Structural studies of Iron (II) spin crossover compounds

Abstract

The drive for ever smaller and faster computers has, in recent years, caused much research interest to be focussed on the development of new materials in which individual molecules or assemblies of molecules can be used for information processing. Materials which show spin crossover behaviour have great potential for use not only in molecular computing but also in applications such as optical switches and display devices and are of fundamental interest due to their importance in biological and geological systems. The results of comprehensive variable temperature and excited state crystallographic studies into the spin crossover behaviour of a family of iron (II) spin crossover complexes based on the 2,6-di(pyrazol-l-yl)pyridine ligand are presented herein. A fascinating aspect of spin crossover materials is their ability to undergo a transition from the low spin state to a metastable high spin state, with a very long lifetime, on irradiation. Crystallographic information on the structure of the metastable high spin state formed as a result of irradiation is very rare. Full structural analyses of the metastable state are reported for [FeLl(_2)](BF(_4))(_2), [Fe(L3)(_2)](BF(_4))(_2), [Fe(L3)(_2)](C1O(_4))(_2) and [Fe(L4)(_2)](BF(_4))(_2) (LI = 2,6- di(pyrazol-l-yl)pyridine, L3 = 2,6-(dipyrazol-l-yl)-4-hydroxymethylpyridine, L4 = 2,6-di(3-methylpyrazol-1 -yl)pyridine). These studies have shown that, unlike other reported materials, the metastable high spin state is structurally identical to that reached as a result of the thermal spin transition. [Fe(L4)(_2)](BF(_4)) (_2)1/3H(_2)O is shown to have a fascinating complexity of spin crossover behaviour including the existence of a number of metastable states. The effect of dehydration on the spin crossover behaviour has been determined. Spin crossover compounds are extremely sensitive to changes in pressure; nonetheless there have been very few studies of the effect of pressure on the structure of these materials. The structure of the pressure induced low spin state at ambient temperature and 4.5 kbar is reported for [FeLl2](BF4)2. The crystallographic results are supported throughout by SQUID magnetometry studies.