Colorimetric fluoride detection in dimethyl sulfoxide using a heteroleptic ruthenium(ii) complex with amino and amide...

A bis-heteroleptic ruthenium(ii) complex, [Ru(Hdpa)₂(H₂pia)]X₂ (1·X₂; X = Cl, OTf, or F; Hdpa = di-2-pyridylamine; H₂pia = 2-pycolinamide; OTf^- = CF₃SO₃ ^-), was synthesized and spectroscopically and crystallographically characterized. The crystal structures of 1·Cl₂·2.5H₂O and 1·F₂·2EtOH revealed essentially identical geometries for the 1²⁺ dication; however, the dihedral angle between the two pyridyl groups in the Hdpa ligands, which represented the degree of bending of the bent conformation, was affected by hydrogen-bonding interactions between the NH group and counterions. In 1·F₂·2EtOH, one of the Hdpa ligands had an unusually smaller dihedral angle (15.8°) than the others (29.9°-35.0°). The two NH groups of each Hdpa ligand and the NH₂ group of the H₂pia ligand in 1²⁺ acted as receptors for F^- anion recognition via hydrogen-bonding interactions in a dimethyl sulfoxide (DMSO) solution, and the reaction showed an unambiguous color change in the visible region. Upon the addition of tetra-n-butylammonium fluoride to the red DMSO solution of 1·(OTf)₂·H₂O, the solution turned dark brown. ¹H NMR analysis and absorption spectroscopy of the reaction between 1²⁺ and the added F^- anions revealed that the F^- anions did not distinguish between the two amino groups of Hdpa and the amide group of H₂pia, although they were in different environments in the DMSO solution. A tris-F-adduct with 1²⁺, 1·F₃ ^-, was formed when sufficient F^- anions were present in the solution, despite the presence of four NH protons in 1²⁺. Time-dependent DFT calculations of 1²⁺ and 1·F₃ ^- were consistent with their absorption spectra.