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

Abstract

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)₂Cl₂·2H₂O, followed by treatment with NH₄PF₆, to afford the anion receptor [(bpy)₆Ru₃L](PF₆)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 SO₄²⁻ or H₂PO₄⁻ 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 SO₄²⁻ and H₂PO₄⁻ ions were obtained to be 4.73 and 4.69 M⁻¹ (1 : 1 binding mode), respectively.