The Atypical Hysteresis of [Fe(C F Tp) ]: Overlay of Spin-Crossovers and Symmetry-Breaking Phase Transition.

The [Fe (C F Tp) ] spin-crossover complex is an atypical molecular switch, which can be converted upon annealing between two archetypal spin-crossover behaviours: from an extremely gradual spin-crossover to a broad hysteretic spin-transition (of ca. 65 K). The hysteresis shows an uncommon "rounded shape" that is reproducible upon cycling temperature. In depth structural studies reveal a first crystal phase transition, which occurs upon melting and recrystallizing at high temperature. This first irreversible transition is associated with a radical change in the crystal packing. More importantly, the "rounded and broad" hysteretic transition is shown to occur in a non-cooperative SCO system and is associated with the occurrence of a symmetry-breaking phase transition that appears when roughly ca. 50 % of the SCO complexes are switched.