Host/anion “sandwich

The range and versatility of amide anion binding hosts is very extensive. Macrocyclic amides have also been reported, such as 4.42 which forms a remarkable sandwich compound with sulfate in the solid state. Sulfate binding is enhanced by protonation of the tertiary amine groups and the host is thus a hybrid between amide and ammonium binding sites. The triethylbenzene scaffold has been used to prepare... 

A macrocyclic receptor (100) has also recently been prepared and its crystal structure was elucidated (220). In comparison with its acyclic analogue 101, an anion macrocyclic effect was observed, the stability constants for chloride complex formation [in DMSO] being K = 250 M (100) and K = 20 M (101). Receptor 102 was shown to act as a switchable cobaltocenium based chloridebinding host (221). The free receptor binds chloride anions, but on the addition of potassium ions, the binding is switched off. This effect is probably due to the ability of the potassium ion to form a sandwich complex with the two crown ether substituents, sterically hindering the anion-binding site. 

Kubik and co-workers have developed a series of highly effective anion receptors based upon cyclic peptides. Cyclic hexapeptide receptors such as 17 consist of alternately linked L-proline and 6-aminopicolinic acid subunits [25]. A 1 1 binding stoichiometry for 17 and the sodium salt of ben-zenesulfonate was confirmed by a Job plot but in the case of the halide and sulfate sodium salts 2 1 host/guest complexes were found. This was confirmed by electrospray mass spectrometry and in the case of iodide a crystal structure of the 2 1 complex was obtained where the iodide was sandwiched between two cyclic hexapeptide receptors. 

In one of the crystal forms of the acetate structure of 20, however, a sandwich-like, 2 1 complex is formed (Figure 8d) when [nBu4N]+ is the counterion, while a simple 1 1 complex is formed in the presence of [(CH3)4N]+. In the sandwich complex, each oxygen atom of the carboxy-late group is H-bonded to the host via four NH- O bonds (Figure 8d). In the 1 1 complex of 20 with acetate, the acetate hovers above the macrocyclic cavity, much like the chloride structure, with four H-bonds to the macrocyclic amides (Figure 8e). The anion is also coordinated by an H-bond to an axial water molecule, an occurrence that is frequently seen in complexes with anions. 

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