As the saying goes, all good things take time – and this could be said to be especially true of major scientific breakthroughs. After nearly half a century of speculation and decades of attempts by numerous research groups, David Scheschkewitz, Professor of General and Inorganic Chemistry at Saarland University, and his doctoral research student Ankur – working with Bernd Morgenstern from Saarland University's X-Ray Diffraction Service Centre – have now delivered a breakthrough, which, fittingly, has now been reported in one of the world's leading scientific journals: Science.
What, precisely, has the Saarbrücken team achieved? Put simply, they have managed to synthesize pentasilacyclopentadienide. While a few specialists will react with 'Wow! That's remarkable!', most readers will probably respond with 'Okay, and what exactly is that?' In essence, Scheschkewitz and his colleagues have replaced the carbon atoms in an aromatic compound – a class of particularly stable molecules in organic chemistry – with silicon atoms.
Aromatics play a prominent role in the world around us, for example in the manufacture of plastics. 'In polyethylene and polypropylene production, for example, aromatic compounds help make the catalysts that control these industrial chemical processes more durable and more effective,' explains David Scheschkewitz. As silicon is much more metallic than carbon, it holds on to its electrons far less strongly. So replacing carbon with silicon in pentasilacyclopentadienide could unlock access to new compounds and catalysts with very different properties – opening the door to a new world of chemical possibilities. And David Scheschkewitz, Ankur and Bernd Morgenstern have now pushed that door wide open.
To understand why this goal was so hard to achieve, we need to take a deeper dive into the underlying chemistry. Cyclopentadienide – the carbon-containing model for the silicon analogue pentasilacyclopentadienide – is an aromatic hydrocarbon molecule, which means that its five carbon atoms are arranged in a flat ('planar') ring structure – a configuration that helps to create a uniquely stable molecular framework. (Historical side note: Aromatics were given this name because the first such compounds to be discovered in the second half of the 19th century were found to have particularly distinctive and often pleasant aromas.) 'To be classified as aromatic, a compound needs to have a particular number of shared electrons that are evenly distributed around the planar ring structure, and this number is expressed by Hückel's rule – a simple mathematical expression named after the German physicist Erich Hückel,' explains David Scheschkewitz. As these electrons are shared evenly around the ring and not confined to individual carbon atoms, these delocalized electrons make aromatic molecules particularly stable.
For decades, only one variant of these aromatic molecules was known for silicon. In 1981, the silicon analogue of cyclopropenium was synthesized – an aromatic molecule in which the three-membered ring of carbon atoms was replaced by a three-membered ring of silicon atoms. Beyond that, numerous attempts at creating silicon-based aromatics proved fruitless. Until now, that is: Ankur, Bernd Morgenstern and David Scheschkewitz have synthesized a five-atom molecule that exhibits these complex properties. And in a neat twist of fate, the very same compound was discovered almost simultaneously in Takeaki Iwamoto's laboratory at Tohoku University in Sendai, Japan. By mutual agreement, the research teams in Saarland and in Japan have published their findings side by side in the same issue of Science.
This work paves the way for entirely new materials and processes with potential industrial relevance. But the hardest first step has now been taken.
