Crystalline organization and bonding of N-perfluoroacylpyridiniumaminides

N-Perfluoroacylpyridiniumaminides are stable crystalline solids characterized by a potentially useful set of properties. The molecular geometries resulting from the determination of the crystal structures of three members of the series suggest that, due to the presence of the perfluoroalky chains, the electron density on the formally negatively charged nitrogen of the ylide system is reduced. The N-N and the OC-N bonds in the systems are shortened and the whole molecules are stabilized as compared to non-fluorinated N-alkanoylpyridiniumaminides. The packing in the crystals can be qualitatively understood as a compromise between two distinct effects: 1. the tendency to segregate, of the fluorocarbon groups on one hand and the bipyridinium blocks on the other, as in the case of 4b or c, 2. the formation of weak interactions involving the pyridinium hydrogens and the most effective hydrogen bond accepting atoms (i.e. O and N). The latter effect leads to disorder of the perfluoroalkyl chains in 4a. In 4b and c, even at room temperature, a surprisingly ordered arrangement of the segregated perfluoropropyl chains is observed, showing a clear tendency to helicoidal conformation related to fluorine-fluorine repulsion. (C) 1999 Elsevier Science S.A. All rights reserved.