Thermal induced spin transition in a series of iron(II) layered inorganic-organic solids. Role of the intermolecular interactions in the interlayer region

The understanding of the factors that determine the spin crossover phenomenon in transition metal-containing solids is a relevant subject in order to design new materials with tunable physical and functional properties based on such spin transition. This is a process that has been studied for decades but even not fully understood in terms of the electronic and structural features that could determine its control and tuning for a given potential application. In this contribution, we shed light in that sense, through an evaluation of the iron atom coordination environment from Raman, Mössbauer, XPS, and structural data for a series formed by the intercalation of the pyridine (Py) molecule (L) and its halogen-substituted derivatives (3X-py) between neighboring layers of iron(II) tetracyanonickellate, Fe(L)₂[Ni(CN)₄]. In their structure, the organic molecule (L) is found occupying the axial coordination sites for the iron atom. The halogen atom determines both, the relative orientation of neighboring molecules in the interlayer region and the dominant intermolecular forces, through dispersive interactions. When such interaction generates a local distortion for the iron atom coordination environment, the thermal induced spin transition is inhibited. This is properly supported by the herein discussed experimental data.