Direct Perfluorination of K2B12H12 in Acetonitrile Occurs at the Gas Bubble-Solution Interface and Is Inhibited by HF. Experimental and DFT Study of Inhibition by Protic Acids and Soft, Polarizable Anions

During the optimization of the F-2 perfluorination of H2B12H12 in acetonitrile with continuous bubbling of 20/80 F-2/N-2, it was discovered that (i) HF and other protic acids inhibit each of the 12 fluorination steps (in contradiction to recently published findings) and (ii) the fluorinations appear to take place at the gas bubble-solution interface. Experimental results and DFT calculations suggest that these two phenomena are related by the relative propensities of the various B12H12-xFx2- anions to partition from bulk solution to the interface (i.e., their relative polarizabilities or softness; 0 <= x <= 12). Relative to the previously reported syntheses of K2B12F12 or Cs2B12F12, the new optimized procedure has the following advantages: (i) the scale was increased 10-fold without sacrificing yield (74% for K2B12F12, 76% for Cs2B12F12) or purity (99.5+%); (ii) the reaction/purification time was decreased from ca. 1 week to 2 days; and (iii) the solvent was changed from anhydrous HF to acetonitrile, allowing ordinary glassware to be used. The anhydrous salt Cs2B12F12 was found to be thermally stable up to 600 degrees C.