Chelate effect with fluoride ions

In light of the work carried out on boron systems as described above, a silacrown (28) was synthesized in fair yield via nine steps (87). This transported bromide ions more effectively than chloride, but no evidence for the binding mechanism or evaluation of binding affinity was provided. In recent studies, Compound 29 showed, as would be expected, a chelate effect with fluoride ions, exhibiting a high-binding constant (log K > 9 in acetone-d6), which had to be determined via a stepwise procedure (88). 

Katz (79, 80) used 23 as a receptor for anions, studying its interaction with hydride, fluoride, and hydroxide ions. Comparison with 24 once again indicated a chelate effect compound 23 abstracting hydride or fluoride from complexed 24. The crystal structure of the hydride sponge 23, showed the hydride ion bound between the pair of boron atoms with short strong bonds. A crystal structure of the chloride complex was also elucidated and showed the same bridged structure (81). 

The column medium is a polystyrene-divinylbenzene polymer with octylphenyl-N,N-diisobutyl carbamo-ylphosphine oxide extractant adsorbed on the hydrophobic polymer matrix. The carbamoylphosphine oxide functional group is an avid chelator for actinides in 0.16 M or higher concentration nitric acid, whereas alkali and alkaline earth metals are poorly bound. Lanthanides are only retained on the column at much higher nitric acid concentrations (>6 M). Therefore, even these are eliminated from the final matrix under the rinse conditions employed. A very hard base anion ligand is necessary to compete effectively with the carbamoylphosphine oxide ligands and elute actinides, including uranium, in a small volume. In Protocol 1, the fluoride ion from dilute hydrofluoric acid (HF) was chosen for this purpose. 

HEXAIT.UORINE solution can provide the mechanical removal at the surface of the skin, as water does, and can avoid the wash-in effect due to its physical hyperosmolarity and restores pH toward neutral value because of its additional amphoteric properties, without any exothermic reaction. With its chelating properties, it will avoid fluoride ion diffusion into the skin. HEXAFLUORINE solution is nonirritant to the eye and the skin, nontoxic (oral LDso> 2,000 mg/kg in rats), and nonsensitizing [202]. 

Since non-bound or non-coordinated nucleophiles are even more reactive, crown-ethers [138] and cryptands (polyaminoethers) [139,140] have been used to chelate the alkali metal cations, notably the potassium ion of K[ F]F. This allows the [ F]fluoride anion to be less tightly paired with the cation and therefore to be more reactive, which has been coined the naked ion effect. In practice, the crown-ether (e.g. 18-crown-6) or better the polyaminoether Kryptofix-222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) is added to the aqueous K[ F]F/K2C03 solution which is then concentrated to dryness [139,140]. The complex (KP FIF-K ) can be further dried, if needed, by one or more cycles of addition of dry acetonitrile and azeotropic evaporation. 

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