Colorimetric fluoride detection in dimethyl sulfoxide using a heteroleptic ruthenium(ii) complex with amino and amide groups: X-ray crystallographic and spectroscopic analyses

A bis-heteroleptic ruthenium(ii) complex, [Ru(Hdpa)(2)(H(2)pia)]X(2) (1·X(2); X = Cl, OTf, or F; Hdpa = di-2-pyridylamine; H(2)pia = 2-pycolinamide; OTf(−) = CF(3)SO(3)(−)), was synthesized and spectroscopically and crystallographically characterized. The crystal structures of 1·Cl(2)·2.5H(2)O and 1·F(2)·2EtOH revealed essentially identical geometries for the 1(2+) 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·F(2)·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 NH(2) group of the H(2)pia ligand in 1(2+) 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)·H(2)O, the solution turned dark brown. (1)H NMR analysis and absorption spectroscopy of the reaction between 1(2+) 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 H(2)pia, although they were in different environments in the DMSO solution. A tris-F-adduct with 1(2+), 1·F(3)(−), was formed when sufficient F(−) anions were present in the solution, despite the presence of four NH protons in 1(2+). Time-dependent DFT calculations of 1(2+) and 1·F(3)(−) were consistent with their absorption spectra.