Dinitrogen difluoride

Dinitrogen difluoride is a chemical compound with the formula N2F2. It is a gas at room temperature, and was first identified in 1952 as the thermal decomposition product of the fluorine azide (FN3). It has the structure F\sN\dN\sF and exists in both cis and trans isomers, as typical for diimides.

Isomers
The cis isomer has C2v symmetry and the trans isomer has C2h symmetry. These isomers can interconvert, but the process is slow enough at low temperature that the two can separated by low-temperature fractionation. The trans isomer is less thermodynamically stable but can be stored in glass vessels. The cis isomer attacks glass over a time scale of about 2 weeks to form silicon tetrafluoride and nitrous oxide:


 * 2 N2F2 + SiO2 → SiF4 + 2 N2O

Preparation
Most preparations of dinitrogen difluoride give mixtures of the two isomers, but they can be prepared independently.

An aqueous method involves N,N-difluorourea with concentrated potassium hydroxide. This gives a 40% yield with three times more of the trans isomer.

Difluoramine forms a solid unstable compound with potassium fluoride (or rubidium fluoride or caesium fluoride) which decomposes to dinitrogen difluoride.

It can also be prepared by photolysis of tetrafluorohydrazine and bromine:


 * N2F4 $$ N2F2 + byproducts

Reactions
The cis form of difluorodiazene will react with strong fluoride ion acceptors such as antimony pentafluoride to form the linear [N\tN\sF]+ cation (fluorodiazonium cation ) which forms a salt with the formula [N\tN\sF]+[SbF6]− (fluorodiazonium hexafluoroantimonate(V)).


 * F\sN\dN\sF + SbF5 → [N\tN\sF]+[SbF6]−

Analogous reaction of cis-difluorodiazene with arsenic pentafluoride gives white solid salt with the formula [N\tN\sF]+[AsF6]− (fluorodiazonium hexafluoroarsenate(V)).


 * F\sN\dN\sF + AsF5 → [N\tN\sF]+[AsF6]−

In the solid phase, the observed N\tN and N\sF bond distances in the [N\tN\sF]+ cation are 1.089(9) and 1.257(8) Å respectively, among the shortest experimentally observed N-N and N-F bonds.