Metals in the middle

Angew.Chem.Int.Ed.http://doi.org/f2nqcs(2013)

One of the most iconic structures in chemistry is undoubtedly ferrocene, a compound with an iron atom neatly sandwiched between the faces of two cyclopentadienyl rings. By increasing the surface area of the organic ligands, sandwich structures with more than one metal-atom filling can also be made. For example, two corannulene molecules — a bowl-shaped hydrocarbon with five six-membered rings fused around a central pentagonal hub — can form a sandwich complex with five lithium ions bound between their convex surfaces. In this structure, each ion interacts with a six-membered ring from each ligand.

Now, a team of researchers in the US led by Marina Petrukhina at the University of Albany have prepared and studied mixed-metal corrannulene sandwich complexes and shown that it is possible to squeeze six metal ions between the two organic ligands. When corranulene was reduced with a mixture of lithium and potassium, a LiK₅ complex was formed in which the five potassium ions sit between the eclipsed six-membered rings of the two organic ligands and the lone lithium ion occupies the site between the two five-membered hubs. In this structure, the larger potassium ions push the corannulene ligands further apart, creating space for a lithium ion to bind between them, unlike in the parent Li₅ complex where there is not enough room for a sixth metal to bind at this position. After careful re-examination of the mixed-metal reduction reaction, another sandwich complex — filled with three lithium and three potassium ions — was also isolated and characterized by X-ray crystallography (pictured).

Credit: © 2013 WILEY

The Li₃K₃ assembly retains the lithium ion in the hub site, and also has two lithium ions in adjacent coordination sites between the six-membered rings of the corranulene bowls. Computational studies suggest that the Li₃K₃ complex could be an intermediate in the conversion of the Li₅ system to the LiK₅ structure, with the two non-hub lithium ions acting as the hinge for a clamshell-type opening of the structure that makes the central hub binding site accessible.