The discovery of graphene and its extraordinary properties have motivated research on large polycyclic aromatic hydrocarbons (PAHs), which are basic building blocks of graphene and other carbon-rich nanostructures. Significant attention has focused on PAHs as molecular models and synthetic precursors, and as components for electronic and optoelectronic devices. Their suitability for this purpose is due not only to their unique electronic and photophysical properties, which result from their extended conjugation and rigidity, but also their tendency to assemble into highly ordered structures in the solid state. Thus, a central goal in this area is the synthetic manipulation of electronic properties for nanocarbon materials, by generation of well-defined dimensionalities and edge structures. This is particularly challenging since it requires high-yielding, efficient synthetic methods for multiple ring fusions within the same molecule. Importantly, near-quantitative yields are necessary for each ring-fusion event to avoid difficult separations and/or defects. Although the development of organometallic methods for the difficult ring-fusion steps has received much attention, few are high-yielding enough for application to very large PAHs. This research group has employed selective metal-mediated cycloadditions for extension of π systems and introduction of multiple fused rings in the synthesis of π-conjugated oligomers, polymers, and macrocycles, mainly via the reductive coupling of alkynes with a low-valent zirconocene reagent. In addition to the zirconocene coupling of alkynes, high-yielding and chemoselective [2+2+2] cycloadditions of alkynes (with an iridium catalyst) and/or [2+2+n] cycloadditions of nitriles (with titanocene) have been developed as complimentary synthetic tools for access to and elaboration of nanocarbon structures. Promising features of this approach are: 1) generation of a PAH directly from the precursor (e.g., no subsequent oxidation is required) and 2) divergent introduction of functionality using metal-transfer reactions. This chemistry will be described, along with applications to scalable syntheses of large PAHs and carbon nanostructures, including expanded helicenes and carbon nanobelts.

Metal-mediated cycloadditions and macrocyclizations for scalable syntheses of conjugated nanocarbons

ACS Fall 2021

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