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When it comes to chemical bonding, boron has a reputation for
being unconventional. While covalent bonds are usually formed by
sharing two electrons between two atoms, some compounds—including
diboranes (B2H6) —contain B–H–B bonds in which an electron pair is
distributed over three sites. The electron-deficient nature of these
‘3-center, 2-electron’ bonds can generate a variety of distinct
chemical structures, some of which—such as triple-bonded diborane
derivatives—have only been seen theoretically.
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Figure 1: X-ray measurements (bottom) reveal that a butterfly-shaped
boron compound (top left) has an intense electron distribution in its B2H2
core (red areas) that is stabilized by exterior bulky ligands (white lines).
The human eye (top right) shows the direction from which you look at
the electron density map.
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Kohei Tamao and colleagues from the RIKEN Advanced Science Institute
in Wako and Kyoto University have now isolated the first stable diborane
molecule with butterfly-shaped B–H–B bonds and a boron–boron link with
triple bond characteristics1. This discovery unlocks new insights into the
workings of 3-center, 2-electron boron interactions and puts scientists
one step closer to synthesizing the elusive boron–boron triple bond.
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The key to this approach is a bulky molecule known as ‘Eind’ that contains
a rigid core of fused hydrocarbon rings covered with ethyl side chains.
Previously, the researchers used Eind ligands to stabilize heavy elements
into multiply bonded species2. This time, the team hoped to generate
a neutral boron–boron double bond by substituting Eind groups for
hydrogen atoms in diborane.
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However, after characterizing the structure of the diborane–Eind
compound—a difficult task requiring synchrotron x-rays to detect
hydrogen atom positions—the researchers saw a previously
unidentified arrangement at the B2H2 core: a central boron–boron
connection nearly as short as a theoretical triple bond, flanked by
two symmetric B–H–B ‘wings’ (Fig. 1). “We did not expect this butterfly
-shaped structure at first, and finding it was a kind of serendipity,” says
co-author Yoshiaki Shoji.
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Quantum computations revealed that the Eind ligands enforced a
levels closely related to the triple-bond species. Furthermore, the
bridging hydrogen atoms enhanced the multiple bonding characteristics.
“Based on this analysis, it is possible to consider triple bonding
interactions between the two boron atoms,” says team-member
Tsukasa Matsuo.
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Matsuo notes that the butterfly-shaped molecule already displays unique
chemical reactivity, and the insights gained from this new structure
could lead to additional multiply-bonded diboranes. “We may be able
to synthesize a more triply bonded species in the near future by replacing
the bridging hydrogen atoms with alkali metals,” he says. “At the
moment, this compound is just a dream but I think we have a chance
to obtain it.”
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