Multidentate Lewis acids: synthesis, structure and mode of action of a redox-based fluoride ion sensorElectronic supplementary information (ESI) available: details of NMR and UV/Vis spectroscopic measurements. See http://www.rsc.org/suppdata/cc/b2/b200828a/

Abstract

The mode of action of the bidentate bis(boronate) Lewis acid 2 as a fluoride ion sensor is shown to involve selective anion binding together with an electrochemical response.

Electrochemical Detection

“…The stability constant of 700 M −1 for the fluoride-ferrocenium complex (determined electrochemically) is due to the favorable interaction between boron and F − , a hard acid and a hard base, respectively. Aldridge et al have demonstrated that a similar boryl-ferrocene 47 acts as a redox-activated colorimetric sensor for F − [46]. When F − was added to a CH 2 Cl 2 solution of 47 under aerobic conditions, a color change from orange to pale green was observed.…”

Synthesis and oxidation of hexafluoroisopropoxyborylferrocenes

“…The main product of the reaction was ferrocene. We suggest that compound 2 was formed, but that the B-C bond was cleaved by the HBr released giving ferrocene and the volatile alkoxybromoborane Br-B(Ohfip) 2 . To prevent the formation of HBr, the alcohol was replaced by the alkoxide Li[Ohfip] [37].…”

“…The solvents were dried by using conventional drying agents and directly distilled. NMR spectra were obtained in d 2 -methylene chloride at room temperature on a BRUKER AVANCE II + 400 spectrometer. 1 H and 13 C chemical shifts are given with respect to TMS, 19 F NMR spectra to fluorotrichloromethane, and 11 B NMR spectra to the boron-trifluoride-diethyl-ether-complex.…”

“…The electron-withdrawing effect of the polyfluorinated groups on the B-O π-bonding is probably responsible for the increase of the dip angle α* of 2.2º if compared to FcB(OMe) 2 . The same trend is observed when studying the Fe-B distance of 3.126(1) Å (FcB(Me) 2 = 3.008(3) Å, FcB(OH)(Me) = 3.056(3) Å, and FcB(OMe) 2 = 3.179(3) Å) [28].…”

“…The main advantage of the ferrocene backbone can be assigned to its electrochemical properties paired with the chemical and thermal inertness. Thus, ferrocenylboranes function, for example, as anion sensors for selective fluoride or hydrogen fluoride detection [1][2][3][4]. Owing to the relatively weak Lewis acidity of dialkoxyborylferrocenes, their fluoride binding is weak and recognition by NMR or electrochemical monitoring is possible even in the presence of other potentially competing anions.…”