Extension of conjugation: probing anion binding strength and reporter mechanisms in (phenyl)cyclopentadienyl and indenyl receptors

Abstract

Two families of novel organo-iron functionalized boranes have been developed for anion binding/detection. Synthetically simple phenylferrocene systems, while stable to air and moisture, possess similar binding affinities to FcBMes₂ and no workable discrimination between F⁻ and CN⁻. In addition, reduced electronic communication between receptor and reporter sites brought about by a lack of conformational rigidity means that photochemical and electrochemical responses to anion binding are intrinsically weak. Receptors based on a more rigid indenyl framework are available in two steps from organic precursors (in ca. 50% yield), and feature intrinsically higher binding constants for F⁻ and especially CN⁻, consistent with the lower LUMO energies implied by extended conjugation. One such system, (4-dimesitylboryl-indenyl)cyclopentadienyl-iron(II), can be shown to bind cyanide in protic media with an accompanying green to red/pink colour change. Extremely high selectivity over fluoride and hydroxide and a detection limit of ca. 10 ppm represent a highly desirable combination among borane derived cyanide receptors.