Structural approach of the features of the spin crossover transition in iron(II) compounds

We have determined the crystal structures, both in high and low spin state, of four Fe(PM-L)(2)(NCS)(2) complexes, where PM is N-2'-pyridylmethylene a nd the aromatic subunit L is 4-aminoterphenyl (TeA), 4-(phenylazo) aniline (AzA), 4-aminobiphenyl( BiA) or 4-(phenylethynyl) aniline (PEA). As previou sly reported, these compounds undergo a spin crossover at low temperature w ith different features of transition: very smooth and incomplete for Fe(PM- TeA)(2)(NCS)(2), smooth with almost no hysteresis for Fe(PM-AzA)(2)(NCS)(2) , unusually abrupt for Fe(PMBiA)(2)(NCS)(2), and abrupt with a very large h ysteresis (37 K) for Fe(PM-PEA)(2)(NCS)(2). In Fe(PM-BiA)(2)(NCS)(2), Fe(PM -TeA)(2)(NCS)(2) and Fe(PM-AzA)(2)(NCS)(2) the spin conversion is not assoc iated with a large structural phase transition and the space group is the s ame above and below the temperature of transition: orthorhombic Peen for th e two first and monoclinic P2(1)/c for the third. On the other hand, Fe(PM- PEA)(2)(NCS)(2) undergoes a change in the crystal symmetry from P2(1)/c thi gh spin) to Pccn (low spin) which corresponds to a strong re-organisation o f the iron atom network. The evolution as a function of temperature of the FeN6 core as well as of the intramolecular characteristics are almost ident ical in all four compounds. To a first approximation, the crystal packing i s similar in all of the structures except that the P2(1)/c structures devel op an asymmetrical molecular environment. Nevertheless, a close examination of the intermolecular interactions, classified as intra- and inter-sheet, show some differences. The intrasheet and the intersheet interactions are s tronger in Fe(PM-BiA)(2)(NCS)(2) and Fe(PM-PEA)(2)(NCS)(2) than either in F e(PM-TeA)(2)(NCS)(2) where no 'second' neighbour intrasheet contacts are cr eated, or in Fe(PMAzA)(2)(NCS)(2) where the intersheet interactions are wea k. Thus, the abruptness of the transition is attributed to the combination of close intrasheet and intersheet contacts. The hysteresis effect in Fe(PM -PEA)(2)(NCS)(2) is connected to the phase transition which could occur due to an irregular iron atom network associated with very short carbon-carbon intermolecular contacts at high temperature, not found in Fe(PM-AzA)(2)(NC S)(2) which shows the same irregular iron atom network.