Olefin Metathesis on Metal Surfaces

SURFACE CHEMISTRY

New methods broaden range of chemical reactions on electrodes

A textbook chapter's worth of organic reactions have just been moved within reach of surface chemists studying molecular electronics and nanoscale devices. In separate studies, researchers in Canada and the U.S. have demonstrated procedures for carrying out olefin metathesis reactions on the surface of metals (Science 2005, 309, 588 and 591).

Some methods for connecting molecules to electrodes, such as the one involving gold-thiol linkages, have been employed successfully in many laboratories. But if more general methods for making robust contacts between metal atoms and a variety of molecules were available to scientists, a wider range of functionalized surfaces could be prepared, and new types of molecular devices might follow. The latest studies take a step in that direction.

In one of the investigations, chemistry professor Peter McBreen and graduate student Mohamed Siaj of Laval University, in Quebec, show that a variety of molecules can be attached to individual metal atoms on surfaces via double bonds and that the products can be transformed in subsequent reactions.

By reacting cyclopentanone with the surface of -molybdenum carbide (-Mo₂C), an electrically conducting material, the Laval group prepared a surface with cyclopentylidene groups doubly bonded to individual molybdenum atoms. Then, by exposing the surface to propene, the team caused cross-metathesis reactions to occur between the surface-bound and gas-phase doubly bonded species.

Using vibrational spectroscopy methods and mass spectrometry, McBreen and Siaj identified two pairs of key products: Mo-methylidene and cyclopentane ethylidene, and Mo-ethylidene and cyclopentane methylidene.

The Laval group also carried out ring-opening metathesis polymerizations on -Mo₂C functionalized with cyclopentylidene groups. In that part of the study, the team initiated molecular chain growth by reacting the surface with cyclopentene--and in a separate reaction, with norbornene.

In the other study, Columbia University chemistry professor Colin Nuckolls, grad student George S. Tulevski, research scientist Michael L. Steigerwald, and their coworkers demonstrate that derivatized ruthenium surfaces can be used to catalyze olefin metathesis reactions.

Specifically, the Columbia team treated ruthenium films with 4-bromophenyldiazomethane--and separately with trimethylsilyldiazomethane--to prepare two types of derivatized surfaces featuring molecules that are multiply bonded to ruthenium. Then in separate reactions, each type of derivatized surface is converted to the other type using olefinic reagents in cross-metathesis reactions. The team reports that the products remain stable upon exposure to oxygen, moisture, and various solvents.

On the basis of quantum mechanical calculations, Nuckolls and coworkers conclude that the energy level of the 1⁄4-orbital system in molecular wires, which could be grown through ring-opening reactions using conjugated molecules, would match well with ruthenium's d orbitals. The orbital overlap looks promising as a high-conductance link between the metal and the molecule, they say.