Protonation of Nitramines: Where does the Proton go
Journal article, 2017
The reactions of nitramine, N-methyl nitramine and N,N-dimethyl nitramine with anhydrous HF and the superacids HF/MF5 (M = As, Sb) were investigated at temperatures below -40 °C. In solution, exclusive O-protonation was observed by multinuclear NMR spectroscopy. Whereas no solid product could be isolated from the neat HF solutions even at -78 °C, in the HF/MF5 systems, protonated nitramine MF6- salts were isolated for the first time as moisture-sensitive solids that decompose at temperatures above -40 °C. In the solid state, depending on the counter-ion, O-protonated or N-protonated cations can be formed, in accord with theoretical calculations which show that the energy differences between O-protonation and N-protonation are very small. The [H2N-NO2H][AsF6], [H3N-NO2][SbF6], [MeHNNO2H][SbF6], and [Me2NNO2H][SbF6] salts were characterized by their X-ray crystal structures.