Cryptands molecular recognition

The problem of molecular recognition has attracted biologically oriented chemists since Emil Fischer s lock-and-key theory l0). Within the last two decades, many model compounds have been developed micelle-forming detergents11, modified cyclodextrins 12), many kinds of crown-type compounds13) including podands, coronands, cryptands, and spherands. Very extensive studies using these compounds have, however, not been made from a point of view of whether or not shape similarity affects the discrimination. 

Figure 7.1.4. The scheme of formation of [2.2.2]cryptand. marked the start of molecular recognition studies. As described in Chapters 2 and 3, the Pedersen analysis was later extended by Lehn s studies of the complementarity of sizes and shapes ofthe cryptand cavities and their guests, and by Cram s preorganization studies. In general, crown ethers and cryptands exhibit analogous complexation behaviour. Thus, similarly to the former host molecules, cryptands in the free, uncomplexed state elongate the vacant cavity by rotating a methylene group inward. Thus, the N...N distance in [2.2.2]-cryptand 54 across the cavity is extended to almost 70 pm [18] whilst, in the complexed...

Since the discovery of crown ethers, cryptands, and other macrocyclic ligands by Cram, Lehn, and Pedersen, who were awarded the 1987 Nobel Prize in chemistry for their development and use of molecules with structure-specific interactions of high selectivity [1], a completely new research field was opened supramolecular chemistry [2-4-]. Since then, this research field has been extended in many fields such as molecular recognition, organic sensing, and liquid crystals. 

Wipff, G. (1992) Molecular Modeling Studies on Molecular Recognition - Crown Ethers, Cryptands and Cryptates - from Static Models in Vacuo to Dynamic Models in Solution, J. Coord. Chem. 27, 7-37 and chapter in this volume. 

Keywords Anion coordination chemistry Cryptands Ligands Metal coordination chemistry Molecular recognition Receptors Sarcophagines... 

Supramolecules containing metal-polypyridine units, especially the Ru(dpp)-based dendrimers, could be used as electron reservoirs or components of molecular-electronic devices. Supramolecules in which an electroactive M(N,N) group is attached to a receptor capable of molecular recognition (crown ethers, calixarenes, cryptands etc.) can work as electrochemical sensors. Electrochemical recognition of cations as well as anions has been reported [33-35, 257, 263]. 

As we have implied, the ability of these host molecules to bind guests is often very specific, often linked to the hydrogen-bonding ability of the host, enabling the host to pull just one molecule or ion out of a mixture. This is called molecular recognition In general, cryptands, with their well-defined 3D cavities, are better for this than monocyclic crown ethers or ether derivatives. An example is the host 30, which selectively binds the dication 31 ( = 5) rather than 31 ( = 4), and 31 n = 6) rather than 31 (n = 7). The host 32, which is water soluble, forms 1 1 complexes with neutral aromatic hydrocarbons, such as pyrene and fluoranthene. 

Cram, Donald James. (1919-2001). Awarded the Nobel Prize for chemistry, together with Lehn, in 1987 for work in elucidating mechanisms of molecular recognition, which are fundamental to enzymic catalysis, regulation, and transport. Cram also studied three-dimensional cyclic compounds that maintained a rigid structure, accepting substrates in a structurally preorganized cavity. He called these compounds cavitands, while Lehn named them cryptands. Cram was awarded a doctorate by Harvard University in 1947. 

Thermodynamic aspects of molecular recognition in solutions of crown ethers, cryptands and cyclodextrins 01MI80. 

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