Cooperative Large-Hysteresis Spin-Crossover Transition in the Iron(II) Triazolate [Fe(ta)] Metal-Organic Framework.

The metal-organic framework [Fe(ta)] (Hta = 1-1,2,3-triazole) containing Fe(II) ions and 1,2,3-triazolate ligands shows a reversible phase transition while retaining the cubic crystal symmetry and space group 3 (no. 227). The phase transition between room temperature (RT-[Fe(ta)]; = 16.6315(2) Å, = 4600.39(8) Å) and high temperature (HT-[Fe(ta)]; = 17.7566(4) Å, = 5598.6(1) Å) phases occurs at a temperature above 290 °C, whereas the phase transition between HT- and RT-[Fe(ta)] starts at a temperature below 210 °C. Both [Fe(ta)] polymorphs have identical bond topologies, but they differ by a large increase of the unit cell's volume of 22% for HT-[Fe(ta)]. The compounds are characterized by powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analyses. Additionally, Mössbauer spectroscopy, magnetic studies, and the electronic structure of both phases are discussed in detail with respect to the spin-crossover transition from the low-spin (RT-[Fe(ta)]) to the high-spin phase (HT-[Fe(ta)]).