Polyaza and azaoxa macrocyclic receptors functionalised with fluorescent subunits; Hg2+ selective signalling

Abstract

A polyazacycloalkane containing naphthyl groups, 1,4,8,11-tetrakis(naphthylmethyl)-1,4,8,11-tetraazacyclotetradecane (L1), and the azaoxa cycloalkane receptors containing anthracenyl groups, 7,13-bis(anthracenylmethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L2) and 10-anthracenylmethyl-1,4,7-trioxa-10-azacyclododecane (L3), have been synthesized and their activity as fluorescent chemosensors studied towards the metal cations Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ in THF–water (70∶30 v/v, 0.1 mol dm−3 tetrabutylammonium perchlorate, 25 °C) for L1 and 1,4-dioxane–water (70∶30 v/v, 0.1 mol dm−3 potassium nitrate, 25 °C) for L2 and L3. The crystal structure of the complex [Cd(L1)(Cl)][PF6] has been determined by X-ray single crystal procedures. The co-ordination geometry about the cadmium atom is near square pyramidal with the ligand showing a conformation with the four naphthylmethyl groups pointing above the N4 plane. Potentiometric experiments in THF–water for L1 in the presence of Cu2+, Hg2+ and Pb2+ and in 1,4-dioxane–water for L2 in the presence of Cu2+, Zn2+, Cd2+, Hg2+, Pb2+, and L3 with Cd2+ and Pb2+ have also been carried out. All metals form stable complexes with L1, L2 and L3 with stability constants for the formation of the [M(L1)]2+, [M(L2)]2+ and [M(L3)]2+ species in the sequence Pb2+ < Hg2+ < Cu2+; Zn2+ < Cd2+ < Pb2+ < Cu2+ ≪ Hg2+ and Cd2+ < Pb2+, respectively. The fluorescent behaviour of L1, L2 and L3 in the presence of the Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+ has been studied as a function of the pH: Cu2+ and Hg2+ selectively quench the fluorescence emission intensity of L1 at neutral and acid pH. The emission intensity of L2 is selectively enhanced in the presence of Hg2+ at basic pH. In contrast, the fluorescence intensity of the free receptor L3 at a certain pH is not modified upon addition of any metal ion. The emission sensing behaviour of L1, L2 and L3 towards metal ions is compared with the electrochemical sensing ability of analogous ferrocene-functionalised ligands.