Metallocene: multi-layered molecular rotors.

The electronic and structural prerequisites for a multi-layered molecular rotor have been demonstrated herein in terms of nine 18-valence-electron metallocene sandwich complexes. First, the lack of strong covalent bonds between layers is a key issue to obtain a barrier-free rotation of one layer relative to other layers, where the considerable energetic but unidirectional (such as electrostatic interactions) interactions are needed between layers to keep the structural integrity against fragment separation and structural distortion in a rotation process. Second, one or more layers should possess continuous and delocalized π electron clouds to provide a driving force for the barrier-free rotation. More importantly, besides a negligible rotation barrier, the reasonable rotational period associated with the ultra-soft rotation mode is a critical point for the observability of dynamical behavior in multi-layered molecular rotors.