Spin-state ordering on one sub-lattice of a mononuclear iron(III) spin crossover complex exhibiting LIESST and TIESST.

The two-step spin crossover in mononuclear iron(III) complex [Fe(salpm)2 ]ClO4 ⋅0.5 EtOH (1) is shown to be accompanied by a structural phase transition as concluded from (57) Fe Mössbauer spectroscopy and single crystal X-ray diffraction, with spin-state ordering on just one of two sub-lattices in the intermediate magnetic and structural phase. The complex also exhibits thermal- and light-induced spin-state trapping (TIESST and LIESST), and relaxation from the LIESST and TIESST excited states occurs via the broken symmetry intermediate phase. Two relaxation events are evident in both experiments, that is, two T(LIESST) and two T(TIESST) values are recorded. The change in symmetry which accompanies the TIESST effect was followed in real time using single crystal diffraction. After flash freezing at 15 K the crystal was warmed to 40 K at which temperature superstructure reflections were observed to appear and disappear within a 10 000 s time range. In the frame of the international year of crystallography, these results illustrate how X-ray diffraction makes it possible to understand complex ordering phenomena.