Fluorescence signaling of aromatic oxoanion inclusion within metal-ion activated molecular receptor complexes formed from 2-(9-anthracenylmethylamino) ethyl-appended cyclen

Our observations that 1-[2-[(9-anthracenylmethylamino)ethyl)-4,7,10- tris[(2S)-2-hydroxy-3-phenoxypropyl]-1,4,7,10-tetraazacyclododecane, L ¹, complexes Cd(II) to form fluorescent [CdL ¹] ²⁺ which undergoes a fluorescence change when it acts as an aromatic anion receptor complex has caused us to explore further the potential development of an interesting sequestration/sensor system. Accordingly, three new, octadentate, fluorescent, macrocyclic ligands, 1-[2-[(9-anthracenylmethyl) ((2S)-2-hydroxy-3-phenoxypropyl)amino]ethyl]-4,7,10-tris[(2S) -2-hydroxy-3-phenoxypropyl]-1,4,7,10-tetraazacyclododecane, (L ²), 1-[2-[(9-anthracenyl-methyl)((2S)-2-hydroxy-3-(4′-methyl)phenoxypropyl) amino]ethyl]-4,7,10-tris[(2S)-2-hydroxy-3-(4′-methyl)phenoxypropyl]-1,4,7, 10-tetraazacyclododecane, (L ³), and 1-[2-[(9-anthracenylmethyl)((2S)- 2-hydroxy-3-(4′-t-butyl)phenoxypropyl)amino]ethyl]-4,7,10-tris[(2S) -2-hydroxy-3-(4′-t-butyl)phenoxypropyl]-1,4,7,10-tetraazacyclododecane, (L ⁴), have been prepared with a view to using their metal complexes to study aromatic anion sequestration. The eight-coordinate Cd(II) complexes of L ² and L ³, [CdL ²](ClO ₄) ₂•5H ₂O and [CdL ³](ClO ₄) ₂•2H ₂O•2Et ₂O are both capable of acting as receptors for a range of aromatic oxoanions. This is demonstrated by perturbation of the anthracene derived fluorescence emission intensity as the guest aromatic oxoanion and the receptor complex combine. In 20% aqueous 1,4-dioxane the receptor complex/aromatic oxoanion association constants are in the range of 10 ^3.2 M ^-1 (guest = p-hydroxybenzoate) to 10 ^7.3 M ^-1 (guest = 3,4,5-trihydroxybenzoate).