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 T-1 = 184 K. The high-temperature step, centered around T-2 = 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 Delta H-1 = 5.7 kJ mol(-1) and Delta S-1 = 30.1 J mol(-1) K-1, and Delta H-2 = 6.5 kJ mol(-1) and Delta S-2 = 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 R (3) over bar, 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 Angstrom 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) Angstrom, respectively. At 190 K, the Fel site remains HS while the Fe2 sire is low-spin (LS) with Fe-N bond lengths of 2.007(3) Angstrom. Finally, at 150 K, both the Fel and Fe2 sites are LS with Fe-N bond lengths of 1.987(5) and 1.994(5) Angstrom, 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 Mossbauer spectroscopy. These findings have been discussed and compared to the previously reported cases of two-step spin conversions.