic, optical and catalytic properties. However, their synthesis is non-trivial. We report a new supramolecular route for the synthesis of core@shell nanoparticles, based on an anion coordination protocol--the first to function by binding the shell metal to the surface of the pre-formed primary metal core before reduction. The resultant gold/palladium and platinum/palladium core@shell nanoparticles have been characterized by aberration-corrected scanning transmission electron microscopy (as well as other techniques), giving striking atomic-resolution images of the core@shell architecture, and the unique catalytic properties of the structured nanoparticles have been demonstrated in a remarkable improvement of the selective production of industrially valuable chloroaniline from chloronitrobenzene.