Cyclopentadienyl Superligand Debuts

The first synthesis of metal cyclopentadienyl (Cp) complexes substituted with five other metal cyclopentadienyl groups has been accomplished by K. Peter C. Vollhardt of the University of California, Berkeley, and coworkers (Angew. Chem. Int. Ed., published online Feb. 10, dx.doi.org/10.1002/anie.200504047). These "super Cp" complexes are of interest for fundamental chemical studies and are expected to function as substrates or ligands in magnetic and electronic materials and as metallic "minisurfaces" in catalysis.

A decade ago, Vollhardt's group succeeded in making the pentacyclopentadienyl-substituted manganese complex (C₅H₄)₅C₅Mn(CO)₃. But the researchers' attempts to metalate the radial Cp substituents failed. After some trial and error, the team has now devised a strategy to directly add metal-Cp groups to the C₅Mn(CO)₃ core. For example, reaction of (C₅I₅)Mn(CO)₃ with five equivalents of (CpFeC₅H₄)₂Zn produces the ferrocenyl-substituted manganese complex (CpFeC₅H₄)₅C₅Mn(CO)₃.

In subsequent chemistry, Vollhardt's group was able to "anneal" some of the pendant Cp groups together by eliminating some of the metal atoms and thereby form complex fused-ring systems. The researchers also were able to "liberate" the ferrocenyl-substituted complex by UV irradiation to form (CpFeC₅H₄)₅C₅R, where R represents hydrogen or a phenoxy group. Minus the manganese tricarbonyl group, these compounds have considerably less structural strain and are amenable to further chemistry, such as protonation of the core C₅ ring to form a cyclopentenyl cation.

Synthesis of the pentaferrocenylcyclopentadiene is "just amazing," remarks Guy Bertrand of the University of California, Riverside. "I am quite convinced that very unusual chemistry will be found with these molecules," he says.

Henning Hopf of the Technical University of Braunschweig, in Germany, is similarly enthused. The synthesis of this super Cp, he says, opens the door to everything, "exciting structural work, the study of new physical and chemical properties, and further chemical transformations." This type of curiosity-driven research is what makes applied research possible, he adds.