Protonation of Nitramines: Where Does the Proton Go?

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 counterion, 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 salts [H₂ N-NO₂ H][AsF₆ ], [H₃ N-NO₂ ][SbF₆ ], [MeHNNO₂ H][SbF₆ ], and [Me₂ NNO₂ H][SbF₆ ] were characterized by their X-ray crystal structures.