Anticrowns

and Hawthorne, M. F., Multidentate carborane-containing Lewis acids and their chemistry mercuracarborands . Coord. Chem. Rev. 2003, 240, 111-128 Wuest, J. D., Multiple coordination and activation of Lewis bases by multidentate Lewis acids , Acc. Chem. Res., 1999, 32, 81-89. 

The bidentate coordination behaviour of Lewis acid anion chelating ligands such as 4.69-4.78 suggests that very strong anion binding might be achieved with those Lewis acid groups as part of a macrocyclic 

Following the preparation of 4.79, a nnmber of other cyclic mercnry crown componnds have been synthesised, which do exhibit halide complexation behavionr. Componnd 4.80, forms a 1 1 polymer with bromide in the solid state in which the Br anions perch above the Hgj plane. The Hg—Br distances of 3.07-3.39A are considerably longer than normal Hg—Br covalent bonds (abont 2.54A). The compound also binds SCN with similarly long bonds as shown in Fignre 4.34. ° The analogons chloride complex has a 3 2 stoichiometry snggesting a triple-decker sandwich of type [4.80 Cl 4.80 Cl 4.80] . 

1-decanethiol) to log P12 = 5.67 with the addition of a 1 4 molar ratio of the thiol. ° This experiment highlights that it is not always high binding constants that make for a useful host. It is its ability to discriminate between potential gnests under the desired conditions. 

Variable-temperature NMR spectroscopic studies indicate that the crown structure is retained in solution with the compounds existing as two distinct inter-converting conformers with W and Z shapes in benzene solution. 

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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. 

More recent work has yielded another class of carborane-based mercury-containing macrocycles 43 and 44 related to crown ethers such as I2-crown-4 and 9-crown-3. Anticrown 43 associates with one or two moles of lithium halide, depending on conditions. The X-ray crystal structure of the chloride salt is shown in Figure 15. 

The X-ray crystal structures of the related, though less complex, anticrown mercury-containing macrocycles 45 and 46 have also recently been reported. Complex 45 may form either 1 1 complexes with Br or I" or a 3 2 complex with Cr. In the case of the bromo derivative, crystallographic results reveal an infinite chain of alternating Br and 45 with each halide bridging between six Hg atoms, Hg- Br 3.07-3.39 A. It is postulated that the related 3 2 chloride complex exhibits a similar, though finite layered structure. The related pentameric species 46 forms... 

Lewis acids based on boronic acid derivatives or main group elements such as mercury, tin and silicon form strong bonds to anions with considerable covalency, exemplified by hydride sponge and the anticrowns. 

Radu, A., Bakker, E., Shifting the measuring range of chloride selective electrodes and optodes based on the anticrown ionophore [9]mercuracarborand-3 by the addition of 1-decanethiol. Chemia Analityczna 2005, 50, 71-83. 

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]. 

Anticrowns are peculiar antipodes of crown ethers and their thia and aza analogues. They contain several Lewis acidic centers in the macrocyclic chain and so are able to efficiently bind various anions and neutral Lewis bases with the formation of unusual complexes, wherein the Lewis basic species is simultaneously bonded to all Lewis acidic atoms of the macrocycle. This remarkable property of anticrowns, being reminiscent of the behavior of conventional crown compounds in metal cation binding, makes them prospective aids in the areas of molecular recognition, ion transport, as well as organic synthesis and catalysis. 

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 first example of the successful application of anticrowns in phase-transfer catalysis was reported in 1989, when it was shown that nonfluorinated poiymer-curamacrocycle 3 is able to catalyze the azo-coupling reaction between benzenediazonium halides PhN2" X (X-Cl, Br) and p-naphthol in the two-phase H2O-CH2Br2 system. Diphenylmercury proved to be totally inactive in this reaction. 

Anticrowns are also promising reagents for the development of a new type of anion-selective electrodes. The first success in this important area was achieved by Hawthorne et who described highly sensitive and... 

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