The incorporation of boron in polycyclic aromatic hydrocarbon systems leads to chromophoric and fluorescent materials of interest for optoelectronics, in particular organic light emitting diodes (OLEDS) and field effect transistors, as well as sensors based on polymers. . In the newspaper applied chemistry, a research team has now introduced a new anionic organoborane compound. The synthesis of borafluorene has been successful through the use of carbenes.
Borafluorene is a particularly interesting boron-containing building block. It is a system of three carbon rings joined together at the edges: two six-membered rings and a central five-membered ring, the free tip of which is the boron atom. Although neutral, radical and cationic (positively charged) borafluorene compounds are fairly easy to produce, there have been few examples of anionic (negatively charged) borafluorene compounds to date. A better understanding of their chemistry is important for advancements in redox dependent applications and could lead to new materials with unique binding or optical properties. However, the relatively high reactivity of borafluorene anions makes their synthesis difficult. A team led by Robert J. Gilliard, Jr. at the University of Virginia (Charlottesville, USA) and David JD Wilson at Latrobe University (Melbourne, Australia) has now succeeded in isolating and structurally characterizing these elusive anions .
The starting point for their new synthesis is 9-bromo-9-borafluorene, which has a bromine atom attached to its boron atom. This is treated with a very powerful reducing agent (potassium graphite, sodium naphthalenide or lithium naphthalenide) in the presence of special carbenes (organic compounds with a divalent carbon atom and a pair of free electrons). The anionic borafluorenes formed during the reduction are stabilized by the carbenes.
As the team demonstrated, carbene-borafluorene anions can also be used as chemical building blocks. This allows new compounds to be produced which are not otherwise accessible with previously known raw materials. For example, compounds with bonds between boron and gold, selenium or germanium have been generated. The reaction with a diketone resulted in ring closure and bonding of the boron atom to the two ketone oxygenes, forming what is called a spirocyclic boron compound.
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