Colorimetric Fluoride Ion Sensing by Polyborylated Ferrocenes: Structural Influences on Thermodynamics and Kinetics

The thermodynamic factors underlying the use of ferroceneboronic esters as electrochemical or colorimetric
fluoride ion sensors have been investigated through the synthesis of a range of systematically related derivatives
differing in the number/nature of the boronic ester substituents and in the nature of ancillary ligands. Thus, if the
shift in electrochemical potential associated with the conversion of one (or more) boronic ester group(s) to anionic
boronate(s) on fluoride binding is sufficient to allow oxidation of the resulting host/guest complex by dioxygen,
colorimetric sensing is possible. In practice, while monofunctional systems of the type CpFe[η5-C5H4B(OR)2] offer
selectivity in fluoride binding, electrochemical shifts in chloroform solution are insufficient to allow for a colorimetric
response. Two chemical modification strategies have been shown to be successful in realizing a colorimetric
sensor:  (i) the use of the more strongly electron-donating Cp* ancillary ligand (which shifts the oxidation potential
of both the free receptor and the resulting fluoride adduct cathodically by ca. −400 mV) and (ii) receptors featuring
two or more binding sites and consequently a larger fluoride-induced electrochemical shift. Thus, in the case of
[η5-C5H4B(OR)2]2Fe [(OR)2 = OC(H)PhC(H)PhO, 2s], the binding of 2 equiv of fluoride gives an electrochemical
shift (in chloroform) of −960 mV (cf. −530 mV for the corresponding monofunctional analogue, 1s). Related tris-
and tetrakis-functionalized systems are also shown to be oxidized as the bis(fluoride) adducts, presumably because
of fast oxidation kinetics, relative to the rate of the (electrostatically unfavorable) binding of a third equivalent of
fluoride. Furthermore, the rate of sensor response (as measured by UV/vis spectroscopy) is found to be strongly
enhanced by the presence of pendant (uncomplexed) three-coordinate boronic ester functions (e.g., a rate
enhancement of 1−2 orders of magnitude for 3s/4s with respect to 2s) and/or delocalized aromatic substituents.