Alkoxy substituted halogenated closo-dodecaborates as anions for ionic liquids

Abstract

Alkoxy substituted halogenated closo -dodecaborate anions [B 12 X 11 OR] 2− (X = Cl, Br; R = alkyl) have been synthesized and fully characterized. Thermal analyses were carried out to reveal their properties for possible applications in ionic liquids. The new salt [C 6 mim] 2 [B 12 Cl 11 O-propyl] is an elusive example of an ionic liquid containing a doubly charged anion. Halogenated and alkoxylated closo -dodecaborates [B 12 X 11 OR] 2− (X = Cl, Br; R = propyl, octyl, dodecyl) have been synthesized by halogenation of the known [B 12 H 11 OH] 2− anion followed by alkylation in the superbasic medium DMSO/KOH. The obtained sodium salts were transformed by simple metathesis reactions in aqueous solution to the tetrabutylammonium ([NBu 4 ] + ) and 1-hexyl-3-methylimidazolium ([C 6 mim] + ) salts. All compounds were fully characterized by heteronuclear NMR, IR and Raman spectroscopy, ESI mass spectrometry, and thermal analytical measurements. Selected anions were also structurally characterized as their [Ph 4 P] + salts by single crystal X-ray diffraction. The [C 6 mim] + salts are thermally stable up to more than 300 °C and show clear melting points. Surprisingly, the compound [C 6 mim] 2 [B 12 Cl 11 O-propyl] having the short propyl group bound to the boron cluster shows the lowest melting point (96 °C) of all the investigated compounds. Thus this compound is a rare member of the class of ionic liquids consisting of dianions.