Supersized Sandwiches

Ferrocene and other single-metal metallocenes may soon start to look like the dieter's option in the menu of sandwich complexes. Overstuffed sandwich complexes, filled with a monolayer of metal atoms, have been isolated for the first time by researchers at Japan's Osaka University and Kyoto University (Science 2006, 313, 1104).

The team, led by Osaka-based chemists Tetsuro Murahashi and Hideo Kurosawa, created two of these novel compounds. In the simpler structure, chloride ions cap a triangular array of three palladium atoms, which are sandwiched between two cycloheptatrienyl ligands. The other complex resembles a submarine sandwich, with its planar array of five palladium atoms nestled between two polycyclic naphthacene ligands.

The surprisingly stable compounds can be handled in air, and the simpler compound worked well as a cross-coupling catalyst in preliminary tests. Murahashi points out that the use of palladium as the metal filling is remarkable because palladium isn't regularly used in sandwich chemistry.

"The concept of making discrete monodisperse 'supersandwiches' with alternating organic and metallic layers has been around for decades," comments metallocene expert K. Peter C. Vollhardt of the University of California, Berkeley. "Murahashi and Kurosawa's group is the first to demonstrate its feasibility in a synthetic approach that is startling in its simplicity and highly aesthetically pleasing in its outcome."

Murahashi says he and his colleagues hope to learn more about supersandwiches' fundamental properties by synthesizing a wide range of the compounds, including mixed-metal systems and complexes that employ other unsaturated hydrocarbon ligands. "We believe that different sizes and shapes of monolayer metal sheets could be formed, particularly when different planar carbocyclic ligands are used," he says. "We also expect that different metals can be used" because many metals form simple metallocenes.

"This work is a beautiful illustration of how fundamental research provides a foundation for possible conceptual and concrete advances in catalysis and materials research," Vollhardt concludes.