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

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[eta(5)-C(5)H(4)B(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 [eta(5)-C(5)H(4)B(OR)(2)](2)Fe [(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.

Zwitterionic Mixed Valence: Internalizing Counteranions into a Biferrocenium Framework toward Molecular Expression of Half‐Cells in Quantum Cellular Automata

“…A preliminary investigation by cyclic voltammetry revealed that FcB shows quantitative shifts of −567 and −702 mV for the Fc + /Fc redox couple in the presence of excesses of CN − and F − , respectively, in CH 2 Cl 2 / n Bu 4 NPF 6 (Figure c and Table ), indicating a significant increase in the binding constants upon one‐electron oxidation . This is consistent with the increases in the density functional theory (DFT)‐calculated HOMO levels of FcB–CN − and FcB–F − compared with that of FcB (Figure a), as well as previous reports on the electrochemical responses of FcB and Fc boronic acid derivatives . Thus, conversion of the three‐coordinated boron to the four‐coordinated borate can change the relative electron‐donating abilities of the redox sites, with different guest anions serving to modulate Δ G 0 .…”

Enhancing Phosphorescence and Electrophosphorescence Efficiency of Cyclometalated Pt(II) Compounds with Triarylboron

“…Triarylboron compounds have recently emerged as an attractive and versatile class of optoelectronic materials. By harnessing the electron‐accepting nature of the boron center, researchers have successfully developed highly selective and sensitive chemical sensors,1–3 bright photo‐ and electroluminescent materials,4 and compounds with impressive nonlinear optical properties5 based on triarylboranes. Furthermore, these compounds can act as efficient electron‐transport materials due to stabilization of radical anions by the Lewis acidic boron atom, making them particularly attractive for use as bifunctional active layers for organic light‐emitting diodes (OLEDs) 4…”

Electrochemistry, Charge Transfer Properties, and Theoretical Investigation of a Macrocyclic Boronate Dimer of 1′,1‴-Biferrocenediboronic Acid and Related Ferrocenyl Boronate Complexes

“…This assignment is consistent with ferrocenium itself 26 and the boronate ester of B reported elsewhere. 27 The second band is broad, in the near-infrared (NIR) region, and is assigned to the IVCT transition between the Fc groups. Gaussian deconvolution of the IVCT transition band of Cester + gave the spectroscopic parameters of energy, intensity, and bandwidth at half-height (υ max = 4460 cm −1 , ε max = 752 M −1 cm −1 , Δυ 1/2 = 4260 cm −1 ).…”