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.