Second-Coordination-Sphere Assisted Selective Colorimetric Turn-on Fluoride Sensing by a Mono-Metallic Co(II) Hexacarboxamide Cryptand Complex

Abstract

The preparation of a selective turn-on colorimetric fluoride sensor was achieved through single cobalt(II) ion insertion into a macrobicyclic cryptand. Monometallic [Co(mBDCA-5t-H₃)]^- (1) and [Zn(mBDCA-5t-H₃)]^- (2) complexes were prepared in 74 and 84% yields, respectively. Structural characterization of 1 confirmed the presence of a proximal hydrogen-bonding network consisting of carboxamide N-H donors. The reaction of 1 with F^- was accompanied by a distinct colorimetric turn-on response in mixed aqueous/organic media, and 1 was capable of selective fluoride sensing in the presence of large quantities of potentially competitive anions. Complex 1 represents a unique example of a fluoride sensor wherein selective F^- binding takes place directly at a transition-metal center and induces a color change based upon metal-centered transitions. The metal(II) fluoride complexes [F⊂Co(mBDCA-5t-H₃)]^2- (3) and [F⊂Zn(mBDCA-5t-H₃)]^2- (4) were both fully characterized, including single crystal X-ray analyses. Fluoride binding is synergistic involving hydrogen-bond donors from the second-coordination sphere together with metal(II) ion complexation.