Neutral Ferrocenoyl Receptors for the Selective Recognition and Sensing of Anionic Guests.

A range of neutral, hydrogen-bonding ferrocenoyl anion receptors and redox sensors operable in nonaqueous solvents are reported and a series of anion-binding and -sensing experiments presented. Thioamide-based receptor L2 binds halide anions more effectively than its carboxamide analogue L1, with the thioamide (N-H) group proving to be a better NMR antenna for detecting the recognition event. The binding of this class of neutral hydrogen-bonding receptor has favorable DeltaH degrees and unfavorable DeltaS degrees. Multidentate amide receptor L5 binds halide guests more strongly, with the effect of solvent on this binding process being studied. The introduction of a primary amine functionality (L4) causes remarkably strong HSO(4)(-) binding, the first reasoned report of selectivity for this acidic anionic guest. Analogously to many biological anion recognition processes, different binding modes operate dependent on guest acidity. In this way, the chemical properties of the substrate are addressed, yielding novel anion selectivities. All the receptors investigated exhibit electrochemical anion recognition. Typically, an EC mechanistic response is observed as ferrocene oxidation "switches-on" electrostatic interactions with the bound guest. Remarkable cathodic shifts of the ferrocene oxidation wave are also induced (up to 220 mV with HSO(4)(-) and 240 mV with H(2)PO(4)(-)) as the proximate bound negative charge stabilizes positively charged ferrocenium. Difunctional receptor L8 shows a large, novel UV-visible spectroscopic enhancement with H(2)PO(4)(-).