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.