Influence of Solvent in Crystal Engineering: A Significant Change to the Order–Disorder Transition in Ferrocene

We present the structure of a novel solvate adduct formed by dissolving ferrocene, FeCp₂, in hexafluorobenzene, C₆F₆. This adduct demonstrates the remarkably strong interactions between the five-membered aromatic rings of FeCp₂ and the six-membered aromatic ring of C₆F₆. These molecular interactions are sufficiently strong and anisotropic to change the temperature of the order-disorder transition of the ferrocene molecule from below ca. 164 K to RT. No solvate adduct could be formed between benzene and FeCp₂. These observations will be of particular relevance to the crystal engineering community, whose goal is the design of solids with bespoke properties.