Two-Step Spin Conversion for the Three-Dimensional Compound Tris(4,4‘-bis-1,2,4-triazole)iron(II) Diperchlorate

The title compound, [Fe(btr)3](ClO4)2, has been synthesized. The investigation of its magnetic properties has
revealed a low-spin ↔ high-spin conversion occurring in two steps, each step involving 50% of the Fe2+ ions.
The low-temperature step is very abrupt and occurs with a thermal hysteresis whose width is about 3 K around
T1 = 184 K. The high-temperature step, centered around T2 = 222 K, is rather gradual. Differential scanning
calorimetric measurements have confirmed the occurrence of a two-step spin conversion. The enthalpy and entropy
variations associated with the two steps have been found as ΔH1 = 5.7 kJ mol-1 and ΔS1 = 30.1 J mol-1 K-1,
and ΔH2 = 6.5 kJ mol-1 and ΔS2 = 28.6 J mol-1 K-1, respectively. The crystal structure of [Fe(btr)3](ClO4)2 has
been solved at three temperatures, namely, above the high-temperature step (260 K), between the two steps (190
K), and below the low-temperature step (150 K). The compound crystallizes in the trigonal system, space group
R3̄, at the three temperatures. The structure is three-dimensional. There are two Fe2+ sites, denoted Fe1 and Fe2.
Each of them is located on a 3-fold symmetry axis and an inversion center and is surrounded by six btr ligands
through the nitrogen atoms occupying the 1- or 1‘-positions. Each btr ligand bridges an Fe1 and an Fe2 site, with
an Fe1−Fe2 separation of 8.67 Å at 260 K. The perchlorate anions are located in the voids of the three-dimensional
architecture and are hydrogen bonded to the triazole rings of the btr ligands. These anions do not interact with the
Fe1 and Fe2 sites exactly in the same way. At 260 K, both the Fe1 and Fe2 sites are high-spin (HS) with Fe−N
bond lengths of 2.161(3) and 2.164(3) Å, respectively. At 190 K, the Fe1 site remains HS while the Fe2 site is
low-spin (LS) with Fe−N bond lengths of 2.007(3) Å. Finally, at 150 K, both the Fe1 and Fe2 sites are LS with
Fe−N bond lengths of 1.987(5) and 1.994(5) Å, respectively. It turns out that the two-step spin conversion is
associated with the presence of two slightly different Fe2+ sites. The spin conversion regime has also been followed
by Mössbauer spectroscopy. These findings have been discussed and compared to the previously reported cases
of two-step spin conversions.