ABSTRACT
The title compound, [Fe(btr)(3)](ClO(4))(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 Fe(2+) 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 DeltaH(1) = 5.7 kJ mol(-)(1) and DeltaS(1) = 30.1 J mol(-)(1) K(-)(1), and DeltaH(2) = 6.5 kJ mol(-)(1) and DeltaS(2) = 28.6 J mol(-)(1) K(-)(1), respectively. The crystal structure of [Fe(btr)(3)](ClO(4))(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&thremacr;, at the three temperatures. The structure is three-dimensional. There are two Fe(2+) 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 Fe(2+) 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.
ABSTRACT
The new spin-crossover compound Fe(PM-BiA)(2)(NCS)(2) with PM-BiA = N-(2-pyridylmethylene)aminobiphenyl has been synthesized. The temperature dependence of chi(M)T (chi(M) = molar magnetic susceptibility and T = temperature) has revealed an exceptionally abrupt transition between low-spin (LS) (S = 0) and high-spin (HS) (S = 2) states with a well-reproducible hysteresis loop of 5 K (T(1/2) downward arrow = 168 K and T(1/2) upward arrow = 173 K). The crystal structure has been determined both at 298 K in the HS state and at 140 K in the LS state. The spin transition takes place without change of crystallographic space group (Pccn with Z = 4). The determination of the intermolecular contacts in the LS and HS forms has revealed a two-dimensional structural character. The enthalpy and entropy variations, DeltaH and DeltaS, associated with the spin transition have been deduced from heat capacity measurements. DeltaS (= 58 J K(-)(1) mol(-)(1)) is larger than for other spin transition bis(thiocyanato) iron(II) derivatives. At 10 K the well-known LIESST (light-induced excited spin state trapping) effect has been observed within the SQUID cavity, by irradiating a single crystal or a powder sample with a Kr(+) laser coupled to an optical fiber. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect with 35 < T(1/2) < 77 K. The HS --> LS relaxation after LIESST has been found to deviate from first-order kinetics. The kinetics has been investigated between 10 and 78 K. A thermally activated relaxation behavior at elevated temperatures and a nearly temperature independent tunneling mechanism at low temperatures have been observed. The slow rate of tunneling from the metastable HS state toward the ground LS state may be explained by the unusually large change in Fe-N bond lengths between these two states.
