Improving the Solubility of Halogenated 1-Ammonio-closo-dodecaborate Anions

The partly halogenated and N-alkylated closo-dodecaborates [B₁₂Cl₆H₅N(propyl)₃]^- and [B₁₂Br₆H₅NR₃]^- (R = ethyl-pentyl) were prepared by alkylation of [B₁₂H₁₁NH₃]^- and subsequent halogenation with elemental chlorine or N-bromosuccinimide. Simple metathesis reactions yielded the [HNMe₃]⁺, [C₆mim]⁺, [NBu₄]⁺, and Na⁺ salts, which were characterized by heteronuclear NMR and IR spectroscopy as well as electrospray ionization mass spectrometry. The crystal structures of the salts [HNMe₃][B₁₂Br₆H₅N(ethyl)₃]·CH₃CN, [HNMe₃][B₁₂Br₆H₅N(propyl)₃], Na[B₁₂Br₆H₅N(butyl)₃], and [HNMe₃][B₁₂Cl₇H₄N(propyl)₃]·CH₃CN were determined by single-crystal X-ray diffraction. The [C₆mim]⁺ salts are thermally stable to temperatures higher than 300 °C. The melting points are between 57 and 80 °C, which classify the [C₆mim]⁺ salts of [B₁₂Cl₆H₅N(propyl)₃]^- and [B₁₂Br₆H₅NR₃]^- (R = propyl-pentyl) as ionic liquids. The anions are oxidized only at potentials higher than 2 V versus Fc^0/+ as determined by cyclic voltammetry. The solubility of the sodium salts in CH₂Cl₂ solution was determined by NMR spectroscopy. With the increasing length of the alkyl chain attached to the ammonio group the solubility is significantly enhanced. A solubility up to 125 mmol/L for Na[B₁₂Br₆H₅N(pentyl)₃] in dichloromethane was determined. In addition, the trialkylation of the perchlorinated anion [B₁₂Cl₁₁NH₃]^- was investigated in detail. A Hofmann elimination was observed to occur at higher temperatures, when alkyl groups with β-hydrogen atoms were introduced. Organic substituents without β-hydrogen atoms gave more stable compounds; however, trialkylation proved to be difficult presumably due to steric hindrance. The crystal structure of the byproduct [PPh₄]₂[B₁₂Cl₁₁N(propargyl)₂] was determined.