About 75 years ago, scientists accidentally synthesised a compound called ferrocene in which the iron (Fe) atom is sandwiched between two C5H5 rings – (C5H5)Fe(C5H5). This compound opened up a new era in transition metal chemistry, and became an important reagent in catalysis, materials, biology, and medicine. For a long time, researchers have tried to create a version of this sandwich that contains no carbon at all, using boron instead – achieving this would prove that these complex structures are not limited to carbon-based chemistry.
In a new study published in Science, researchers at the Indian Institute of Technology Madras (IITM) and Indian Institute of Science (IISc) have successfully created a carbon-free boron alternative to ferrocene for the first time. The team used a metal called osmium (Os), which falls in the same group of the periodic table of elements as Fe, to hold together rings made of boron and hydrogen. They discovered that this new compound – ((B5H10)Os(B5H10)) – looks very similar to the original ferrocene sandwich. They also found that the bond holding the boron sandwich together is stronger than the carbon version because the boron rings have B-H and BHB hydrogen atoms placed strategically that help them grab the metal atom more effectively than carbon rings.
The new compound shows that boron can mimic carbon’s ability to form stable rings and complex structures. Understanding how such elements bond could also lead to the development of new types of materials in the future.
For many years, researchers including Eluvathingal D Jemmis, National Science Chair-ANRF in the Department of Inorganic and Physical Chemistry (IPC), IISc, have been working on the possibility of replacing carbon in the C5H5 ring of ferrocene by B-1 (anion of boron atom which has the same number of electrons as carbon). Jemmis has been collaborating with Sundargopal Ghosh, Professor at the Department of Chemistry, IITM, for more than 15 years on ways of stabilising polyhedral boranes with other elements, including transition metals. Ideas from orbital engineering led them to try (B5H10)Os(B5H10) as a preferred target, which Ghosh’s students were able to succesfully synthesise in the lab.
To synthesise the compound, the team used a chemical process called thermolysis in which they heated an osmium precursor with a boron-hydrogen source at 100°C. Following this reaction, they were able to isolate the new compound as a colourless solid. Using X-ray analysis and NMR spectroscopy, the team was able to confirm that they had successfully built the “sandwich” structure. “Just as ferrocene started a new era in organometallics, these results will start a new era in inorganometallics and will soon be a part of textbooks of inorganic chemistry. Our efforts are on to study the reactions of these new compounds,” says Ghosh of IIT Madras, one of the corresponding authors.
In the new structure, the osmium atom sits between the two flat boron rings. The rings, however, are closer together than those in the carbon version and the bonding is also stronger. While synthesising the compound, the team also discovered a different version of the molecule (isomer) in which a ring is attached in a unique way that has not been seen in the carbon original. This alternative structure shows that boron molecules can connect to metals in more ways than carbon can.
“With the renaissance in the 2D chemistry of boron during the last decade – with borophenes, bilayer borophenes, and multilayer borophenes on the horizon – the possibility of metal sandwiched/intercalated bilayers and multilayers will be a reality soon,” says Jemmis, one of the corresponding authors. These materials can potentially rival graphene in many applications.
