Metal cations, synthetic receptors

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

Early in the development of the field, compounds were often prepared to define the limits of both the synthetic methods and the stable products that could be formed. To date, many thousands of heteromacrocycles have been prepared. The dominant application of the vast family of host or receptor molecules has been to bind or complex a guest structure. The guests can be metal cations, organic cations, neutral substrates, anions, or complementary molecules. The complexation process can be understood from the simple example of 18-crown-6 complexing K+Cl- in solution. Ignoring structural and solvation/desolvation issues, the process can be described simply as... 

Metal cations are essential in many biological processes. The functioning of the nervous system depends on the control of Na and ions, while transition metal cations are present in the active site of many enzymes often playing catalytic roles. Furthermore, the selective extraction of metal salts from aqueous systems is important for both industrial and environmental reasons. Receptors that are able to extract precious metal ions from aqueous solution or detect/remove toxic and polluting cations (such as Cs and Pb ) are highly desirable. In response to these diverse applications, the study of synthetic receptors for cationic guest species has become a well-established field with many organometallic examples to draw upon. 

The well-developed and highly adaptable synthetic chemistry of ferrocene, together with its accessible ferrocene/ ferrocenium (Fc/Fc ) redox couple has led to its abundant use in redox-active receptors. The first organometallic cation receptors to be studied were the ferrocene-modified crown ethers of Saji 1 and Beer 2-4. The electron-rich crown ethers provide the cation-binding site—they are potent ionophores for group 1 and 2 metal cations. NMR (in methanol/acetone solution) and solid-state X-ray crystal structure studies demonstrated that 2-4 bind Na, K, and Cs. The binding stoichiometry varies depending on the receptor and cation used. Both Na and form 2 1 cation receptor complexes with 3, as does Na with 2. The ability of ferrocene to swivel around the CpFeCp axis also allows the formation of a 1 1 intramolecular sandwich complex, as seen in 2 Na, 2 K, 2 Cs and 3 K. ... 

The chapter is split into two major sections, ionic recognition and molecular covalent recognition. Ionic recognition discusses synthetic receptor molecules designed to bind cations (metal, ammonium, bipyridinium ions) and anions (halide, azide, sulphate, phosphate, dicarboxylate ions etc.). The section on molecular covalent recognition describes the complexation of neutral (un-... 

The cation-TT interaction involves binding of a cation to Ji-electrons (e.g., benzene, acetylene). Early mass spectrometry and ion cyclotron resonance studies have established that alkali-metal cations (e.g., Na+) bind strongly to simple aromatics. Later, Deakyne, and Meolner showed that organic ions (e.g., alkylammoniums) display affinities for aromatics. Moreover, extensive work by Dougherty and CO workers has established models of the cation- n- interaction that have been successfully demonstrated in a variety of synthetic receptors. ... 

Free amino acids are able to coordinate transition metal cations via the carboxylate and the amino gronps. This is a well-estabUshed fact, which has been utilized for the design of synthetic receptors since a long time ago. In this regard, a number of examples can be fonnd in which metal complexes are at the core of the design of synthetic receptors for amino acids or other zwitterionic gnests... 

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