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

X-ray crystallographic investigation of the tertiary structure of simple 1-methylimidazolium (1-Meim) salts reveals that cation-cation face-to-face pi-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 angstrom. The two imidazolium rings are slipped to the extent that the interaction can be regarded as a manifestation of C-H center dot center dot center dot C-H dipole interactions. 1-Meim-NO3 exists as a one-dimensional (1-D) polymer with interplanar separations of 3.65 angstrom. 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 pi-pi stacking in both 1 and 2. Crystal data: 1-Meim-CF3SO3, 1, triclinic, P (1) over bar, a = 6.416(3) angstrom, b = 7.617(4) angstrom, c = 9.569(4) angstrom, alpha = 85.36(4)degrees, beta = 86.08(3)degrees, gamma = 85.18(4)degrees, V = 463.6 (4) angstrom,(3) Z = 2, D-c = 1.66 g cm(-3), mu = 3.7 cm(-1), T = 17 degrees C, R = 0.054 and R-w = 0.076 for 1241 reflections; 1-Meim-NO3, 2, monoclinic, P2(1)/c, a = 9.009(7) angstrom, b = 9.988(6) angstrom, c = 7.308(5) angstrom, beta = 94.93(6)degrees, V= 655.2(8) angstrom,(3) Z = 4, D-c = 1.47 g cm(-3), mu = 1.2 cm(-1), T = 17 degrees C, R = 0.060 and R-w = 0.068 for 483 reflections.