It is possible that fluorous compounds could be utilized as directing forces in crystal engineering for applications in materials chemistry or catalysis. Although numerous fluorous compounds have been used for various applications, their structures in the solid state remains a lively matter for debate. The reaction of 4-[(2,2,2-trifluoroethoxy)methyl]pyridine with HX (X = I or Cl) yielded new fluorous ponytailed pyridinium halide salts, namely 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium iodide, C8H9F3NO+·I-, (1), and 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium chloride, C8H9F3NO+·Cl-, (2), which were characterized by IR spectroscopy, multinuclei (1H, 13C and 19F) NMR spectroscopy and single-crystal X-ray diffraction. Structure analysis showed that there are two types of hydrogen bonds, namely N-H...X and C-H...X. The iodide anion in salt (1) is hydrogen bonded to three 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium cations in the crystal packing, while the chloride ion in salt (2) is involved in six hydrogen bonds to five 4-[(2,2,2-trifluoroethoxy)methyl]pyridinium cations, which is attributed to the smaller size and reduced polarizability of the chloride ion compared to the iodide ion. In the IR spectra, the pyridinium N-H stretching band for salt (1) exhibited a blue shift compared with that of salt (2).
Keywords: IR spectroscopy; bifurcated hydrogen bonding; blue shift; crystal structure; fluorous ponytailed pyridine; hydrogen halides; synthon.