Synthesis and Stability of Reactive Salts of Dodecafluoro-closo-dodecaborate(2−)

The synthesis and characterization of several salts of the B₁₂F₁₂^2- anion are reported. The potassium salt was prepared in 72% recrystallized yield by treating K₂B₁₂H₁₂ with liquid HF at 70 °C for 14 h and 20% F₂/N₂ in liquid HF at 25 °C for 72 h. The CPh₃⁺, N(n-Bu)₄⁺, NH(n-C₁₂H₂₅)₃⁺, NH₄⁺, and Li⁺ salts were prepared by metathesis reactions. The [NH(n-C₁₂H₂₅)₃]₂[B₁₂F₁₂] salt is soluble in aromatic hydrocarbon solvents. The B₁₂F₁₂^2- anion is remarkably stable. The salts Li₂B₁₂F₁₂ and [NH₄]₂[B₁₂F₁₂] were stable when heated to 450 and 480 °C, respectively. The B₁₂F₁₂^2- anion did not react with 98% H₂SO₄, 70% HNO₃, 3 M KOH, a 10-fold excess of Ce(NH₄)₂(NO₃)₆ in aqueous solution, or metallic sodium in THF. In addition, B₁₂F₁₂^2- did not react with metallic lithium in a mixture of ethylene carbonate and dimethyl carbonate, was not reduced at 0 V versus Li^+/0 in that solvent, and underwent a quasi-reversible oxidation at 4.9 V versus Li^+/0. The structure of [CPh₃]₂[B₁₂F₁₂] was determined by single-crystal X-ray diffraction:  tetragonal, space group I4₁/acd, a = 19.102(2), b = 19.102(2), c = 20.535(3) Å, V = 7492.2(2) ų, Z = 8, T = 173(2) K, R₁ = 0.064. The B₁₂F₁₂^2- anion weakly interacts with the two symmetry related CPh₃⁺ cations via F···C contacts of 3.087(2) Å, which are very close to the 3.17 Å sum of van der Waals radii for these two atoms. Taken together, the data suggest that B₁₂F₁₂^2- may be useful as a very robust weakly coordinating anion.