The N2F+ cation. An unusual ion containing the shortest presently known nitrogen-fluorine bond

Article abstract of DOI:10.1021/ja00010a023

The N₂F⁺AsF₆^- salt was prepared in high yield from trans-N₂F₂ by thermal trans-cis isomerization in the presence of AsF₅ at 70°C. A displacement reaction between N₂F⁺AsF₆^- and FNO yields exclusively cis-N₂F₂. The Lewis acids BF₃ and PF₅ do not form a stable adduct with cis-N₂F₂ at temperatures as low as -78°C and do not catalyze the N₂F₂ trans-cis isomerization. A semiempirical molecular orbital model is used to explain the puzzling differences in the reaction chemistry of cis- and trans-N₂F₂. The crystal structure of N₂F⁺AsF₆^- (monoclinic, C2/m, a = 9.184 (5) A?, b = 5.882 (2) A?, c = 5.160 (2) A?, β = 90.47 (4)°, Z = 2) was determined. Alternate space groups (Cm and C2) can be rejected on the basis of the observed vibrational spectra. Since in C2/m the N₂F⁺ cations are disordered, only the sum of the N-F and N-N bond distances could be determined from the X-ray data. Local density functional calculations were carried out for N₂F⁺ and the well-known isoelectronic FCN molecule. The results from these calculations allowed the sum of the N₂F⁺ bond lengths to be partitioned into the individual bond distances. The resulting N-F bond length of 1.217 A? is by far the shortest presently known N-F bond, while the N-N bond length of 1.099 A? is comparable to the shortest presently known N-N bond length of 1.0976 (2) A? in N₂. The surprising shortness of both bonds is attributed to the high s-character (sp hybrid) of the σ-bond orbitals on nitrogen and the formal positive charge on the cation. Thus, the shortening of the N-F bond on going from sp³-hybridized NF₄⁺ (1.30 A?) to sp-hybridized N₂F⁺ (1.22 A?) parallels those found for the C-H and C-F bonds in the CH₄, CH₂=CH₂, CH≡CH and CF₄, CF₂=CF₂, FC≡N series, respectively. The oxidative power of N₂F⁺ has also been studied. The N₂F⁺ cation oxidized Xe and ClF to XeF⁺ and ClF₂⁺, respectively, but did not oxidize ClF₅, BrF₅, IF₅, XeF₄, NF₃, or O₂.