Cooperative Spin-Crossoverin a Neutral DinuclearComplex Based on a Functionalized Triazole Ligand

A novel spin-crossover (SCO) dinuclear complex, [Fe₂(μ₂-cp-trz)₃(cp-trz)₂(tcm)₄] (cp-trz = 4-cyclo-pentyl-1,2,4-triazole, tcm- = tricyanomethanide anion), was synthesized and thoroughly characterized. Crystallographic analyses at 250 and 180 K revealed a centrosymmetric dinuclear structure with Fe(II) centers bridged by triazole ligands. The complex exhibits a cooperative hysteretic thermally induced abrupt one-step spin transition between high-spin (HS) and low-spin (LS) states. Magnetic measurements (Magn), X-rays diffraction (XRD), and optical microscopy (OM) show the following switching temperatures: <i>T</i>_1/2↓^Magn = 202 K and <i>T</i>_1/2↑^Magn = 212 K, and <i>T</i>_1/2↓^OM ≃ <i>T</i>_1/2↓^XRD = 203 K and <i>T</i>_1/2↑^OM ≃ <i>T</i>_1/2↑^XRD = 207 K. Moreover, structural and OM studies reveal significant anisotropic lattice changes, driven by strong hydrogen bonding along the <i>c</i> axis, which underpins the cooperative behavior of the SCO transition, and propagation of the HS/LS interface along the spin transition.