Bifunctional colorimetric chemosensing of fluoride and cyanide ions by nickel-POCOP pincer receptors

Abstract

Three Ni(II)-POCOP pincer complexes [NiCl{C₆H₂-4-OH-2,6-(OPPh₂)₂}], 1; [NiCl{C₆H₂-4-OH-2,6-(OPtBu₂)₂}], 2 and [NiCl{C₆H₂-4-OH-2,6-(OPiPr₂)₂}], 3 were studied as bifunctional molecular sensors for inorganic anions and acetate. In CH₃CN, fluoride generates a bathochromic shift with a colorimetric change for 1–3 with a simultaneous fluorescence turn on, this optical effect is based on deprotonation of the para-hydroxy group of the POCOP ligand. On the other hand, in a neutral aqueous solution of 80 vol% CH₃CN, additions of cyanide produce a distinct change of color by forming very stable complexes with the nickel-based receptors 1–3 with log K_a in the range of 4.38–5.03 M⁻¹ and pronounced selectivity over other common anions such as iodide, phosphate, and acetate. Additionally, bromide shows a modest spectral change and affinity, but lower than those observed for cyanide. On the basis of ¹H NMR experiments, UV-vis titrations, ESI-MS experiments, and the crystal structure of the neutral bromo complex of 1, it is proposed that the colorimetric change involves an exchange of chloride by CN⁻ on the Ni(II) atom. The Ni(II)-based sensor 1 allows the fluorescent selective detection of fluoride with a limit of 5.66 μmol L⁻¹ and colorimetric sensing of cyanide in aqueous medium in the micromolar concentration range.