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

Abstract

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 Fc0/+ 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.