Fluorescent signaling provides deeper insight into aromatic anion uptake by metal-ion activated molecular receptors

Abstract

Two new, octadentate, fluorescent, macrocyclic ligands, 1-(2-(9-anthrylmethylamino)ethyl)-4,7,10-tris((2S)-2-hydroxy-3-phenoxypropyl)-1, 4,7,10-tetraazacyclododecane (L1) and 1-(2-(9-anthrylmethylamino) ethyl)-4,7,10-tris((2S)-2-hydroxy-3-[4′-(methyl)phenoxy]propyl)-1,4,7, 10-tetraazacyclododecane (L2), have been prepared with a view to using them to study aromatic anion sequestration. The eight-coordinate Cd(II) complexes of L1 and L2, [CdL1](ClO 4)2·2H2O and [CdL2](ClO 4)2·4H2O, have both been shown capable of acting as receptors for a range of aromatic oxoanions. This has been demonstrated by perturbation of both 1H NMR chemical shift values and the anthracene derived fluorescence emission intensity as the potential guest anion and the receptor are combined. Non-linear least squares regression analysis of the resulting titration curves leads to the determination of binding constants in 20% aqueous 1,4-dioxane which lie in the range 102.3 M-1 (benzoate) to 107.5 M-1 (2,6-dihydroxybenzoate). By reference to earlier, X-ray determined structures of related, but non-fluorescent, inclusion complexes the primary anion retention force is known to arise from hydrogen bonding between the anion and four convergent hydroxy groups that exist at the base of a cavity that develops in L1 and L2 as their aromatic groups juxtapose upon metal ion coordination. This work reveals significant stability enhancement when hydroxy groups are positioned on the anion at points where O-H⋯π hydrogen bonding to the aromatic rings that constitute the walls of the cavity becomes geometrically possible. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.