Molecular structure and hydrolytic stability amidinium salts derived from triazatricyclo[5.2.1.04,10]decane

Five amidinium salts have been prepared from triazatricyclo[5.2.1.04,10]decane (tacnoa) and characterised by mass spectrometry, NMR spectroscopy and X-ray crystallography. The X-ray structures revealed a long distance between the methine carbon and the ammonium nitrogen, viz., C–N distance 1.64–1.70 Å, cf. other C–N distances of 1.40–1.50 Å. An NMR study of 1-ethyl-4,7-diaza-1-azoniatricyclo[5.2.1.04,10]decane and 1-benzyl-4,7-diaza-1-azoniatricyclo[5.2.1.04,10]decane, confirmed that these amidinium salts hydrolyse in aqueous solution, the latter 60 times faster than the former. Tacnoa, which has C–N distances typical of single bonds, showed no evidence of hydrolysis after several days at 80 °C. Molecular modeling calculations indicate that the preferred gas phase structure of the salts is one where the positive charge is delocalised over the two secondary amines and the methine carbon. The calculated distance between this carbon and the ammonium nitrogen is 0.15–0.4 Å longer than in the crystal structure. The energy difference between the preferred gas phase and solid state conformations is 2 kJ mol−1 and presents little barrier to nucleophilic attack of the methine carbon. Further analysis of the methine carbon geometry (C(7)) reveals that the bond angles in the benzyl salt are closer to those expected for an sp2 centre than in the ethyl salt and that this could be the origin of the faster hydrolysis rate.