Anticrowns host-guest chemistry

Host-guest chemistry-carborane anticrowns, 4 216-217 Host-guest complexes, binding constants for, 24 39t... 

Metalloboranes, 4 172 exopolyhedral, 4 208-210 main group element, 4 207-208 transition element, 4 205-207 Metallo-carbohedrene clusters, 4 648 Metallocarboranes, 4 170 as catalysts, 4 217-218 economic aspects, 4 229 exopolyhedral, 4 215-216 f-block element, 4 225-226 host-guest chemistry-carborane anticrowns, 4 216-217 structural systematics, 4 176-179 transition metal, 4 210-215 Metallocene catalysis, MAO in, 16 92-93. 

In the present article, host-guest chemistry of anticrowns as well as available data on their applications in catalysis and as ionophores will be briefly reviewed. Strictly speaking, only macrocycles with three and more Lewis acidic centers in the chain can be considered as genuine anticrowns. Nevertheless, the binding properties of some macrocycles containing only two Lewis acidic atoms in the ring will also be discussed. 

The anticrown chemistry term has been coined to describe the way in which many electroneutral hosts were built to recognize anions. That is the incorporation of multiple and convergent Lewis-acid centers into preorganized cyclic molecular scaffolds. The Lewis-acid centers, usually transition metals or elements like B, Si, Sn or Hg, should expose their electron-deficient sites for interaction with the lone electron pairs of the anions. The hydrogen-bonding interaction of an anion with an amide N - H mentioned above has also been widely used in the preparation of neutral receptors for anionic guests [8]. 

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