Dinitrogen Difluoride Chemistry. Improved Syntheses of cis- and trans-N2F2, Synthesis and Characterization of N2F+Sn2F9-, Ordered Crystal Structure of N2F+Sb2F11-, High-Level Electronic Structure Calculations of cis-N2F2, trans-N2F2, F2N=N, and N2F+, and Mechanism of the trans-cis Isomerization of N2F2

N2F+ salts are important precursors in the synthesis of N-5(+) compounds, and better methods are reported for their larger scale production. A new, marginally stable N2F+ salt, N2F+Sn2F9-, was prepared and characterized. An ordered crystal structure was obtained for N2F+Sb2F11-, resulting in the first observation of individual N N and N-F bond distances for N2F+ in the solid phase. The observed N N and N-F bond distances of 1.089(9) and 1.257(8) angstrom, respectively, are among the shortest experimentally observed N-N and N-F bonds. High-level electronic structure calculations at the CCSD(T) level with correlation-consistent basis sets extrapolated to the complete basis limit show that cis-N2F2 is more stable than trans-N2F2 by 1.4 kcal/mol at 298 K. The calculations also demonstrate that the lowest uncatalyzed pathway for the trans-cis isomerization of N2F2 has a barrier of 60 kcal/mol and involves rotation about the N = N double bond. This barrier is substantially higher than the energy required for the dissociation of N2F2 to N-2 and 2 F. Therefore, some of the N2F2 dissociates before undergoing an uncatalyzed isomerization, with some of the dissociation products probably catalyzing the isomerization. Furthermore, it is shown that the trans-cis isomerization of N2F2 is catalyzed by strong Lewis acids, involves a planar transition state of symmetry C-s, and yields a 9:1 equilibrium mixture of cis-N2F2 and trans-N2F2. Explanations are given for the increased reactivity of cis-N2F2 with Lewis acids and the exclusive formation of cis-N2F2 in the reaction of N2F+ with F. The geometry and vibrational frequencies of the F2N = N isomer have also been calculated and imply strong contributions from ionic N2F+ F- resonance structures, similar to those in F3NO and FNO.