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

The partly halogenated and N-alkylated closo-dodecaborates [B ₁₂ Cl ₆ H ₅ N(propyl) ₃ ] ^- and [B ₁₂ Br ₆ H ₅ NR ₃ ] ^- (R = ethyl-pentyl) were prepared by alkylation of [B ₁₂ H ₁₁ NH ₃ ] ^- and subsequent halogenation with elemental chlorine or N-bromosuccinimide. Simple metathesis reactions yielded the [HNMe ₃ ] ⁺ , [C ₆ mim] ⁺ , [NBu ₄ ] ⁺ , 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 [HNMe ₃ ][B ₁₂ Br ₆ H ₅ N(ethyl) ₃ ]·CH ₃ CN, [HNMe ₃ ][B ₁₂ Br ₆ H ₅ N(propyl) ₃ ], Na[B ₁₂ Br ₆ H ₅ N(butyl) ₃ ], and [HNMe ₃ ][B ₁₂ Cl ₇ H ₄ N(propyl) ₃ ]·CH ₃ CN were determined by single-crystal X-ray diffraction. The [C ₆ mim] ⁺ salts are thermally stable to temperatures higher than 300 °C. The melting points are between 57 and 80 °C, which classify the [C ₆ mim] ⁺ salts of [B ₁₂ Cl ₆ H ₅ N(propyl) ₃ ] ^- and [B ₁₂ Br ₆ H ₅ NR ₃ ] ^- (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 CH ₂ Cl ₂ 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[B ₁₂ Br ₆ H ₅ N(pentyl) ₃ ] in dichloromethane was determined. In addition, the trialkylation of the perchlorinated anion [B ₁₂ Cl ₁₁ NH ₃ ] ^- 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 [PPh ₄ ] ₂ [B ₁₂ Cl ₁₁ N(propargyl) ₂ ] was determined.