Solid-state 1H and 13C NMR studies of novel ionic rotator-crystals of [NEt x Me(3−x)R][BEt4] (R = Pr, Bu. x = 1, 2)

Abstract

Abstract Four new ionic rotator-crystals of [NEt x Me(3−x)R][BEt4] (R = Pr, Bu; x = 1, 2) were observed. Rotator crystals (two-dimensional plastic crystals) are mesophases between solid and isotropic liquid phases. Solid-state 1H and 13C nuclear magnetic resonance (NMR) measurements revealed that the ellipsoidal cations of [NEt x Me(3−x)R]+ undergo uniaxial rotation about their N–R axis and libration motion of the axis, and the anions perform isotropic reorientations in the highest-temperature solid-phase (rotator phase). Differential scanning calorimetry (DSC) measurements showed small entropy changes of 8–11 J K−1 mol−1 at the melting point of the compounds. These results suggest that the cations and anions have large degrees of freedom of motion in the rotator phase. The diffraction patterns of X-ray diffraction (XRD) could be indexed to the trigonal structure (space group of P31c). Compared with the reported data for [NEt x Me(3−x)Pr][BEt3Me] (x = 1, 2) compounds, which also have rotator-crystal phases and transform to a plastic crystalline phase, a model that explains why the cations of [BEt4] salts hardly perform isotropic reorientation in the solid phases was proposed.