Boroles – five-‐membered heterocycles with a boron atom – are 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 prepared 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 – making them “aromatic”, and chemists have isolated this form as well. The group of Prof. Holger Braunschweig wondered: what about the missing borole with five π-‐electrons? By pushing the protection of the boron atom 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: “Boroles are 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
fundamental concepts of aromaticity and antiaromaticity.” Using Electron Paramagnetic Resonance spectroscopy, they showed that the unpaired electron is located on the boron, confirmed by its reactivity as a boron-‐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/
Supramolecular interactions – the way a molecule interacts with other molecules – 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 with 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 – 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 atomic force microscopy (AFM) to study and visualise the long fibres of pi-‐pi-‐stacked
squaraines that form in non-‐polar solvents. They found that this organisation begins with 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/2012/sc/c2sc00996j Würthner Research Group: http://www-‐organik.chemie.uni-‐wuerzburg.de/lehrstuehlearbeitskreise/wuerthner/
Defects in polymeric materials like graphene are unavoidable, and often annoying. But in graphene, defects can change the way the material responds to external stimulus – 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 atoms. 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” with a tribenzotriquinacene core. But while the defects occur naturally in graphene, synthesising a molecular version is very tricky indeed.
The first hurdle to overcome was the synthesis of a tribenzotriquinacene with 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 arene rings to create three more rings, beckons. Link to article: http://pubs.rsc.org/en/content/articlelanding/2012/cc/c1cc14703j Krueger Research Group: http://www-‐organik.chemie.uni-‐wuerzburg.de/lehrstuehlearbeitskreise/krueger/startseite/
Even high-‐school students will tell you that four-‐coordinate carbon is tetrahedral, and this concept holds too for carbon’s neighbours boron 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 boron atom, they have prepared two complexes in which the boron 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 boron atom, 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 atoms to boron was even possible came as a big surprise to us.” Since forcing boron to contort and form unsual geometries is a founding principle of the Braunschweig research group,
what is next on the list? “I really would love to see someone trying to add a fifth metal to the boron”, 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/
