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SYNTHESIS AND APPLICATIONS OF CRYPTANDS Shobhana K. Menon*, Sanjay V. Hirpara and Uma Harikrishnan Chemistry Department, School of Sciences, Gujarat University, Ahmedabad, India-380 009. ABSTRACT Cryptands find applications in many fascinating areas of chemistry, biochemistry and material science due to their architectural and functional plasticity. This review discusses in detail different methods of synthesis of the recently reported cryptands and their applications, viz. in the recognition of cations, anions and neutral molecules, as carriers in ion-transportation, as stationary phases in column chromatographic separations, as redox active material, in photophysical studies and in non-linear optics, as dopant in sol- gels and finally as structure-directing agents in synthesis. INTRODUCTION The early chemists tried to understand nature at a level that was purely molecular; they considered only structures and functions involving strong covalent bonds; the linkages that connect atoms to form molecules. But, with the understanding of the lock-key mechanism of enzyme action1, it was discovered that most of the biological structures are usually made from loose aggregates that are held together by weak, non-covalent interactions and are responsible for most of the processes occurring in living systems. The slow shift towards this new approach of structures held by non- covalent interactions became a new subject, "supramolecular chemistry". The term supramolecular chemistry was coined by Jean-Marie Lehn in his study of inclusion compounds and cryptands2. Lehn defined supramolecular chemistry as "The chemistry of the intermolecular bond." Just as molecules are built by connecting atoms with covalent bonds, supramolecular 233 Vol. 23, No. 4, 2004 Synthesis and Applications of Cryptands compounds arc built by linking molecules with intermolecular interactions, like vanderwaals forces, hydrogen bonding etc. Supramolecular structures are the result of not only additive but also co-operative interactions, and their properties generally follow from their supramolecular character. Their properties are important in both material sciences3 (magnetism, conductivity, sensors, nonlinear optics) and biology4 (receptor protein binding, drug design, protein folding). Cyclodextrins, crown ethers, cryptands, calixarenes, rotaxanes etc. are some of the important macrocyclic compounds studied for their supramolecular functions. Of these, crowns and cryptands form a very interesting and important class of macroheterocycles which have been widely studied due to their increasing use in various improbable chemical and physical processes. Crown ethers are a group of macroheterocyclic polyethers in which the ethereal Ό' atoms are separated by methylene (-CH2) groups. On the other hand, cryptands are bicyclic or oligocyclic macroheterocycles, which are considered to be three-dimensional analogs of crown ethers. They arc appropriately cross-linked with donor atoms correctly positioned in the bridging groups to encapsulate metal ions in cage-like structures. They are more potent, selective and even stronger complexing agents than crowns. Due to their architectural and functional plasticity, cryptands have a wide range of applications not only in co-ordination chemistry5"7 but also as ion- selective electrodes8 as molecular switches9, in chromatographic columns111"12 etc. Several reviews and books have been published on cryptands and their applications. Some of the reviews published during the last five years are listed below: 1) M. Formica11, V. Fusi, M. Micheloni, R. Pontellini, P. Romani, "Cryptand ligands for selective lithium coordination", 1999. The binding properties, in aqueous solution, towards lithium ion of aza- and azaoxo- macrocycles with cage and cyclindrical molecular topology have been reviewed. 2) H. Takshi14, T. Norio, K. Toshiharu, N. Shigeo, Y. Miromaso, N. Hiroshi, "Chromoionophores based on crown ethers and related structures for alkali metal ion sensing in aqueous media", 1999. This review covers the recent progress in chromoionophores based on spherands, hemi- spherands, cryptands and crowns which selectively respond to alkali metal ions in aqueous media depending on their molecular structure and photometric function. 234 S.K. Menon, S.V. Hirpara & U. Harikrishnan Reviews in Analytical Chemistry 3) X. Zhong'\ R. Μ. Izatt, J. S. Bradshaw, Κ. E. Krakowiak, "Approaches to improvement of metal ion selectivity by cryptands", 1999. A review covering the factors influencing metal ion selectivity by cryptands having only Ο and Ν donor atoms. 4) N. G. Lukyanenko"', "New methods and approaches to the effective synthesis of crown ethers and cryptands", 2000, A review on the effective synthesis of crown ethers, cryptands, hemispherands and crypto hemispherands. 