Simple anion hosts

The earliest anion-binding ligands to be prepared were those of Park and Simmonds [10,11] whose katapinands were similar to Lehn s later, and better known, cryptands but lacked coordinating oxygen atoms in the linkages connecting the 

REACTS VIOLENTLY WITH WATER] Rotary evaporator Methanol [FLAMABLE] Glassware for extraction and work up 

Note This reaction should be carried out in a well-ventilated laboratory. Exercise due care when handling potassium borohydride. 

Dissolve A1, A1, A1 - tri s - (2 - (2 - a m i n o e t h y I i m i n o) m e t h y I p h e n o I) (7) (1 g, 2.2 mmol) in methanol (100 mL) in a 250 mL round-bottomed flask equipped with a stirrer bar. Gentle heating may be necessary to effect complete dissolution. Add potassium borohydride (0.4 g, 7.4 mmol) in small portions to the bright yellow solution. During the addition the solution completely decolorizes. Continue to stir at room temperature for a further 1 h after the last portion of potassium borohydride has been added. Upon completion remove the solvent under vacuum to leave a white solid. Add a solution of ammonium chloride (20mL of a 2m aqueous solution) to the crude product, cautiously at first, to ensure all the potassium borohydride has been consumed. Extract with chloroform (50 mL 

Potassium borohydride [CORROSIVE REACTS VIOLENTLY WITH WATER] 

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For anions, it is tempting to try and attribute a preferential coordination geometry analogous to that so well established for various metal cations. In many cases simple anions such as the halides exist in approximately tetrahedral or octahedral environments, but it is clear from the diversity of examples reviewed herein that anion coordination geometry is highly flexible and may be adjusted to fit the properties of the various host systems. 

From Cation Hosts to Anion Hosts - a Simple Change in pH... 

Dual-host approaches have also been used to good effect for CsN03 extraction. Nitrate is a common anion found upon nitric acid extraction of 137Cs+ in the nuclear industry. Cs+ is conveniently com plexed by a large crown ether such as tetrabenzo[24] crown-8 while simple tripodal amide hosts of type 5.31 (where R is a long alkyl group to impart lipophilicity) are effective at binding N03 . Extraction efficiency of Cs+ from water into 1,2-dichloroethane was found to be enhanced by a factor of up to 4.4 in the presence of the anion host.28... 

These hosts include simple anion sensor systems containing urea, thiourea, amine, amide, alcohol, and pyrrole groups linked to chromophores. Chro-mophores used in this type of chromogenic anion sensors are mainly organic dyes such as azobenzene, nitrobenzene, indoaniline, and anthraquinone or... 

In contrast to host molecules for binding cations examples for anion hosts are still very scarce One simple reason becomes immediately obvious on comparison of the molecular dimensions of the guest species (Table 4). 

These data reveal some important aspects of the inclusion phenomenon displayed by the anion hosts 24 and 25 Many small and heavily hydrated anions can be bound in water with stability constants exceeding those with simple cyclodextrinsby factors of 20-150. The specificity of binding, however, is rather poor. The stability pattern does not follow the hydration enthalpies of the anions as is most obviously realized in the halide series. Though bromide ion possesses an intermediate enthalpy of hydration it forms the most stable inclusion complex with 24. 

Oxacalixarenes have been explored as host molecules for guest species beyond simple anions and cations. The development of oxacalix[2]arene[2]naphthyridines 47 as molecular receptors was first investigated by Katz (Fig. 15.9). In the preferred... 

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