Organotin Compounds Ph2XSnCH2-[19]-crown-6 (X = Ph, F, Cl, Br, I, SCN) and Ph2ISnCH2Sn(I)PhCH2-[19]-crown-6 as Ditopic Receptors for Potassium Salts

Macrocycles as Ion Pair Receptors

Cation and anion recognition have both played central roles in the development of supramolecular chemistry. Much of the associated research has focused on the development of receptors for individual cations or anions, as well as their applications in different areas. Rarely is complexation of the counterions considered. In contrast, ion pair recognition chemistry, emerging from cation and anion coordination chemistry, is a specific research field where co-complexation of both anions and cations, so-called ion pairs, is the center of focus. Systems used for the purpose, known as ion pair receptors, are typically di- or polytopic hosts that contain recognition sites for both cations and anions and which permit the concurrent binding of multiple ions. The field of ion pair recognition has blossomed during the past decades. Several smaller reviews on the topic were published roughly 5 years ago. They provided a summary of synthetic progress and detailed the various limiting ion recognition modes displayed by both acyclic and macrocyclic ion pair receptors known at the time. The present review is designed to provide a comprehensive and up-to-date overview of the chemistry of macrocycle-based ion pair receptors. We specifically focus on the relationship between structure and ion pair recognition, as well as applications of ion pair receptors in sensor development, cation and anion extraction, ion transport, and logic gate construction.

Organotin-based receptors for anions and ion pairs

The field of anion recognition has developed into an area of tremendous significance over the recent decades due to the role of anions in biological and environmental systems, contributing significantly to the more general domain of supramolecular chemistry. So far, a number of receptors have been designed for anion recognition, synthesized and evaluated, most involving hydrogen bonding donors (urea, amide, pyrrole, imidazolium and hydroxyl groups), π-acidic aryl rings, Lewis acidic metals (boron, tin, aluminium, mercury and uranium) and positively charged polyammonium moieties. With the rapid progress in this field, the role of counterions in modulating the binding strength and selectivity of a specific ion has been recognized, leading to the design and discovery of more robust ion pair receptors. Among various recognition strategies that are presently available for anions and ion pairs, the development of Lewis acidic element-based receptors offers an attractive alternative to the hydrogen bond donor-based receptors, by which both anion and Lewis base recognition can be achieved. Consequently, researchers have focused a great deal of attention on such receptors and this sub-branch of recognition chemistry is expanding rapidly. In recent years, the desired selectivity and binding strength have been achieved for various anions by tuning the Lewis acidity through variation of substituents about the metal center. The easy access, rich molecular diversity and strong Lewis acidity of organotin compounds led to the development of organotin-based molecular receptors for anions and ion pairs. This feature article highlights the advances in the design, synthesis and applications of organotin-based receptors, mainly focusing on our group's contributions.

Interplay of Lewis acidity, intramolecular O→Sn interactions and selectivity: organotin-functionalized crown ethers as ditopic hosts for sodium and potassium halides

Not only ditopic complexation, but also selectivity has been realized by increasing the Lewis acidity of tin atom in organotin-functionalized crown ethers.

Liquid membrane transport of potassium fluoride by the organotin-based ditopic host Ph2FSnCH2SnFPh-CH2-[19]-crown-6

A novel tin-based ditopic host efficiently transports KF through an organic membrane.

Polyether complexes of groups 13 and 14

Notable aspects of the chemistry of polyether complexes of group 13 and 14 elements are reviewed.

Sensing of aqueous fluoride anions by cationic stibine-palladium complexes

Turn on the lantern! The stibine donor ligand of a cationic palladium complex acts as a Lewis acid and reacts with a fluoride anion to afford the corresponding fluorostiboranyl-palladium species (see scheme). Bindung of the fluoride anion to the antimony center induces a change in denticity of the triphosphine unit and leads to a bright-orange trigonal-bipyramidal d(8) lantern complex.

A bidentate Lewis acid with a telluronium ion as an anion-binding site

The search for receptors that can selectively capture small and potentially toxic anions in protic media has sparked a renewed interest in the synthesis and anion-binding properties of polydentate Lewis acids. Seeking new paradigms to enhance the anion affinities of such systems, we synthesized a bidentate Lewis acid that contains a boryl and a telluronium moiety as Lewis acidic sites. Anion-complexation studies indicate that this telluronium borane displays a high affinity for fluoride in methanol. Structural and computational studies show that the unusual fluoride affinity of this bidentate telluronium borane can be correlated with the formation of a B-F → Te chelate motif supported by a strong lone-pair(F) → σ*(Te-C) donor-acceptor interaction. These results, which illustrate the viability of heavier chalcogenium centres as anion-binding sites, allow us to introduce a novel strategy for the design of polydentate Lewis acids with enhanced anion affinities.