Observation of the structural consequences of a reversible S = 3/2 and S = 1/2 spin crossover in the single crystal

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 d_z2 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.