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 and the aromatic subunit L is 4-aminoterphenyl (TeA), 4-(phenylazo) aniline (AzA), 4-aminobiphenyl( BiA) or 4-(phenylethynyl) aniline (PEA). As previously reported, these compounds undergo a spin crossover at low temperature with 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 hysteresis (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 associated with a large structural phase transition and the space group is the same above and below the temperature of transition: orthorhombic Peen for the 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 thigh spin) to Pccn (low spin) which corresponds to a strong re-organisation of the iron atom network. The evolution as a function of temperature of the FeN6 core as well as of the intramolecular characteristics are almost identical in all four compounds. To a first approximation, the crystal packing is similar in all of the structures except that the P2(1)/c structures develop 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 stronger in Fe(PM-BiA)(2)(NCS)(2) and Fe(PM-PEA)(2)(NCS)(2) than either in Fe(PM-TeA)(2)(NCS)(2) where no 'second' neighbour intrasheet contacts are created, or in Fe(PMAzA)(2)(NCS)(2) where the intersheet interactions are weak. 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)(NCS)(2) which shows the same irregular iron atom network.