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 invest igation of its magnetic properties has revealed a low-spin <-> high-spin co nversion occurring in two steps, each step involving 50% of the Fe2+ ions. The low-temperature step is very abrupt and occurs with a thermal hysteresi s whose width is about 3 K around T-1 = 184 K. The high-temperature step, c entered around T-2 = 222 K, is rather gradual. Differential scanning calori metric measurements have confirmed the occurrence of a two-step spin conver sion. The enthalpy and entropy variations associated with the two steps hav e 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, resp ectively. The crystal structure of [Fe(btr)(3)](ClO4)(2) has been solved at three temperatures, namely, above the high-temperature step (260 K), betwe en the two steps (190 K), and below the low-temperature step (150 K). The c ompound 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 symmet ry axis and an inversion center and is surrounded by six btr ligands throug h the nitrogen atoms occupying the 1- or 1'-positions. Each btr ligand brid ges 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-dimensi onal 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 bo nd lengths of 2.007(3) Angstrom. Finally, at 150 K, both the Fel and Fe2 si tes are LS with Fe-N bond lengths of 1.987(5) and 1.994(5) Angstrom, respec tively. It turns out that the two-step spin conversion is associated with t he presence of two slightly different Fe2+ sites. The spin conversion regim e has also been followed by Mossbauer spectroscopy. These findings have bee n discussed and compared to the previously reported cases of two-step spin conversions.