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

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Abstract
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(PM-BiA) 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 Pccn 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 (high 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 FeN 6 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(PM-AzA) 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.

This publication has 1 reference indexed in Scilit:

  • Mithril
    Published by Association for Computing Machinery (ACM) ,2017