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 (L¹1) and 1-(2-(9-anthrylmethylamino)ethyl)-4,7,10-tris((2S)-2-hydroxy-3-[4′-(methyl)phenoxy]propyl)-1,4,7,10-tetraazacyclododecane (L²2), have been prepared with a view to using them to study aromatic anion sequestration. The eight-coordinate Cd(II) complexes of L¹1 and L²2, [CdL¹1](ClO₄)₂·2H₂O and [CdL²2](ClO₄)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 10^2.3 M⁻¹ (benzoate) to 10^7.5 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 L¹1 and L²2 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.