Bifunctional Indenyl-Derived Receptors for Fluoride Chelation and Detection

Anion receptors based on a [CpFe(indenyl)] scaffold offer the possibility for the incorporation of adjacent Lewis acidic functions onto a six-membered carbocyclic framework, while at the same time retaining the colorimetric/electrochemical reporter mechanisms available to synthetically simpler ferrocene systems. Thus, [CpFe(indenyl)] systems featuring mutually ortho BMes2 and PPh2 Me(+) substituents (with either 4,5 or 5,6 regiochemistry) are accessible which are capable of cooperative fluoride ion fixation. Simultaneous binding at the borane and phosphonium centres can be established by spectroscopic, structural and computational approaches, and is responsible for the favourable thermodynamics associated with F(-) uptake. Thus, in contrast to simple BMes2 systems, the binding of fluoride is found to be more favourable than the uptake of cyanide (which interacts only with the borane Lewis acid). Moreover, in the case of a 4-(MePh2 P)-5-(Mes2 B)-7-Me-indenyl derivative, fluoride chelation is signalled not only by a large cathodic shift in the Fe(II) /Fe(III) potential (>500 mV in THF), but also by a distinct colour change from green (for the free receptor) to maroon for the adduct.