The structural chemistry of energetic compounds containing geminal-difluoramino groups

This report discusses the structural chemistry of four molecules containing geminal-difluoramine (NF2) groups. They are: 1,1,4,4-tetrakis(difluoramino)cyclohexane (1), monoclinic, P21/n, a=6.277(1), b=10.502(2), c=7.910(1) Å, β=99.50(1)°; 1-{3-[5,5-bis(difluoramino)-2-oxopyrrolidinyl]-2-oxopropyl}pyrrolidine-2,5-dione (2), monoclinic, P21/c, a=12.994(1), b=8.4799(8), c=13.142(1) Å, β=102.998(7)°; 1-[2,2-bis(difluoramino)propyl]-pyrrolidine-2,5-dione (3), monoclinic, P21, a=6.488(1), b=9.508(2), c=8.612(1) Å, β=105.52(1)°; and 2-[2,2-bis(difluoramino)propyl]isoindoline-1,3-dione (4), orthorhombic, Pbca, a=15.718(1), b=7.0818(5), c=22.039(2) Å. The pyramidal difluoramine group sometimes displays extensive torsional disorder in crystals, but in each of the structures reported here, the geminal-NF2 groups are primarily ordered, due to interactions with their steric environments. There are increases in the thermal parameters of the fluorine atoms indicating libration in some cases, where there are fewer steric demands in the NF2 environment. The conformation adopted by each NF2 group can be explained in terms of minimization of non-bonded F⋯H and F⋯F contacts. All NF2 groups are nonplanar and show unusual metrical parameters compared to amines containing no fluoro substituents. The interior FNF and CNF angles are very small, indicating a high degree of pyramidalization. This type of behavior has been attributed to pπ–s resonance and electrostatic effects.