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

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 CuCu 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.