Selective anion sensing by a ruthenium(ii)-bipyridyl-functionalized tripodal tris(urea) receptor

A tripodal tris(urea) ligand with 2,2′-bipyridyl (bpy) substituents (L) has been designed and synthesized, which coordinates with three equivalents of Ru(bpy) ₂Cl ₂·2H ₂O, followed by treatment with NH ₄PF ₆, to afford the anion receptor [(bpy) ₆Ru ₃L](PF ₆) ₆ (1). The anion-binding behavior of the ligand L and the Ru ^II-bpy functionalized receptor 1 toward different anions was investigated by ¹H NMR (for L and 1), fluorescence, and UV-vis spectroscopy (for 1). Both compounds showed selective recognition of SO ₄ ^2- or H ₂PO ₄ ^- ions in the 1:1 binding mode in the NMR studies. The Ru ^II complex 1 displayed the metal-to-ligand charge transfer emission at 600 nm, which was quenched on addition of the sulfate and dihydrogen phosphate ions. Quantitative fluorescence titration experiments were carried out and the stability constants (log K) of the complex 1 with SO ₄ ^2- and H ₂PO ₄ ^- ions were obtained to be 4.73 and 4.69 M ^-1 (1:1 binding mode), respectively.