Boroles – five-‐membered heterocycles with a boron atom – are on

<strong>Boroles</strong> <strong>–</strong> <strong>five</strong>-­<strong>‐membered</strong> <strong>heterocycles</strong> <strong>with</strong> a
<strong>boron</strong> <strong>atom</strong> <strong>–</strong> <strong>are</strong> one of the more curious
families of molecule. They have four π-­‐electrons,
making them “antiaromatic” and, at least on
paper, unlikely to be stable. Yet, synthetic
chemists prep<strong>are</strong>d them by bulking up the
groups around the ring, creating a shield of
aromatic groups that stops them from
decomposing. If you add two
electrons to a borole you increase
the π-­‐electron count to six <strong>–</strong>
making them “aromatic”, and
chemists have isolated this form as
well. The group of Prof. Holger Braunschweig
wondered: what about the missing borole <strong>with</strong>
<strong>five</strong> π-­‐electrons?
By pushing the protection of the <strong>boron</strong> <strong>atom</strong>
to the extreme and adding just one electron to
the molecule, doctoral student Johannes Wahler
isolated the 5-­‐electron borole, a radical anion. In
his words: “<strong>Boroles</strong> <strong>are</strong> real multi-­‐talents in
terms of reactivity, which is governed by the
antiaromatic nature of this class of molecules. By
the synthesis of a borole radical anion we
intended to create a junction between the two
Supramolecular interactions <strong>–</strong> the way a
molecule interacts <strong>with</strong> other molecules <strong>–</strong> can
have a huge effect on the properties of
functional materials. Squaraines, promising
molecules for applications as fluorescent dyes,
have both large flat pi-­‐systems ideally suited to
intermolecular pi-­‐pi stacking and the possibility
for hydrogen bonding.
These structural traits result in the well-­‐
known, property-­‐altering aggregation of
Squaraines. Despite their potential as molecular
materials, no studies of the aggregation of
squaraines have been performed in the absence
of water, which can interfere <strong>with</strong> non-­‐covalent
interactions. Recognising this, the group of Prof.
Dr. Frank Würthner set out to study the
aggregation of squaraines in exclusively non-­‐
polar solvents <strong>–</strong> conditions where the weak
intermolecular interactions can truly shine.
In a publication in the new journal Chemical
Science, Dipl. Chem. Ulrich Mayerhöffer and
Prof. Würthner use UV-­‐visible spectroscopy and
<strong>atom</strong>ic force microscopy (AFM) to study and
visualise the long fibres of pi-­‐pi-­‐stacked
fundamental concepts of aromaticity and
antiaromaticity.”
Using Electron Paramagnetic Resonance
spectroscopy, they showed that the unpaired
electron is located on the <strong>boron</strong>, confirmed by
its reactivity as a <strong>boron</strong>-­‐centred radical.
According to Mr. Wahler, there is still a lot of
work to be done: “Future work will include
synthesis of related borole radical anions and
other fancy borole derivatives -­‐ a job that is
challenging but rewarding.” The results were
published recently in Angewandte Chemie,
International Edition.
Link to article:
http://onlinelibrary.wiley.com/doi/10.1002/anie
.201108632/abstract
Braunschweig Research Group:
http://www-­‐anorganik.chemie.uni-­‐
wuerzburg.de/Braunschweig/
squaraines that form in non-­‐polar solvents. They
found that this organisation begins <strong>with</strong> the
coupling of two squaraines to form a dimer,
followed by stacking of the dimers to form long
chains about three nanometres in width, which
eventually clump together in bundles about nine
nanometres wide.
Link to article:
http://pubs.rsc.org/en/content/articlelanding/2
012/sc/c2sc00996j
Würthner Research Group:
http://www-­‐organik.chemie.uni-­‐
wuerzburg.de/lehrstuehlearbeitskreise/wuerthn
er/

Defects in polymeric materials like graphene <strong>are</strong>
unavoidable, and often annoying. But in
graphene, defects can change the way the
material responds to external stimulus <strong>–</strong>
sometimes in desirable ways.
These defects occur in graphene when the
usual honeycomb-­‐like pattern of hexagons is
interrupted by pentagons or heptagons. As we
all learn as children, there’s no way to make flat
networks of pentagons or heptagons, so these
defects create blisters in an otherwise dead-­‐flat
sheet of carbon <strong>atom</strong>s.
To study how such a defect disturbs the
electronic and magnetic properties of graphene,
the group of Prof. Dr. Anke Krueger has
targetted an isolated, molecular version of the
irregularity, a “defective graphite flake” <strong>with</strong> a
tribenzotriquinacene core. But while the defects
occur naturally in graphene, synthesising a
molecular version is very tricky indeed.
Even high-­‐school students will tell you that four-­‐
coordinate carbon is tetrahedral, and this
concept holds too for carbon’s neighbours <strong>boron</strong>
and nitrogen. However, a new report in
Angewandte Chemie, International Edition from
the research group of Prof. Dr. Holger
Braunschweig suggests otherwise. By attaching
four transition metals to a <strong>boron</strong> <strong>atom</strong>, they
have prep<strong>are</strong>d two complexes in which the
<strong>boron</strong> is essentially flat.
The students who performed the syntheses,
Dr. Katharina Kraft and Dipl. Chem. Sebastian
Östreicher, spent over a year trying to add the
crucial fourth metal fragment to the <strong>boron</strong> <strong>atom</strong>,
but did not expect that both complexes would
turn out to be planar.
As Mr. Östreicher
explains, “The sheer fact that
coordination of four metal
<strong>atom</strong>s to <strong>boron</strong> was even
possible came as a big
surprise to us.” Since forcing <strong>boron</strong> to contort
and form unsual geometries is a founding
principle of the Braunschweig research group,
The first hurdle to overcome was the
synthesis of a tribenzotriquinacene <strong>with</strong> six
substituents at para-­‐positions (i.e. the portion
shown in black in the figure). After much
tribulation, two variations of the desired
structure were isolated, and their progress has
recently been published in the journal Chemical
Communications. The next step in the process,
connecting the three <strong>are</strong>ne rings to create three
more rings, beckons.
Link to article:
http://pubs.rsc.org/en/content/articlelanding/2
012/cc/c1cc14703j
Krueger Research Group:
http://www-­‐organik.chemie.uni-­‐
wuerzburg.de/lehrstuehlearbeitskreise/krueger/
startseite/
what is next on the list? “I really would love to
see someone trying to add a fifth metal to the
<strong>boron</strong>”, said Mr. Östreicher.
Link to article:
http://onlinelibrary.wiley.com/doi/
10.1002/anie.201107248/abstract
Braunschweig Research Group:
http://www-­‐anorganik.chemie.uni-­‐
wuerzburg.de/Braunschweig/