Twisted and Bent out of Shape: Symmetry and Chirality Analysis of Substituted Ferrocenes

Ferrocene symmetry is commonly described as either eclipsed (D₅h) or staggered (D₅d), but this symmetry does not hold when substitution is involved. Here we examine and quantify the effect of substitution on the geometry of the core structure of ferrocene, and provide means to distinguish between its various distortion paths. Continuous symmetry analysis of the core structure of 7418 substituted ferrocenes extracted from the Cambridge Crystallographic Database was used to explore its twisting and bending distortion paths, as well as asymmetric bond stretch that deforms the symmetry of its cyclopentadienyl rings. Gas-phase density functional theory calculations provided a theoretical background to describe these distortion paths and define symmetry profiles for conformer interconversion processes and the interplay between them. Our results show that the distortion of ferrocene can often be substantial. Furthermore, its geometry is highly flexible, and almost always chiral to some degree. Handedness in terms of the P/M helicity convention is discussed.