Sulfonium Boranes for the Selective Capture of Cyanide Ions in Water

Abstract

Owing to the presence of low lying s* orbitals, sulfonium ions are inherently Lewis acidic and can interact with electron-rich substrates to form donor-acceptor complexes. Although this phenomenon has been documented, [1] efforts to use sulfonium ions as a binding site in Lewis acidic hosts have not been reported. As part of our fundamental interest in the chemistry of polydentate Lewis acidic boranes, [2] we have become interested in probing the synthesis and properties of anion receptors containing accessible sulfonium ions. As an added motivation for these studies, we anticipated that the anion binding properties of sulfonium boranes would also benefit from attractive Coulombic effects similar to those occurring in other cationic boron-based anion receptors. [3,4] To test the validity of the aforementioned concepts, we synthesized the cationic boranes (Figure 1).[9]Despite the similarity of this boron-sulfur separation, the two cationic boranes differ by the respective disposition of the boryl and sulfonio moieties which are oriented in a more convergent fashion in [1] + . A natural bond orbital (NBO) analysis carried out at the density functional theory (DFT) optimized geometry of [1] + indicates the presence of a lp(S)! p(B) donor-acceptor interaction (Figure 1) whose deletion leads to an increase of the total energy of the molecule by E del = 6.8 kcal mol À1 . Therefore, in addition to being more sterically crowded, the boron center of [1] + experiences electron donation from the neighboring sulfur atom, a phenomenon which would be expected to reduce its electron deficiency. Analogous computations on [2] + show that this lp(S)!p(B) interaction (Figure 1), although present, is much weaker (E del = 2.3 kcal mol À1 ), which is in agreement with the more divergent orientation of the lp(S) and p(B) orbitals. Altogether, this analysis shows that [2] + features a more accessible and electron-deficient boron atom.Next, we decided to compare the Lewis acidity of these boranes by studying their behavior in aqueous solution as a function of pH. Since hydroxide binding to the boron center is expected to interrupt the p conjugation mediated by the vacant p-orbital on the boron atom, [10] we monitored the absorbance of the boron-centered chromophore as a function of pH in MeOH/H 2 O (5:95 v/v; see the Supporting Information) and observed that [1] + and [2] + are stable up to pH 9.5 and 7.0, respectively. These experiments indicate that [2] + is more acidic than [1] + by at least two orders of magnitudes.

The Future of Main Group Chemistry

“…[123] In the European Union (EU) the corresponding limit for cyanide is 50 ppb (1.9 mM). [124] A successful strategy for meeting these stringent requirements involves Lewis acidic boron-based hosts that incorporate both acceptor sites and cationic ammonium, phosphonium, or sulfonium groups to provide water solubility. For example, the phosphonium borane 46 binds fluoride ions below the EPA contaminant level and provides a basic design for molecules that could be used in drinking water analysis.…”

A class of fascinating optoelectronic materials: Triarylboron compounds

“…Owing to the presence of low lying * orbitals, sulfonium ions are inherently Lewis acidic and could interact with electron-rich substrates to form donor-acceptor complexes. Gabbaï et al [55] designed a CN  receptor 66 (Figure 18) containing accessible sulfonium ions. Addition of 50 ppb of CN  to a 4.0 M solution of 66 in pure water resulted in a 33% quenching of the fluorescence after one hour.…”

Rapid visual detection of blood cyanide

“…Many chemosensors have been recently developed for the detection of cyanide, [25][26][27][28] but only a few work in biological samples.…”