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Article

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

Supporting Info

Joanna K. Day,^†^‡ Christopher Bresner,^†^‡ Natalie D. Coombs,^†^‡ Ian A. Fallis,*^‡ Li-Ling Ooi,^‡ and Simon Aldridge*^†

Inorganic Chemistry, University of Oxford, South Parks Road, Oxford, U.K., OX1 3QR, and School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, U.K., CF10 3AT

Inorg. Chem., 2008, 47 (3), pp 793–804

DOI: 10.1021/ic701494p

Publication Date (Web): October 12, 2007

†

University of Oxford.

‡

Cardiff University.

To whom correspondence should be addressed. E-mail: simon. aldridge@chem.ox.ac.uk (S.A.); fallis@cardiff.ac.uk (I.A.F.).

Abstract

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, 2^s], the binding of 2 equiv of fluoride gives an electrochemical shift (in chloroform) of −960 mV (cf. −530 mV for the corresponding monofunctional analogue, 1^s). 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 3^s/4^s with respect to 2^s) and/or delocalized aromatic substituents.