57Fe Mössbauer spectroscopy and SQUID magnetometry of a 3D Iron(II) Spin crossover framework

Some transition metal complexes, particularly those of the first transition series with configurations 3d4 to 3d7, can undergo a spin crossover (SCO) when induced by an external stimulus such as light, temperature, pressure, electric field, or guest molecules 1. SCO compounds have been studied extensively over the last few decades for potential applications in high-density information storage devices, quantum computers and spintronics 2. In this work, a new three-dimensional iron(II) complex based on tris(4,4’-bis-1,2,4-triazole), hereafter abbreviated as btr, has been synthesized with the formula Fe(btr)3(anion)2. Single-crystal X-ray diffraction showed that the complex crystallized in the R3 space group, and the structure is three-dimensional 3. Two crystallographically independent iron sites were revealed, denoted as Fe1 and Fe2, respectively. The magnetic properties were studied using SQUID (Superconducting Quantum Interference Device). This compound exhibits SCO behavior. The low-spin to high-spin transition occurs in two steps with an unprecedented ca. 67 K plateau, each involving around 50% of Fe2+ ions. The low-temperature step is abrupt and undergoes a thermal hysteresis with a width of 12 K from 116 K to 128 K. The high-temperature step, is centered on 187 K without thermal hysteresis. Differential scanning calorimetry (DSC) measurements have confirmed the occurrence of the two-step spin transition. In addition, 57Fe Mössbauer spectra were obtained at selected temperatures, which allowed us to follow the temperature dependence of the HS and LS molar fractions in an accurate fashion.