Highly saddle shaped iron(III) porphyrin complex 1 showing a novel spin crossover process between the S = 3/2 and S = 1/2 has been crystallographically analyzed at 298, 180, and 80 K. As the temperature is lowered, the lattice contraction has occurred specifically along the b-axis. Correspondingly, the iron-pyridine bonds, which tilt slightly from the b-axis, have decreased by 7.3%. In contrast, the lattice contractions along the a- and c-axes are much smaller and the iron-porphyrin bonds, which almost coincide with the a- and c-axes, have shown much smaller contraction, ca. 2.2%. The large contraction of the specific bonds caused by packing force raises the energy level of the dz2 orbital and induces the spin transition. The detailed structural and lattice changes in the single crystal, which may be regarded as a superstructure parallel to a protein matrix, have been clarified.
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