Cooperative Recognition of a Copper Cation and Anion by a Calix[4]arene Substituted at the Lower Rim by a beta-Amino-alpha,beta-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-(1-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 ligands I and 2 were highly selective for Cu-II ions. Upon titration with Cu-II, the fluorescence of I 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 Cu-I 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 center dot Cu-I, which showed further enhancement of its fluorescence intensity upon complexation with anions such as acetate or fluoride.