The phenomenon of spin-crossover has a large impact on the physical properties of a solid material, for example on magnetic moment and electrical resistance. Some practical applications of this effect include display and memory devices, electrical and electroluminescent devices, and MRI contrast agents. As such the relationship between intermolecular interactions and spin-crossover features, such as the abruptness of the spin transition and cooperativity, in crystalline complexes of iron(II) and iron(III) is of great interest.

Keith S. Murray, Monash University, Clayton, Australia, and colleagues have prepared a series of homoleptic complexes of the formula [Fe(qsal)(thsa)]·solvent (qsal = quinolylsalicylaldimine, thsa = thiosemicarbazonesalicylaldiminate, solvent = CH₃CN or THF). The complexes with CH₃CN show spin-crossover properties.

One of the things that the team has discovered is a fascinating array of intermolecular interactions that occur in the various crystals, which all contain two distinct Fe sites. They discuss how these interactions relate to differences in spin states of neighbouring [Fe(qsal)(thsa)] molecules and hence their spin crossover properties.

• Spin Crossover, Polymorphism and Porosity to Liquid Solvent in Heteroleptic Iron(III) {Quinolylsalicylaldimine/Thiosemicarbazone-Salicylaldimine} Complexes,
  Wasinee Phonsri, Casey G. Davies, Guy N. L. Jameson, Boujemaa Moubaraki, Keith S. Murray,
  Chem. Eur. J. 2015.
  DOI: 10.1002/chem.201504266