Research - Molecular Gears

Controlling the rotation of molecules by the construction of gears and motors offers novel approaches to the assembly of catalysts and other nanoscale devices. Carbon based gears have been known for some time, a result of the pioneering work of Mislow and Iwamura who synthesised oxygen or methylene linked triptycenes to give three-toothed bevel gears 1. To increase the number of cog-teeth available in simple rotors, and to generate molecular gears with non-equal gearing ratios, we became interested in the use of the cobalt metallocene 2. Here the low energy barrier to rotation of the p-fragment about the metal is analogous to rotation about a low-friction ball-bearing.
We first synthesised 3, the first example of a metallocene based molecular gear, where the absence of a high three-fold torsional barrier common to bridgehead substituted triptycenes is likely due to the availability of a low energy correlated gearing mechanism between the two intermeshing four- and three-toothed cogs. [1] This is clearly evident in the X-ray structures of 4, the related pentaphenylferrocene gear 5 [X-ray], and larger 4:3:4 gearing arrays represented by 6. [2]
[1] A. M. Stevens and C. J. Richards, Tetrahedron Lett., 1997, 38, 7805. [2] D. C. D. Butler and C. J. Richards, in preparation.