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Progress in chemistry is often gradual, with some of its most important advances taking years – sometimes decades – to unfold. A case in point is the discovery of a novel ‘ferrocenophane’ from the class of compounds known as ‘sandwich molecules’ – so named because of their particular structure. In a ferrocenophane, the ‘bread slices’ are two carbon rings that enclose an iron atom as the sandwich ‘filling’. A team of chemists at Saarland University has now succeeded in developing a highly unusual bent sandwich molecule that opens up new possibilities for designing iron-containing materials.
This result counters the long-held and generally accepted assumption that it would be impossible to produce a stable molecule with this structure.
Metallocenes, or ‘sandwich molecules’ as they are often known, consist of two flat rings of carbon atoms with a metal atom sandwiched between them. If, for example, the metal atom is iron (Latin: ferrum), the compound is known as ferrocene. A further class of compounds derived from ferrocene are the so-called ferrocenophanes, in which the carbon rings are bridged by other atoms such as silicon, boron or sulfur. The two rings can also be linked by a bridge comprising two or more carbon atoms. For decades, however, these ferrocenophanes remained little more than a laboratory curiosity with little or no practical application.
In the 1990s, the British chemist Ian Manners discovered that the bending – especially when the rings are bridged by a single atom – leads to substantial ring strain, which can be exploited in polymerization reactions to produce iron-containing plastics. This synthetic strategy opened up entirely new possibilities for the production of metallopolymers, and over the past 30 years a wide range of ferrocenophanes with many different bridging elements have been prepared. However, one molecule that remained elusive during this period was that in which the two rings are bridged by a single carbon atom. A carbon atom is very small – even smaller than boron or sulfur – so that the two rings would have to be bent at a very sharp angle, creating high ring strain. ‘Short bridging bonds force the molecule to bend – something it doesn’t like,’ explains Dr. André Schäfer, a senior staff scientist who conducts research and teaches inorganic chemistry at Saarland University.
This was the reason why some scientists believed that a ferrocenophane molecule with a single carbon atom as the bridging element would not be stable, and in fact could not exist at all. That view now looks set to change. Aylin Feuerstein, a doctoral research student in Schäfer’s group, has been working on the topic for several years and last year, she achieved what many had thought impossible: she successfully synthesized a ferrocenophane with a single bridging carbon atom.
At Saarland University, André Schäfer’s research group has been studying bent sandwich molecules of this type for a number of years. Together with researchers from Professor Markus Gallei’s group, the Saarbrücken team is also investigating how these ring-strained precursors can be used to produce metal-containing polymers.
‘Metal-containing polymers are interesting for a number of reasons,’ says Markus Gallei, professor of polymer chemistry at Saarland University. ‘By incorporating metals into organic polymers, we combine the properties of two different worlds, opening up entirely new applications in areas such as switchable optical materials, membranes and electrically switchable polymer surfaces.’ With the synthesis of this new ferrocenophane in Saarbrücken, the range of applications of these materials could once gain expand significantly.
The Saarbrücken success story was no lucky accident, but the result of a combination of theoretical and experimental approaches. It began with computer-based modelling to try and answer the key underlying question: Is the ring strain really so high that the molecule is unstable? ‘There is no one blanket answer to this question. It depends on the precise structure of the molecular framework; small changes make a big difference,’ explains Aylin Feuerstein. ‘Once we had understood that, we were able to design a ferrocenophane molecule on the computer that was likely to be stable.’
This was when the real laboratory work began, with the synthesis taking many months to complete. First, the team constructed the appropriate molecular framework. At this stage, the metal atom was still missing. ‘We initially set out to incorporate a magnesium atom. From our earlier work, we already had experience of doing this and we knew that it could later be replaced relatively easily by an iron atom,’ explains Feuerstein. Eventually, the research team succeeded in isolating a red powder. Analysis quickly showed that it was indeed a ferrocenophane bridged by a single carbon atom. ‘The big surprise was the molecule’s exceptionally high thermal stability,’ says André Schäfer.
‘At the beginning, I was extremely cautious,’ Aylin Feuerstein recalls. ‘We thought it might decompose even at room temperature. But we were able to show that the molecule is remarkably stable even at elevated temperatures. In fact, the problem was never really the ring strain, but the synthesis itself, as no one really knew how to make the molecule. Once you’ve isolated it, you can heat it to over 200 °C without anything happening,’ explains Aylin Feuerstein.
Having now been synthesized, the molecule then needed to be characterized – a process that involved a whole team of scientists. Dr. Sergi Danés Pibernat from the research group of Professor Julio Lloret-Fillol at the Institut Català d’Investigació Química (ICIQ) in Tarragona, Catalonia, Spain, carried out quantum-chemical calculations to understand why certain structural features of the molecular framework lead to such high stability.
The team has now published its discovery in the journal Angewandte Chemie.
Dr. André Schäfer
Tel.: +49 681 302-70668
Email: andre.schaefer@uni-saarland.de
'The Missing Link in Ferrocenophane Chemistry: Isolation of a Carba[1]Ferrocenophane'; A. Feuerstein, S. Danés, K. Kolling, B. Morgenstern, M. Gallei, A. Schäfer, Angew. Chem. Int. Ed. 2026 , e2211037; https://doi.org/10.1002/anie.2211037
Aylin Feuerstein, a Ph.D. student in André Schäfer's research group, has succeeded in synthesizing f ...
Source: Thorsten Mohr
Copyright: Universität des Saarlandes
Dr André Schäfer, private lecturer in inorganic Chemistry at Saarland University
Source: Thorsten Mohr
Copyright: Universität des Saarlandes
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