Five-membered bismuth ring opens up ‘a new era for aromatic compounds’

A five-membered aromatic bismuth ring, prepared by researchers in Germany, addresses a long-standing challenge in synthetic chemistry. The isolation of a complex containing the motif means that analogues of the classic cyclopentadiene anion have now been produced using all of the naturally occurring pnictogen elements.

Cyclopentadiene is a widely used ligand in transition metal chemistry that features an aromatic five-membered carbon ring, with the formula C₅H₅. Perhaps the most famous example of its use is in ferrocene, where two cyclopentadiene ligands sandwich an iron atom.

Over the last few decades, analogues of cyclopentadiene have been synthesised that feature group-15 pnictogen elements instead of CH- groups. An example comprised of five arsenic atoms was isolated in 1982, with phosphorus, antimony and nitrogen versions all made in the years since.

Now, a cyclopentadienyl analogue made from bismuth has been isolated by a team led by Karlsruhe Institute of Technology’s Florian Weigend and Stefanie Dehnen. The researchers note that one reason the new species has proven more difficult to make than those of the other group-15 elements, is that the ‘enhanced metallic character of bismuth often hinders viable starting materials for Bi₅^-’. With metal atoms tending to form clusters rather than rings, all-metal aromatic cyclic structures are exceedingly rare – a four-membered bismuth ring synthesised last year by a group including Weigend was the first such example to be isolated.

However, the researchers write that with a growing understanding of bismuth chemistry that has developed over the last couple of years, a targeted synthesis of the five-membered bismuth ring is now feasible. The final complex that they produced takes the form of an ‘inverse sandwich’, where the Bi₅^- ligand is trapped between two cobalt atoms. The species was characterised by x-ray crystallography, with density-functional theory calculations confirming its π-aromatic nature.

The team believes that the isolation of the bismuth species will open up new possibilities for synthetic chemists. ‘Capturing Bi₅⁻ represents a landmark in the chemistry of all-metal aromatic molecules and defines a new era for aromatic compounds,’ they write.