X-RAY DIFFRACTION UNDER CONSTRAINT (T, P,H) TO UNDERSTAND THE SPIN TRANSITION IN IRON(II) COMPOUNDS

J.A.K. Howard¹, P. Guionneau¹, J.F. Létard², D.S. Yufit¹, G. Bravic², Y. Barrans², A.E. Goetaa¹, D. Chasseau², O. Kahn²

¹Durham Crystallography Group, Chemistry Department, Science site, Durham DH13LE, U.K.
² Laboratoire des Sciences Moléculaires, Institut de Chimie de la Matière condensée de Bordeaux, UPR CNRS 9048, 33608 Pessac, France.

The features of the spin transition in iron(II) complexes, especially the temperature of transition and the hysteresis effect, can strongly change from one compound to the other. The control of such characteristics and the ability to design required compounds for industrial application go through the previous understanding of the relation between structural properties and magnetic behaviours. The series of the Fe(PM-L)2(NCS)z spin crossover complexes, provided us the occasion to study similar molecules with very different features of transition.

The crystal structures of the four studied compounds were determined in the high spin state and in the low spin state. The analysis of a set of 9 crystal structures allows us to link the topology of the intermolecular interactions to the features of the transitions. For instance, the strong combination of intersheet and intrasheet intermolecular contacts between phenyl rings seems to be the relevant parameter which govern the abruptness of the transition. In the same way, the increase of the hysteresis effect is related to the presence of strong intermolecular contacts associated with an irregular iron network in the high spin state.

The magnetic behaviour depending also on the applied pressure, high pressure X-ray diffraction experiments are under progress on this family of compounds in our laboratories. Moreover, data collection at very low temperature together with Laser irradiation may give us, for the first time, structural information corresponding to the LIESST effect.