Potential anion sensing properties by a redox and substitution series of [Ru(bpy)3−n(Hdpa)n]2+, n = 1–3; Hdpa = 2,2′-dipyridylamine: selective recognition and stoichiometric binding with cyanide and fluoride ions

Abstract

Three mononuclear ruthenium complexes of composition [Ru(bpy)_3−n(Hdpa)_n]²⁺, n = 1–3, where bpy = 2,2′-bipyridine, and Hdpa = 2,2′-dipyridylamine have been developed for selective recognition of cyanide (CN⁻) and fluoride (F⁻) ions. All the complexes have been characterised using standard analytical and spectroscopic techniques. The potential anion sensing features of complexes [1](ClO₄)₂, [2](ClO₄)₂ and [3](ClO₄)₂ have been thoroughly investigated by spectrophotometry, electrochemistry and ¹H NMR spectroscopy using a wide variety of anions, such as F⁻, Cl⁻, Br⁻, PF₆⁻, NO₃⁻, ClO₄⁻, HSO₄⁻, AcO⁻ and CN⁻. Cyclic voltammetry and differential pulse voltammetry established that [1](ClO₄)₂ and [2](ClO₄)₂ are excellent electrochemical sensors for the selective recognition of CN⁻ and F⁻ anions. The complexes display intense ligand-centred absorption bands in the UV region and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region. All the complexes act as selective colorimetric sensors for CN⁻ and F⁻ anions due to modulation of MLCT band position of the receptors in the presence of CN⁻ or F⁻ resulting distinct colour change visible by naked eyes. Further, emission quenching of 1²⁺ and 2²⁺ in the presence of CN⁻, F⁻ and AcO⁻ ions make them suitable luminescence-based sensors. The Stern–Volmer plot revealed static quenching mechanism of emission of the receptor 1²⁺ in presence of cyanide and fluoride ions, while dynamic quenching dominates in case of AcO⁻ anion.