Ph(3-n)XnSnCH2-16-crown-5 (X = F, Cl, Br, I, SCN; n = 1, 2): Intramolecular O → Sn Coordination versus Ditopic Complexation of Sodium Thiocyanate

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.

Selective sensing of a Cu(ii) ion by organotin anchored keto-enamine ligands

New organotin carboxylates [{(n-Bu)₂Sn(HL)}₂O]₂ (1), [(t-Bu)₂Sn(HL)₂] (2) and [Ph₃Sn(HL)] (3) have been synthesized and used for sensing of Cu(ii) ion. Complex 3 shows highest binding constant and better limit of detection than other complexes.

Organotin metalloligands for selective sensing of metal ions

Four new organotin carboxylates, [(t-C₄H₉)₂Sn(L₁)₂] (1), [(t-C₄H₉)₂Sn(L₂)₂] (2), [{(n-C₄H₉)₂Sn(L₁)}₂O]₂ (3) and [{(n-C₄H₉)₂Sn(L₂)}₂O]₂ (4), are reported. All complexes selectively recognize Cu(ii) and Fe(iii) ions in solution. Complex 1 was found to be the best one for sensing those ions among all complexes.

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.

Ion pair receptors

Compared with simple ion receptors, which are able to bind either a cation or an anion, ion pair receptors bearing both a cation and an anion recognition site offer the promise of binding ion pairs or pairs of ions strongly as the result of direct or indirect cooperative interactions between co-bound ions. This critical review focuses on the recent progress in the design of ion pair receptors and summarizes the various binding modes that have been used to accommodate ion pairs (110 references).