Ditopic anion electrochemical sensors containing calix[4]aene

Abstract

Two heteroditopic electrochemical anion sensors 1 and 2 have been synthesized. p-tert- Butylcalix[4]arene was used as a building block. Pseudo-crown ether and amidoferrocene group were attached to the p-tert-butylcalix[4]arene framework for binding cations and anions, respectively. Compounds 1 and 2 are different in the position of the amidoferrocene units attached to the benzene ring. Syntheses of compounds 1 and 2 were carried out in four steps giving products 1 and 2 as orange solid in 12% and 14% yields, respectively. The complexation properties of 1 and 2 were studied by NMR titration experiments recorded in 5% CD3CN:CDCl3 at room temperature. We found that both compounds simultaneously complexed alkali metal salts in ion-pair fashions in which metal cations were encapsulated in pseudo-crown ether cavity and anions were hydrogen bonded to amide protons. In the presence of metal cations, the anion binding affinities of the receptors 1 and 2 were enhanced due to the strong electrostatic interactions of co-bound cation and anion. Interestingly, both compounds possessed highest selectivity for Br- in the presence of Na+. However, in the presence of Na+, the binding constants of receptor 2 with anions were much higher than that of receptor 1. The electrochemical properties of 1 and 2 were studied by cyclic and square wave voltammetry using 40% CH3CN:CH2Cl2 with 0.1 M TBAPF6 as supporting electrolyte and using a Pt working electrode, a Ag/Ag+ reference electrode and a Pt coil counter electrode. Studies showed that in the presence of metal cations, the interactions of heteroditopic receptor 2 towards anions, especially chloride and acetate are higher than its free form whereas no significant electrochemical change was observed when Na+ was added to receptor 1. This may result from the long distance between binding sites and ferrocene moiety of receptor 1.