Adam Capriola
“A Stable Neutral Diborene Containing a B=B Double Bond”
Unlike the well know and oft studied chemistry of double bonds between carbons,
the chemistry of boron-boron double bonds is for the most part unexplored. It is believed
that boron should behave similarly to carbon due to its relativity to the element on the
periodic table. Anions containing boron double bonds, specifically [R2BBR2]2-, have in
the past been predicted to be possible structures of interest to synthesize in the laboratory,
however such efforts have failed for the most part.
It was then proposed to explore neutral diborenes, even though they in theory
should be highly reactive compounds due to their triplet ground states and two oneelectron π-bonds according to molecular orbital theory. The electron deficiency in this
structure could however be stabilized by the addition of Lewis base ligands. The
stabilizing ability of different ligand groups were assessed, including CO and NHC,
which were chosen based on their strong electron donating capabilities. The ligand group
that ultimately experimentally produced an actual neutral diborene was :C{N(2,6PRi2C6H3)CH}2. Previous work from using this ligand group for stabilizing carbenes
suggested that this would be a potential stabilizing ligand for a diborene.
This compound, R(H)B=B(H)R, where R is the aforementioned ligand group, was
synthesized beginning with RBBr3 and KC8 in diethyl ether. Two products were isolated
from this reaction, including the desired diborene R(H)B=B(H)R. It was shown that a
ratio of 1:5.4 of RBBr3 to KC8 yielded the highest percentage of R(H)B=B(H)R (12%).
Any excess amount of RBBr3 over this ratio resulted in a decrease of R(H)B=B(H)R and
thus in increase of the other product, R(H)2B-B(H)2R.
A few methods were utilized in order to determine the chemical makeup of these
products. NMR resonances of RBH3, R(H)2B-B(H)2R, and R(H)B=B(H)R were
respectively reported to be -35.38, -31.62, and +25.30 ppm. The 11B signal of
R(H)B=B(H)R produced a quartet, while the other two compounds elicited singlets. This
alone could suggest double bond character between borons.
X-ray structural analysis shows a bond distance of 1.828 Å for R(H)2B-B(H)2R.
This number seems to be on point with calculated B-B bond lengths for similar structures
such as the CO-ligated analogue (1.819 Å) and an activated m-terphenyl based diborate
(1.83 Å). Crystallization of R(H)B=B(H)R reveals B-C bond distances of 1.547 Å,
which is marginally shorter than that of the other molecules. In addition to this, it is
calculated that the angles between the C3N2 carbene rings and the core are strikingly
different than that of the other compounds used and produced. Finally, the B=B bond
distance in R(H)B=B(H)R was measured to be much shorter than the B-B distance
reported in R(H)2B-B(H)2R, again implying a double bond.
DFT computations were also used to support the nature of R(H)B=B(H)R. The
analysis was performed on the simplified model, where R=:C(NHCH)2. The
experimental bond lengths for the non-simplified model seem to be in concordance with
the computed B-B and B-C bond lengths, and well as the B-B-C bond angle calculated
from the simplified model analyzed using DFT. The bond character of these bonds was
also delved into via HOMO representations of the compounds among other
computational techniques.
In conclusion, the authors of the paper were able to successfully prove that they
had synthesized and characterized the first neutral diborene compound. They also
ventured into the nature of the elusive boron-boron double bond. Though it was not
necessarily expected that this phenomenon could feasibly be synthesized due to the
expected reactivity of the boron-boron double bond, these chemists found a way to isolate
the compound. In context to the larger field of chemistry, I suppose that the authors
could determine other possible ligand groups that would produce a stable neutral
diborene. They could also venture into increasing the percent yield, as 12% is on the low
side. Finally, they could explore other group 13 elements, such as Al and Ga to see if
they can replicate similar double bond properties.