The design of iron(II) spin crossover (SCO) complexes which exhibit wide thermal
hysteresis have attracted much attention lately, since their bistable nature could allow their use as
molecular switches in new electronic devices. In this aim, it is crucial to understand the main factors
which control the spin crossover phenomenon.
We were interested in investigating the cooperative effects in the [Fe(dpp)2(NCS)2]py (dpp
= dipyrido[3,2-a:2,3,-c]phenazine and py = pyridine) compound (Figure 1a) described in 1998 [1]
which presents a fairly abrupt spin transition at 123 K in the cooling mode and 163 K in the heating
mode, with a resulting hysteresis of 40 K (Figure 1b).
In this study we used high-pressure single-crystal X-ray diffraction and Raman
spectroscopy to study the spin crossover (SCO) between high spin (HS) and low spin (LS) states in
the mentioned above compound. A single geometrical mechanism is responsible not only for the
strongest negative linear compression (NLC) behaviour yet observed in a molecular material,
combined with negative thermal expansion (NTE) and extreme positive linear compressibility, but
also for the high cooperativity of the SCO in [Fe(dpp)2(NCS)2]·py. The scissor-like motion of
individual molecules is propagated through the lattice via the physical intercalation of ligands and
is also responsible for suppressing the expected HS→LS SCO under pressure [2].

References
1. Z. J. Zhong, J.-Q. Tao, Z. Yu, C.-Y. Dun, Y.-J. Liu, X.-Z. You, J. Chem. Soc. Dalton Trans.,
327 – 328, 1998.
2. H. J. Shepherd, T. Palamarciuc, P. Rosa, P. Guionneau, G. Molnar, J.-F. Letard, A. Bousseksou,
Angew. Chem. 124, 3976-3980, 2012.