A tripodal amide based ligand, tris-{(2-carbamoyl-5-carbomethoxy-pyridine)-2-ethyl}amine (H3L, 1), was synthesized and structurally characterized by single crystal X-ray diffraction. Investigation of the cation recognition behavior showed that the ligand has selective colorimetric sensing properties for cobalt(II) ions by an obvious color change from colorless to yellow. To investigate the sensing mechanism of H3L for Co2+ ions, UV-vis absorption spectroscopy and single-crystal structural analysis were performed. The mixture of the ligand and cobalt(II) ions displayed selective colorimetric sensing properties for weak acid anions, such as CO32−, Ac−, HCO3−, SO32−, and PO43−. Detailed 1H NMR experiments revealed that the basicity of the anions played an important role in the intensity of the interaction between the ligand and anions. The structures of compounds CoL (2), Co-Ac-HL (3), H4L-NO3 (4), and H4L-ClO4 (5) were also determined by single crystal diffraction studies.
