Interaction of copper with dinitrogen tetroxide in 1-butyl-3-methylimidazolium-based ionic liquids

Abstract

Ionic liquids that are stable toward oxidation and nitration and are based on the 1-n-butyl-3-methylimidazolium cation (BMIm⁺) can be used as solvents and reaction media for copper dissolution in liquid dinitrogen tetraoxide N₂O₄. The ionic liquid not only favors the dissociation of N₂O₄ into NO⁺ and NO₃⁻, but also takes part in the formation of different crystalline products. Thus, NO[BF₄], NO[Cu(NO₃)₃] and (BMIm)₂[Cu₂(CF₃COO)₆] were prepared using (BMIm)A, A⁻ = [BF₄]⁻, (CF₃SO₂)₂N⁻, CF₃COO⁻, respectively. The formation of a certain product is determined by the nature of the anion A⁻ and the relative solubility of the reaction products in the ionic liquid. Crystals of NO[BF₄] were also prepared directly from a mixture of N₂O₄ and BMImBF₄. According to XRD single-crystal structure analysis, the structure of NO[BF₄] consists of tetrahedral [BF₄]⁻ anions and nitrosonium NO⁺ cations; the formation of these ions prove the heterolytic dissociation of N₂O₄ dissolved in the ionic liquid. The crystal structure of the earlier unknown binuclear copper trifluoroacetate (BMIm)₂[Cu₂(CF₃COO)₆] were determined by X-ray diffraction. The peculiarity of this dimer compared to the majority of known dimeric copper(II) carboxylates is the unusually long Cu⋯Cu distance (3.15 Å), with Cu(II) ions demonstrating an atypical coordination of a distorted trigonal bipyramid formed by five O atoms of five trifluoroacetate groups.