Receptors for the ammonium ion

A logical extension to this strategy is to encapsulate the ammonium guest within a cryptand. Cram illustrated this with a series of hemispheraplex complexes though, given the size of the central cavity, it is likely that the cation is not encapsulated but binds closely between two strands of the host molecules [5]. A better example is one in which a capped cyclotriveratrylene encapsulates an ammonium cation [6]. Needless to say these compounds will not be able to bind terminal groups on protonated natural and synthetic amines. 

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Spherical cryptand (13) contains four nitrogen atoms in a tetrahedral arrangement this is the topologically ideal receptor for the ammonium ion (NH4+) which has a tetrahedral... 

By far the most common type of anion-binding host is based upon numerous substituted forms of the ammonium ion. Protonated alkyl ammonium receptors have the advantages of positive charge (for anion binding via electrostatic attrac-... 

Fig. 5. Conjectured cross-sectional representation of complex for cucur-bituril plus C HjNHfCHjj NH-(CHjj NHj in acidic and in alkaline solution. Outlines are drawn as for Fig, 4. In all probability a slight buckling of the hexyl chain actually occurs in the upper situation, better filling the cavity and bringing the ammonium ions into improved alignment with the carbonyl dipoles of the receptor, from [8] with permission...

This indicates a lack of dynamic cohesion within the adducts i.e. the substrate has considerable freedom for reorientation within the receptor. The apparent reason for an absence of mechanical coupling is the nearly cylindrical symmetry of cucurbituril, which allows the guest an axis of rotational freedom when held within the cavity. Hence, the bound substrates show only a moderate increase in tc relative to that exhibited in solution. No relationship exists between values and the thermodynamic stability of the complexes as gauged by K (or K, cf. Tables 1 and 2). It must be concluded that the interior of cucurbituril is notably nonsticky . This reinforces previous conclusions that the thermodynamic affinity within adducts is chiefly governed by hydrophobic interactions affecting the solvated hydrocarbon components, plus electrostatic ion-dipole attractions between the carbonyls of the receptor and the ammonium cation of the ligands. 

Table 3.17 shows the complexation free energies for various ammonium ions with three corand receptors. It is immediately apparent that NH4+ is bound more strongly than either of its alkylated analogues, and methylammonium is bound more strongly than f-butylammonium, with a particularly large difference between NH4+ and CH3NH3+ complexes of 3.95 and 3.96 (10 kj mol-1). 

Phenolate resorcin[4]arenes of the type IV-1 (Scheme 12.15) are efficient hosts for organic ammonium ions in protic solvents in strongly alkaline organic media by simultaneous charge-charge and cation- r interaction. A drawback of receptors such as IV-2 is the low selectivity towards alkyltrimethylammonium and bulkier tetraalkylam-monium ions. ... 

The simplest molecular cation is the ammonium ion which cannot be discriminated from by size very effectively. However, a clear difference in charge distribution exists, being spherical for and tetrahedral for NH4. Selective binding of a tetrahedral guest requires the construction of a receptor... 

The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

As in the previous categories in this section, there are numerous compounds which have been prepared based on a sugar subunit. Examples may be found in Refs. 7,35,42-45, 57, 82-85, 117—121,175,176,193 and 208. Much of the work in these references has been reported by Stoddart and his coworkers, who have pioneered this field. As with the compounds prepared by Cram, the goal was to prepare a chiral receptor for ammonium ions which could be utilized in enzyme model studies. 

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