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 Fd (3) over barm (no. 227). The phase transition between room temperature (RT-[Fe(ta)(2)]; a = 16.6315(2) angstrom, V = 4600.39(8) angstrom(3)) and high temperature (HT-[Fe(ta)(2)]; a = 17.7566(4) angstrom, V = 5598.6(1) angstrom(3)) phases occurs at a temperature above 290 degrees C, whereas the phase transition between HT- and RT-[Fe(ta)(2)] starts at a temperature below 210 degrees 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)]).