Unexpected boron coupling reaction

Two positively charged compounds of boron have been joined to form a chain of four boron atoms in a previously unknown coupling reaction.

In synthetic chemistry, element-element bonding is exploited to assemble small building blocks into larger, more complex molecules. In carbon-carbon chemistry, for example, steam cracking of crude oil will – via various processes – eventually lead to the production of plastics, fuels and pharmaceutical agents.

Researchers from Heidelberg University’s Institute of Inorganic Chemistry have been focussing on element-element coupling reactions of this kind, only with boron instead of carbon. New element-element combinations usually result from a reaction between two electrically neutral, or differently polarised atoms, explained Professor Hans-Jörg Himmel, Chair of Inorganic Chemistry. However, Himmel’s team have discovered a coupling reaction in which two positively charged molecules bond via multi-centre bonding, which plays a significant role in boron chemistry.

“The product of this reaction is a compound with four boron atoms,” said Himmel. “This in its turn is a promising precursor on the route toward the making of complex boron chains.”

The team created dicationic tetraborane – two boron-boron units connected by three-centre, two-electron bonds.

Himmel’s team are now investigating further combinations of the unprecedented four-atom boron chain to form boron chain polymers, which are expected to possess high electrical conductivity, as well as other useful properties. These chains could lead the way to long sought-after boron chain polymers with a structure similar to silicon chains in ?-SiB₃. Such materials would be of interest for electronic and optoelectronic applications.

The work has been published in Nature Chemistry: A boron-boron coupling reaction between two ethyl cation analogues http://www.nature.com/nchem/journal/v5/n12/abs/nchem.1776.html