Protonmotive Force: Development of Electrostatic Drivers for Synthetic Molecular Motors

Ferrocene has been investigated as a platform for developing protonmotive electrostatic drivers for molecular motors. When two 3‐pyridine groups are substituted to the (rapidly rotating) cyclopentadienyl (Cp) rings of ferrocene, one on each Cp, it is shown that the (Cp) eclipsed, π‐stacked rotameric conformation is preferred both in solution and in the solid state. Upon quaternization of both of the pyridines substituents, either by protonation or by alkylation, it is shown that the preferred rotameric conformation is one where the pyridinium groups are rotated away from the fully π‐stacked conformation. Electrostatic calculations indicate that the rotation is caused by the electrostatic repulsion between the charges. Consistently, when the π‐stacking energy is increased π‐stacked population increases, and conversely when the electrostatic repulsion is increased π‐stacked population is decreased. This work serves to provide an approximate estimate of the amount of torque that the electrostatically driven ferrocene platform can generate when incorporated into a molecular motor. The overall conclusion is that the electrostatic interaction energy between dicationic ferrocene dipyridyl systems is similar to the π‐stacking interaction energy and, consequently, at least tricationic systems are required to fully uncouple the π‐stacked pyridine substituents. Torque of the town: Ferrocene has been investigated as a platform for developing protonmotive electrostatic drivers for molecular motors (see scheme). When two 3‐pyridine groups are added to the (rapidly rotating) cyclopentadienyl (Cp) rings of ferrocene, one on each Cp, the (Cp) eclipsed, π‐stacked rotameric conformation is preferred both in solution and in the solid state.