Browsing by Autor "S.A. Kumalah"

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Properties of metal-organometallic frameworks derived from facially η<sup>6</sup>metalated terephthalates
(Wiley, 2008) S.A. Kumalah; K.T. Holman
The title compound (I) crystallizes in the orthorhombic space group P21212 (a=10.7028 , b= 12.8508 , c= 6.5844 , V= 905.62 3 ) at 298 K (phase I). The Ti complex resides on a two-fold axis with Z'=0.5. Complex (I) undergoes two sharp enantiotropic phase transitions upon cooling. The first transformation occurs at 293 K to yield a unit cell with a doubled c axis (a=10.5539 , b= 12.8757 , c= 13.0817 , V= 1777.66 3 at 175 K) and the orthorhombic space group P212121 (phase II). The doubling of the unit cell volume results in the addition of a translational component to the two-fold rotational symmetry in the c direction, and now the complex is in a general position. The second transition takes place at 147 K to convert the single crystal into a merohedrally twinned crystal with two monoclinic components of half the size (a= 10.5529 , b= 12.7403 , c=6.5486 , = 92.327, V= 879.71 3 at 100 K) in the monoclinic space group P21 (phase III). The Ti complex remains in a general position. The concomitant phase transition and twinning necessarily result in the loss of symmetry along the a and c axes and can be thought of as taking the b axis in phase II and "pulling" (shifting) it in the positive direction to generate the twinning in phase III (see diagram). The unique angle widens as the temperature is lowered. The variable temperature study of the selected crystal was conducted in an automated mode on a Bruker SMART APEX2 diffractometer with Cu K radiation while the exact transition temperatures were manually pinpointed with Mo K radiation. The molecular symmetry evaluations and exact refinement procedures are also discussed.