Cooperative recognition of a copper cation and anion by a calix[4]arene substituted at the lower rim by a ß-amino-a,ß-unsaturated ketone

We report herein a new ditopic calix[4]arene receptor 25,27-bis{[4-amino-4- ( 1 -naphthyl) -2-oxo-3-butenyl]oxy}-26,28-dihydroxycalix[4]arene (2) for the simultaneous complexation of anionic and cationic species. The host molecule 25,27-bis{[3-(l-naphthyl)-5-isoxazolyl]methoxy}-26,28-dihydroxycalix[4]arene (1) was synthesised first and was followed by a [Mo(CO)₆]mediated ring-opening reaction to give the target receptor 2. The binding properties of ligands 1 and 2 towards metal ions in CH₃CN were investigated by UV/Vis and fluorescence spectroscopies. The results showed that both Iigands 1 and 2 were highly selective for Cu11 ions. Upon titration with Cu_II, the fluorescence of 1 was severely quenched, whereas 2 showed strong fluorescence enhancement because the metal ions help to lock the conformation of the fluorophores. During the complexation of 2 with Cu_II, the Cu_II was reduced to Cu₁ by the free phenolic OH of 2, whereas the phenol was oxidised by Cu_II after which it assisted in the trapping of Cu_I. Ditopic behaviour was observed for the complex 2»Cu_I, which showed further enhancement of its fluorescence intensity upon complexation with anions such as acetate or fluoride.