Cooperative Large-Hysteresis Spin-Crossover Transition in the Iron(II) Triazolate [Fe(ta)2] Metal-Organic Framework.

The metal–organic framework [Fe­(ta)2] (Hta = 1H-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 Fd3m (no. 227). The phase transition between room temperature (RT-[Fe­(ta)2]; a = 16.6315(2) A, V = 4600.39(8) A3) and high temperature (HT-[Fe­(ta)2]; a = 17.7566(4) A, V = 5598.6(1) A3) phases occurs at a temperature above 290 °C, whereas the phase transition between HT- and RT-[Fe­(ta)2] starts at a temperature below 210 °C. Both [Fe­(ta)2] polymorphs have identical bond topologies, but they differ by a large increase of the unit cell’s volume of 22% for HT-[Fe­(ta)2]. The compounds are characterized by powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analyses. Additionally, Mossbauer 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)2]) to the high-spin phase (HT-[Fe­(ta)2]).