Twenty-year-old molecular prediction comes true – chemists have finally managed to synthesize an unusual and elusive molecule

André Schäfer and Inga Bischoff

Advances in metallocene chemistry led to the synthesis of ‘heterobimetallic’ sandwich molecules, challenging to create but offering the potential for new chemical discoveries and industrial applications. André Schäfer and Inga Bischoff in the laboratory with a sample of their new dimetallocene. Credit: University of Saarland/Thorsten Mohr

The first and best known metallocene is ‘ferrocene’, which contains a single iron atom. Sandwich complexes are now standard topics in inorganic chemistry textbooks, and the bonding and electronic structure of metallocenes are covered in undergraduate chemistry courses. These sandwich molecules are also important in industry, where they serve as catalysts and are used in the creation of unique metallopolymers.

No one knows exactly how many sandwich molecules there are today, but the number is certainly in the thousands. And they all have one thing in common: a single metal atom located between two flat rings of carbon atoms. At least that was the thinking until 2004, when a research group from the University of Seville made a surprising discovery.

The Spanish research team managed to synthesize a sandwich molecule that contained not one but two metal atoms. For a long time, this ‘dimetallocene’ with two zinc atoms remained the only example of its kind, until last year a group in Britain managed to synthesize a very similar molecule containing two beryllium atoms. But now Inga Bischoff, PhD student in Dr. André Schäfer at the University of Saarland, has taken a big step further. She has managed to synthesize the world’s first ‘heterobimetallic’ sandwich complex in the laboratory: a dimetallocene containing two different metal atoms.

Theoretical and practical breakthroughs

Shortly after the discovery of the first dimetallocene in 2004, theoretical work indicated that dimetallocenes do not necessarily have to contain two identical metal atoms, and that a complex with two different metal atoms should also be stable. These predictions were made based on quantum chemical modeling calculations using powerful computers. Despite this predicted stability, all attempts to create such a molecule in the laboratory had failed until Inga Bischoff’s current breakthrough.

‘It is very exciting and special when you realize what you have in your hands. To the naked eye it looks like just another white powder. But I can still clearly remember the moment when we first saw the experimentally determined molecular structure on the computer screen and we knew that we had a sandwich molecule with two different metal atoms,” says Dr. André Schäfer.

‘Which carbon rings you choose is just as important as which metal atoms you want to enclose between them. This is crucial because the electronic structures of the cyclic carbon rings and the metal atoms must fit together,” explains Inga Bischoff. ‘The metals in our ‘heterobimetal dimetallocene’ are lithium and aluminium. Calculations predicted that these two metals would be suitable candidates because their electronic structure is in some sense similar to that of two zinc atoms, which we knew could form a stable dimetallocene.’

But what sounds so simple and straightforward took months to achieve. The molecule turns out to be so reactive that it can only be synthesized, stored and analyzed under an inert nitrogen or argon blanket. If it came into contact with air, it would simply decompose. Once synthesized, the molecule had to be characterized, which involved an entire team of scientists from the University of Saarland. The results of their work have now been published in the highly respected journal Natural chemistry.

“Our heterobimetallic dimetallocene represents what is essentially an entirely new class of sandwich molecules,” said group leader Dr. André Schäfer. ‘Who knows, it might one day appear in a student’s textbook. But first we need to study it further. At this point, we have a fairly good understanding of its structure, but still know very little about its reactivity. If we find other suitable combinations of metal atoms, it may prove possible in the future to synthesize other heterobimetallic dimetallocenes.’

The enormous significance of this class of molecules is underlined by the award of the Nobel Prize in 1973 to the German chemist Ernst Otto Fischer and the British chemist Geoffrey Wilkinson ‘for their pioneering work, carried out independently, in the field of organometallic chemistry, so-called sandwich joints.’

Reference: “A lithium-aluminum heterobimetallic dimetallocene” by Inga-Alexandra Bischoff, Sergi Danés, Philipp Thoni, Bernd Morgenstern, Diego M. Andrada, Carsten Müller, Jessica Lambert, Elias CJ Gießelmann, Michael Zimmer and André Schäfer, May 14, 2024, Natural chemistry.
DOI: 10.1038/s41557-024-01531-y

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