Solvent Effect on the Spin State of an Iron(II)‐Triazole Trimer

We report a triazole‐based trinuclear iron(II) compound with bridging 4‐(p‐methylphenyl)‐1,2,4‐triazole ligands with solvatochromic spin crossover (SC) behavior. The magnetic properties of this trimer depend on crystallization solvent molecules. Pink crystals (color suggesting low spin state), showing a triclinic structure can be obtained by slow diffusion of diethyl ether in acetonitrile solution. As in other SC materials, immersion of these crystals in different solvents promote color change. In this case, we have been able to define the microscopic effect of the solvent substitution following the single crystal to single crystal transformation by UV/Vis absorption spectroscopy and X‐ray diffraction. Furthermore, the solvent‐induced switching can be explained by weak intermolecular interactions provoking a variation in the relative orientations of the two aromatic rings in the ligands, which, in turn, determines the ligand field on the central Fe(II), and its SCO behavior. This study brings important details to the origin of solvatochromism in SCO materials, offering alternatives for better control of this switching and, potentially, for their application as colorimetric sensors. An iron triazole‐bridged trimer capable of changing color and magnetic properties with solvent was synthesized and characterized. A study of its single crystals in different solvents provides evidence of the structural reasons behind this intriguing behavior.