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).
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.
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Cantrill, S. Metals in the middle. Nature Chem 6, 4 (2014). https://doi.org/10.1038/nchem.1839