Efficient direct fluorination of the B12H11(NH3)- anion in acetonitrile and comparison of the structures of Na(H2O)4(B12F11(NH3)), Na(H3O) (H2O)3(B12F12), and Na2(H2O)4(B12F12)

An efficient procedure for the direct per-B-fluorination of the potassium salt of the mono-ammonioborane anion B12H11(NH3)(-) in anhydrous CH3CN with 20:80 F-2:N-2 is reported. It differs significantly from the previously reported procedure in that anhydrous CH3CN (a-CH3CN) replaced anhydrous HF (a-HF) as the solvent and the yield of recrystallized K(B12F11(NH3)) increased from 40% to 70%. It also differs from the published procedure for the direct per-B-fluorination of K-2(B12H12) to form K-2(B12F12) in CH3CN containing 2% H2O (wet CH3CN) in that (i) a-CH3CN was used instead of wet CH3CN and (ii) addition of excess KF at the start of the reaction to sequester the HF byproduct was deleterious in the case of K(B12H11(NH3)) fluorination, not advantageous as in the case of K-2(B12H12) fluorination. The addition of excess KF resulted in decomposition of B12H11-nFn(NH3)(-) intermediates as the average value of n approached 11, and even small amounts of K(B12F11(NH3)) free of incompletely fluorinated K(B12HF10(NH3)) could not be isolated. In contrast to the direct fluorination of K-2(B12H12) in wet CH3CN, which is inhibited by the buildup of HF, the fluorinations of both K(B12H11(NH3)) and K-2(B12H12) in a-CH3CN were not inhibited by HF. A competition experiment in which approximatetly equimolar amounts of K(B12H11(NH3)) and K-2(B12H12) were fluorinated together in a-CH3CN showed that BH vertex fluorination was ca. 50% faster for B12H122- than for B12H11(NH3)(-). The single-crystal X-ray structures of Na(H2O)(4)(B12F11(NH3)) and Na(H3O)(H2O)(3)(B12F12) were determined. They both contain trans-Na(H2O)(2)F-4 moieties, with ranges of NaO and NaF distances of 2.331(1)2.339(1) and 2.264(1)2.456(1) angstrom, respectively, in contrast to the cis-Na(H2O)(4)F-2 moieties in the published structure of Na-2(H2O)(4)(B12F12), with ranges of NaO and NaF distances of 2.368(9)2.473(9) and 2.313(8)2.381(7) angstrom, respectively. The O atoms of the nearly-square array of four hydrogen-bonded H2O molecules in Na(H2O)(4)(B12F11(NH3)) or the H3O+ ion and three H2O molecules in Na(H3O)(H2O)(3)(B12F12) are co-planar to within 0.016 angstrom and have O center dot center dot center dot O distances of 2.742.83 angstrom and O center dot center dot center dot O center dot center dot center dot O angles of 80-100 degrees.