Use of Topotactic Phase Transformations To Obtain Solutions of the Crystal Structures of Highly Disordered Materials

A straightforward
procedure is outlined for prediction of the complete three-dimensional
coordinate set for a highly disordered phase. For a mother and daughter
phase, where one of the pair has an “unsolvable” structure,
one needs only to (a) establish the topotaxy using previously published
techniques, (b) obtain the topotactic transformation matrix, φ,
between the ordered and highly disordered phase, and (c) apply the
transpose of φ–1 to obtain a trial set of
coordinates for refinement using the reflection data set of the highly
disordered phase. For the inclusion compound [Fe­(η-C5H5)]­·3­(NH2)2CS (1), which contains highly disordered ferrocene molecules above
160 K (polymorph 1_I), we found a more ordered structure
at low temperature. At 135 K (polymorph 1_II), two ferrocene
moieties are present in the thiourea channel in an approximately 1:1
ratio. One is nearly orthogonal (87.0°) to the channel axis,
while the other is tipped 16.2° from that direction. Using steps
(a–c) outlined above, a trial structure may be obtained for 1_I, and refinement leads to R1 = 4.25%. The structure of 1_1, containing 12-fold disordered
ferrocene molecules, is similar to that found at temperatures below
the phase transition, with a greater amount of the orthogonal orientation
(55:45 vs 51:48), consistent with, but lower than, amounts found using
solid-state NMR techniques. The low temperature polymorph is a trill,
with an approximately 3:1:1 ratio of twin components. The exact alignment
of the mother phase and the three daughters has been established using
the methods of topotactic analysis described previously.