Spin crossover in homoleptic Fe(II) imidazolylimine complexes

The straightforward synthesis of a diverse variety of imidazolylimine ligands has led to the preparation of a large number of molecular iron(II) complexes, many of which undergo spin crossover between the high-spin (HS) and low spin (LS) states. This review surveys these compounds, with particular emphasis placed on homoleptic mononuclear, dinuclear (triple helicate), tetranuclear (tetrahedral cage) and octanuclear (cuboid cage) complexes, each of which features Fe(II) centres with octahedral geometry and tris-bidentate imidazolylimine ligand coordination. We present a wider discussion of their design and synthesis and give special emphasis to the characterisation of their magnetic properties. The realisation of a series of chemically similar spin crossover materials has enabled a fuller understanding of their magnetic behaviour and permitted structure-function correlations to be made.