Fluoroborate Scaled Up, Ready For Inaction

As weakly coordinating anions go, B(CF₃)₄^– is a chemist’s dream. It superbly stabilizes partner cations, and with a sheath of nonreactive fluorine atoms surrounding the boron-carbon core, it just hangs around and does nothing—no other anion is more weakly coordinating. That property would make B(CF₃)₄^– a perfect electrolyte in lithium-ion batteries or counteranion for electrophilic cationic catalysts, except no one has been able to synthesize enough of the anion to see whether it can live up to its promise. German chemists Eduard Bernhardt and Helge Willner of Bergische University, in Wup­pertal, and Maik Finze of Julius Maximilians University, in Würzburg, thus set to work trying to solve this problem (Inorg. Chem., DOI: 10.1021/ic201319h). The researchers had previously found that simply exchanging CF₃ groups for CN or other groups wouldn’t work but that K[B(CF₃)₄] could be made by stepwise fluorination of K[B(CN)₄] using toxic and hard-to-handle ClF gas in aqueous HF. By studying the reaction mechanism and measuring reaction rates and thermodynamics, they determined how to safely control reaction conditions—using low ClF pressure at temperatures from –78 °C to 30 °C and stopping periodically to remove the Cl₂ and N₂ by-products. Their improved synthesis now allows them to make 60 g of K[B(CF₃)₄] at a time.