Manifestations of noncovalent interactions in the solid state. Dimeric and polymeric self-assembly in imidazolium salts via face-to-face cation—cation π-stacking

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Abstract
X-ray crystallographic investigation of the tertiary structure of simple 1-methylimidazolium (1-Meim) salts reveals that cation—cation face-to-face π—stacking with interplanar separations in the range typically seen for molecule—molecule and molecule—cation interactions are possible. Two salts are reported. 1-Meim-CF3SO3, 1, exists as a centrosymmetric dimer with an interplanar separation of only 3.16 Å. The two imidazolium rings are slipped to the extent that the interaction can be regarded as a manifestation of C—H…C—H dipole interactions. 1-Meim-NO3 exists as a one-dimensional (1-D) polymer with interplanar separations of 3.65 Å. The cations are not as severely slipped as for 1 and the interactions can be regarded as the result of cation—cation and anion—anion complementary electrostatics. Semi-empirical calculations are used to rationalize the π-π stacking in both 1 and 2. Crystal data: 1-Meim-CF3SO3, 1, triclinic, P1, a=6.416(3) Å, b=7.617(4) Å, c=9.569(4) Å, α=85.36(4)°, β=86.08(3)°, γ=85.18(4)°, V=463.6(4) Å,3 Z=2, Dc =1.66 g cm−3, μ=3.7 cm−1, T=17°C, R=0.054 and R w=0.076 for 1241 reflections; 1-Meim-NO3, 2, monoclinic, P21/c, a=9.009(7) Å, b=9.988(6) Å, c=7.308(5) Å, β=94.93(6)°, V=655.2(8) Å,3 Z=4, Dc =1.47 g cm−3, μ=1.2 cm−1, T=17°C, R=0.060 and R w=0.068 for 483 reflections.