X-ray and 2H NMR studies of structure and dynamics in the Hofmann-type and the Hofmann-(en)-type pyrrole clathrates

Structure and dynamics in the Hofmann and Hoffmann-en type pyrrole clathrates, Cd(NH₃)₂Ni(CN)₄·2C₄H₄NH and Cd(en)Ni(CN)₄·2C₄H₄NH, have been studied by single crystal X-ray crystallography, solid state ²H NMR, PXRD, TGA and DSC. Both show in-plane reorientation of the pyrrole in an inequivalent six-site potential, with two low energy minima where the N of the pyrrole is oriented up or down with respect to the crystal c axis, followed by out-of-plane 4-fold motion about an axis parallel to the crystal c axis. There is also evidence for an intermediate dynamic state where there is 2-fold rather than 4-fold motion. The NH₃ or en groups of the host frameworks rotate about the crystal c direction. The Hofmann-en pyrrole clathrate shows two phase transitions at 196 and 259 K, and the dynamics of the host en and pyrrole guest appear to be intimately linked to the latter structural transition. The results for these pyrrole clathrates have been compared with those for the two equivalent benzene clathrates, obtained previously and augmented with new ²H NMR lineshape data for Cd(en)Ni(CN)₄·2C₆D₆ above 300 K.