Thermally and photo-induced spin crossover behaviour in an Fe(II) imidazolylimine complex: [FeL3](ClO4)2

Herein we report the synthesis, structure and magnetic properties of [FeL3](ClO4)(2), 1, where L is the bidentate ligand N-(4-methoxyphenyl)-1H-imidazol-2-yl-methanimine. Complex 1 crystallises as the mer-isomer which is stabilised by intramolecular pi-pi interactions between the methoxyphenyl 'tail' and imidazole 'head' of adjacent ligands centred around Fe(II). The crystal lattice is devoid of any significant intermolecular pi-pi interactions between neighbouring complexes, although hydrogen bonding between two imidazole N-H groups and two perchlorate anions links two adjacent complexes together and allows them to buttress up against one another. Structural data collected at 116 and 292 K suggest a spin-crossover (SCO) behaviour for 1 as Fe-N bond lengths increase by ca. 10% at the higher temperature. SCO behaviour is confirmed by magnetic susceptibility measurements. The chi T vs. T data reveals a complete and reversible SCO for 1 with T-1/2 of 158 K, and this is further confirmed by Mossbauer spectroscopy (at 4.2 and 293 K) and variable temperature reflectivity measurements. Indeed, upon irradiation with red light (647 nm) at 10 K, 1 shows a photo-induced spin-crossover and undergoes full switching between the LS and HS states.