5) Κ. E. Kralowiak17, R. M. Izatt, J. S. Bradshaw, "One step synthesis of macrocyclic compounds: a short review", 2001. This short review covers one-step cyclization reactions involving the formation of four to twenty covalent bonds. The new macrocycles include cyclophanes, biscrown ethers, cryptands, calixarenes and super cryptands etc 6) V. Amemdala1*, L. Fabbrizzi, C. Manzano, P. Pallavicini, A. Passi, A. Taglietti, "Anionic recognition by dimetallic cryptates", 2002. This review covers studies on bis-tren cryptands able to incorporate two metal ions and then an ambident anion. 7) B. Sarkar1', P. Mokhopadhyay, P. K. Bharadwaj, "Laterally non- symmetric aza-cryptand: synthesis, catalysis and derivatization to new receptors", 2003. This review covers the synthesis of laterally non- symmetric aza-cryptands and use of their metal cryptates in homogenous catalysis, in the photochemical splitting of water to generate H2, in the cleavage of nucleic acids as chemical nucleases. 8) V. Mckee20, J. Nelson and R. M. Town, "Caged Oxoanions", 2003. The association between azacryptand hosts and oxoanion guests has been reviewed along with studies on charge based selectivity and pH dependence. 9) A. Y. Tsivadzu21, "Crown compounds : Selective receptors of ions", 2003. A review on the main classes of crown compounds including cryptands, cavitands, spherands, hemispherands, calixarenes, podands etc,. Their physico chemical properties, structural features, selectivity, and complex formation properties are analysed. In addition, a few books have been published22 2s which discuss the basic principles of synthesis and applications of crowns, cryptands, calixarenes and cyclodextrins. The present review covers the most recent work reported on the synthesis and applications of cryptands which are not discussed in the earlier reviews. 235 Vol. 23, No. 4, 2004 Synthesis and Applications of Cryptands SYNTHESIS OF CRYPTANDS A number of approaches have been made and a large number of cryptands reported in the literature. The methods commonly employed for the synthesis of cryptands are: High dilution technique This is the most extensively used technique and is accomplished by addition of reactants into a large amount of solvents2''"28. The rationale for this approach is that in dilute solutions the formation of a cyclic product by intramolecular reaction (i.e. one end of a molecule bumping into itself) is more likely and hence faster, than the formation of a polymer, which requires a collision between two separate reactants (an intermolecular reaction). The method considerably reduces the scope of forming undesired side products but takes a long time for completion of reaction. Template synthesis A template organizes the respective reactants to achieve a particular linking to form the desired compound2'"10. The template effect of a metal ion can be of kinetic or of thermodynamic origin or a combination of both. When the kinetic template effect is operative, the geometrical arrangement of ligands within the co-ordination sphere of the templating ion provides constraints that can be used to control the structure of a product formed from the reactions of coordinated ligands. The thermodynamic template effect on the other hand acts to stabilize a ligand structure that might otherwise be disfavored in the pure organic chemical system at equilibrium. Rigid group principle Macrocycles are formed from acyclic precursors which can rotate about their bonds and thus effectively lower the possibility of reactive groups coming in proximity to yield the desired macrocycle31. To lower the conformational mobility of an acyclic compound, rigidity may be increased in the structure by incorporating aromatic groups or unsaturation to aid in pre-organizing the reactive fragments to react in the directed fashion, thereby reducing the possibility of oligomerization. 236 S.K. Menon, S.V. Hirpara & U. Harikrishnan Reviews in Analytical Chemistry Low temperature synthesis At low temperature movement of the reactive groups slows down; this minimizes polymerization to a great extent, simultaneously facilitating macrocyclization even if there is no templating ion used32. RECENT TRENDS A survey of the recent literature on the synthesis of cryptands reveals that high dilution or the template route or a combination of both are still an option for cryptand synthesis. Many different modifications can be done on cryptands such as changing the ring size, kind of substituents, the types of donor atoms and the introduction of different functional groups. A recent report by Chaffin et . describes the synthesis of cryptands incorporating four thiophene rings (Fig. 1), wherein the stepwise approach was used commencing from the readily available α, ω,