Weak hydrogen bonding and fluorous interactions in the chloride and bromide salts of 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium

Neutralization of 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridine with hydrohalo acids HX (X = Cl and Br) yielded the pyridinium salts 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium chloride, C₉ H₁₀ F₄ NO⁺ ·Cl^-, (1), and 4-[(2,2,3,3-tetrafluoropropoxy)methyl]pyridinium bromide, C₉ H₁₀ F₄ NO⁺ ·Br^-, (2), both carrying a fluorous side chain at the para position of the pyridinium ring. Single-crystal X-ray diffraction techniques revealed that (1) and (2) are isomorphous. The halide anions accept four hydrogen bonds from N-H, ortho-C-H and CF₂-H groups. Two cations and two anions form a centrosymmetric dimeric building block, utilizing complimentary N-H..X..H-Csp³ connections. These dimers are further crosslinked, utilizing another complimentary Csp 2-H..X..H-Csp₂ connection. The pyridinium rings are-stacked, forming columns running parallel to the a axis that make angles of ca 44-45° with the normal to the pyridinium plane. There are also supramolecular C-H..F-C interactions, namely bifurcated C-H..F and bifurcated C-F..H interactions; additionally, one type II C-F..F-C halogen bond has been observed.