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

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. l-Meim-CF₃SO₃, 1, exists as a centrosymmetric dimer with an interplanar separation of only 3.16 A. 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-N0₃ exists as a one-dimensional (1-D) polymer with interplanar separations of 3.65 A. 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: l-Meim-CF₃SO₃, 1, triclinic, PI, a = 6.416(3) A, b = 7.617(4) A, c = 9.569(4) A, a = 85.36(4)°, ß = 86.08(3)°, γ = 85.18(4)°, V = 463.6(4) A,³Z = 2, D_c= 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-N03, 2, monoclinic, P2₁/c, a = 9.009(7) A, b = 9.988(6)A, c = 7.308(5) A, ß = 94.93(6)°, V= 655.2(8) A,³Z = 4, D_c = 1.47 g cm^-3, µ = 1.2 cm ^-1 = 17°C, R = 0.060 and R_w = 0.068 for 483 reflections.