REVERSIBLE PHASE TRANSITION IN CRYSTALS WITH A SUPRAMOLECULAR AGGREGATE. Isabel Pascual(2), Larry R. Falvello(1), Arthur J. Schultz(2), Dianna M. Young(2), (1)University of Zaragoza, Department of Inorganic Chemistry, Faculty of Science, Plaza San Francisco s/n, E-50009 Zaragoza, Spain, (2)Intense Pulsed Neutron Source, Bldg. 360, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4814, USA

Crystals of trans-[Ni(cyan-N)₂(NH₃)₄], which possess a stable supramolecular ribbon formed by self-assembly of cyanurate ligands, undergo a reversible, temperature-induced phase transition. At 23[[ordmasculine]] C., crystals of this molecular material are orthorhombic, space group Fmmm, with a = 12.0551 (6), b = 7.2825 (6), c = 16.0779 (7) Å, and Z = 2. The molecule lies on a site of symmetry mmm, with the cyanurate rings coplanar. At -173[[ordmasculine]] C., the crystals are orthorhombic, space group Cmcm, with a = 11.998 (4), b = 7.203 (6), c = 15.736 (5) Å; and the molecule lies on a site of symmetry m2m. At the lower temperature, the molecule is distorted by bending of the cyanurate rings out of their common plane, such that the angle between the two ligand rings is 33[[ordmasculine]]. Upon warming to room temperature, the structure reverts to its original form. A given crystal can be cycled through the transitions more than once. At intermediate temperatures, an intermediate structure emerges. At -50[[ordmasculine]] C., the crystal has space group Cmcm, with a = 12.033 (2), b = 7.253 (1), and c = 15.937 (3) Å, and the same structural type as that at -173[[ordmasculine]] C.; but the distortion of the molecule is less pronounced, with an angle of 21.6[[ordmasculine]] between the planes of the two cyanurate ligands. Structural characterizations at temperatures from -250 to +80[[ordmasculine]] C. are presented. The very low temperature determination was by neutron diffraction. The phase transitions have been followed in real time, using profiles of reflections that change in the transition. It has been possible to isolate a metastable Cmcm phase at room temperature. The qualitative as well as quantitative differences between the two phases are described, and the nature of the transition is discussed.