Some guidelines for the design of anion receptors

Anions can be recognized by either positively charged or neutral artificial receptors. Positive charges within receptor's cavity can be provided by transition metal ions (e.g. CuII), which offer a binding site to one donor atom of the envisaged anion. Dicopper(II) bistren cryptates are ideal receptors for ambidentate anions, capable to bridge the two CuII centres: the size of the ellipsoidal cavity and consequent inclusion selectivity can be modulated by varying the length of the spacers linking the two tren subunits. Examples are discussed of the selective recognition of halides, polyatomic anions, aromatic and aliphatic dicarboxylates. Among neutral receptors, attention is centred to systems containing the urea subunit. Urea behaves as a bifurcate H-bond donor towards oxoanions. On the basis of equilibrium studies in aprotic solvents (mainly MeCN and DMSO), it is shown that the energy of the hydrogen bonding interaction and selectivity are solely related to the acidic tendencies of the receptor and to the basic properties of the anion. In particular, the H-bond interaction can be conveniently viewed as a more or less advanced (and ‘frozen’) proton transfer from the NH fragment of urea and the oxygen atom of the anion. Addition of excess fluoride may induce deprotonation of the NH fragment, essentially due to the unique stability of the [HF2]− self-complex which forms.