Rational direct synthesis of [Fe(Htrz)2(trz)](BF4) polymorphs: temperature and concentration effects

The [Fe(Htrz)2trz](BF4) compound is probably the most studied in the spin crossover (SCO) community since it exhibits switching properties with a large temperature range of memory effect, just above room temperature. We present in this paper a detailed study of the influence of the synthetic parameters on the polymorphism of this compound. While the crystal structure of polymorph I is already known, we report the crystal structure of polymorph II and discuss the differences to allow further studies to discriminate between the two. This was achieved through the resolution of the crystal structure of the [Fe(Htrz)2(trz)](PF6) analogue. This study allows us to draw a phase diagram and to discuss the effect of the reaction conditions on the formation of polymorphs, the size of the particles and their properties.