Crystal-Structure Studies of Mononuclear Iron(II) Complexes with Two-Step Spin Crossover: [Fe{5-NO2-sal-N(1,4,7,10)}] Revisited

In the region of the spin transition of spin-crossover (SCO) compounds there coexist molecules in both high-spin (HS) and low-spin (LS) states. In the case of two-step spin transition, theoretical predictions and computer simulations have shown that HS and LS complex molecules should be ordered in the plateau region. Several examples that support this hypothesis are already presented in the literature. Herein, we discuss a mononuclear complex of iron(II) with two-step spin transition in terms of long-range ordering of HS and LS molecules. In such compounds, spin transition is associated with the formation of a superstructure. However, previous studies of the [Fe{5-NO2-sal-N(1,4,7,10)}] [5-NO2-sal-N(1,4,7,10) = 5-nitrosalicylaldehyde-1,4,7,10-tetraazadecane] coordination complex have not revealed the occurrence of the satellite reflections in the SCO plateau region as a typical sign of the formation of a superstructure. Re-investigations of this complex by using strong X-ray sources and a charge-coupled device (CCD) camera show that satellite reflections appear in the plateau region. This is evidence that also in this case the superstructure is formed in the plateau region with long-range ordering of the HS and LS molecules. The two-step spin transition is connected to the change of the conformer and a dramatic change in the hydrogen-bonding system.