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

A polyazacycloalkane containing naphthyl groups, 1,4,8,11-tetrakis(naphthylmethyl)-1,4,8,11-tetraazacyclotetradecane (L-1), and the azaoxa cycloalkane receptors containing anthracenyl groups, 7,13-bis(anthracenylmethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L-2) and 10-anthracenylmethyl-1,4,7-trioxa-10-azacyclododecane (L-3), 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 degrees C) for L-1 and 1,4-dioxane-water (70 : 30 v/v, 0.1 mol dm(-3) potassium nitrate, 25 degrees C) for L-2 and L-3. The crystal structure of the complex [Cd(L-1)(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 N-4 plane. Potentiometric experiments in THF-water for L-1 in the presence of Cu2+, Hg2+ and Pb2+ and in 1,4-dioxane-water for L-2 in the presence of Cu2+, Zn2+, Cd2+, Hg2+, Pb2+, and L-3 with Cd2+ and Pb2+ have also been carried out. All metals form stable complexes with L-1, L-2 and L-3 with stability constants for the formation of the [M(L-1)](2+), [M(L-2)](2+) and [M(L-3)](2+) species in the sequence Pb2+ < Hg2+ < Cu2+; Zn2+ < Cd2+ < Pb2+ < Cu2+ much less than Hg2+ and Cd2+ < Pb2+, respectively. The fluorescent behaviour of L-1, L-2 and L-3 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 L-1 at neutral and acid pH. The emission intensity of L-2 is selectively enhanced in the presence of Hg2+ at basic pH. In contrast, the fluorescence intensity of the free receptor L-3 at a certain pH is not modified upon addition of any metal ion. The emission sensing behaviour of L-1, L-2 and L-3 towards metal ions is compared with the electrochemical sensing ability of analogous ferrocene-functionalised ligands.