Fully Enclosed Molecular Rotors

ORGANOMETALLIC CHEMISTRY

Single-molecule rotors, propellers, turnstiles, and related molecular assemblies that could one day function as tiny machines in nanoscale devices have been popping up in the chemical literature for several years. The most recent examples, devised by a research team in Germany, are the first fully enclosed rotor systems that resemble a gyroscope. They consist of an iron tricarbonyl core surrounded by a framework of three methylene chains whose length can be varied.

Postdoc Takanori Shima, crystallographer Frank Hampel, and chemistry professor John A. Gladysz of the University of Erlangen-Nuremberg synthesized the molecular gyroscopes from iron tricarbonyl complexes that include one trialkenylphosphine ligand on either side of the iron center [Angew. Chem. Int. Ed., published online Aug. 27, http://dx.doi.org/10.1002/anie200460534].

The primary feature of the synthesis is a series of ring-closing metatheses that connect the loose ends of the phosphine alkene chains from opposite sides of the iron center. The researchers used Grubbs catalyst, a ruthenium complex known for efficient ring closing of dienes, to carry out the reactions. The double bonds that remain at the center of the newly formed methylene chains are hydrogenated in a final step.

The result is an architecturally unique molecule where the Fe(CO)₃ core functions as the rotating disk and center of the axis for the gyroscope, and the methylene chains connecting the axial phosphorus atoms serve as the gyroscope's spokes.

The work "is a nice illustration of the power of synthesis in the preparation of molecules that can emulate the structure and function of macroscopic objects," notes Miguel A. Garcia-Garibay, a chemistry professor at the University of California, Los Angeles.

Garcia-Garibay's group was the first to suggest that molecular gyroscopes would be ideal systems to use in solid-state devices. The UCLA researchers have prepared compounds in which aryl groups attached to triple bonds can spin under the influence of electric, magnetic, or optical fields (C&EN, July 8, 2002, page 32). More recently, they have improved their design and are now studying the dynamics of crystalline arrays of their gyroscopes, Garcia-Garibay says.

However, those gyroscopelike molecules don't have a closed topology like the compounds prepared by Gladysz and coworkers. "They have come a step closer to the topology of a toy gyroscope," Garcia-Garibay says. "Their synthetic approach is amenable to numerous structural modifications, which will allow them to test variations that may lead to some of the desired machine functions."

The iron carbonyl center of Gladysz's molecules can spin around at different rates, modulated by the length of the gyroscope spokes. To determine the rate of rotation, the Fe(CO)₃ core was converted into [Fe(CO)₂(NO)]⁺, which provides a lower molecular symmetry and allowed the researchers to study the gyroscopes' dynamic behavior by nuclear magnetic resonance spectroscopy.

They observed that the iron core in the gyroscopes spins rapidly at room temperature. The direction of the spin is random, Gladysz says, but it likely could be controlled by applying an electric field, as has been done with surface-bound molecular rotors recently prepared by chemistry professor Josef Michl's group at the University of Colorado (C&EN, March 22, page 34). The frequency of rotation could be controlled by adjusting the size of the substituents on the iron center, Gladysz adds.