Borane Compound Turns Off When Sensing Cyanide

A sulfonium borane compound can sense the presence of cyanide ions in water at concentrations less than 1 ppm, an important development for efforts to test drinking water for the poison, reports a group led by François P. Gabbaï of Texas A&M University (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200901275). The compound contains adjacent sulfonium and dimesitylboryl moieties connected by an o-phenylene linker. The trigonal planar boron center is coordinatively unsaturated and mediates the π-conjugation of its aromatic ligands, resulting in a fluorescent blue chromophore. When CN^- is present in solution, it latches onto the boron and quenches the fluorescence. Structural analysis of the B-CN adduct reveals that the sulfonium group stabilizes cyanide through donor-acceptor and Coulombic interactions. The researchers found that the compound's fluorescence is almost completely quenched within an hour for 200 ppb CN, the maximum contaminant level for drinking water set by EPA, and 33% quenched after an hour for 50 ppb CN^-, the maximum contaminant level for drinking water in the European Union. The sulfonium borane "is one of the rare molecular systems competent for cyanide sensing at the sub-parts-per-million level in water," the researchers write.