Chemists discover a safe, green method to process red phosphorus

When it comes to making phosphorus compounds, chemists have traditionally relied on white phosphorus, P₄, a tetrahedral-shaped allotrope of the element. The downside with white phosphorus is that it’s toxic and flammable. Red phosphorus, an air-stable amorphous oligomeric allotrope, is a safer alternative. But chemists have had difficulty processing the relatively inert material in large quantities without resorting to high temperature and strong reducing agents. Florida State University chemists have now solved that problem by discovering an easy way to convert red phosphorus to soluble polyphosphides (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201511186). Alina Dragulescu-Andrasi, a postdoctoral researcher in Michael Shatruk’s group, explained in San Diego how the team simply passes a solution of inexpensive potassium ethoxide in an organic solvent through red phosphorus under mild heating to produce P₅^–, P₁₆^2–, and P₂₁^3–. These variously sized clusters, which the researchers isolate as potassium or tetrabutylammonium salts, could be used to synthesize phosphorus compounds or to make two-dimensional semiconductors and lithium-ion battery anodes. Taking the process a step further, the researchers adapted it to run as a continuous-flow reaction by passing potassium ethoxide through a stainless steel column packed with red phosphorus, generating multigram amounts of the soluble polyphosphides. “This appears to be a relatively safe and convenient methodology for generating soluble salts of polyphosphide anions,” commented MIT’s Christopher C. Cummins, who builds new compounds from elemental phosphorus. “It should open the door to more widespread study and application of these interesting little bits of reduced phosphorus.”