Titanocene-Mediated Dinitrile Coupling: A Divergent Route to Nitrogen-Containing PAHs

Nitrogen-containing polycyclic aromatic hydrocarbons (PAHs) have attracted tremendous interest in organic electronics, as they often are good electron-acceptors and have increased stability. Such compounds are also models or building blocks for nitrogen-doped graphene, which is currently under intense investigation for its applications in electronics and catalysis. We recently developed a new synthetic methodology to access nitrogen-substituted PAHs that enables rational, late-stage control of HOMO and LUMO energy levels and thus photophysical and electrochemical properties. The method involves two key steps: 1) a titanocene-mediated reductive cyclization of an oligo(dinitrile) precursor to form a PAH appended with di(aza)titanacyclopentadiene functionality; and 2) a divergent titanocene transfer reaction, which allows final-step installation of one or more o-quinone, diazole, or pyrazine units into the PAH framework.
The new dinitrile coupling chemistry is especially interesting from a fundamental chemical perspective, as our recent report was the first productive use of di(aza)metallacyclopentadiene intermediates in organic synthesis. Our focus is on application of this methodology to PAHs and carbon nanostructures; however, this chemistry should find applications in other fields where o-quinone, diazole, or pyrazine functionalities are desirable. Current efforts involve application of the method to synthesis of more complex electron-deficient PAHs (e.g. expanded helicenes), especially those containing multiple redox centers.