A Nonalternant Aromatic Belt: Methylene-Bridged [6]Cycloparaphenylene Synthesized from Pillar[6]arene

The
synthesis, structure, and properties of methylene-bridged [6]­cycloparaphenylene
([6]­CPP), a nonalternant aromatic belt, are described. This belt-shaped
methylene-bridged [6]­CPP, in which each phenylene unit is tethered
to its neighbors by methylene bridges, was constructed through 6-fold
intramolecular nickel-mediated aryl–aryl coupling of triflate-functionalized
pillar[6]­arene in 18% isolated yield. As compared to the analogous
[6]­CPP, the methylene bridges coplanarize neighboring paraphenylene
units and enhance the degree of π-conjugation, which results
in a significant decrease in energy gap. Moreover, the incorporation
of small molecules in the defined pocket of methylene-bridged [6]­CPP
makes it an attractive supramolecular architecture. Methylene-bridged
[6]­CPP is characterized by high internal strain energy reaching 110.2
kcal mol–1, attributed to its restricted structure.
This work not only exhibits an efficient strategy to construct a new
family of aromatic belt, but also showcases their properties, which
combine the merits of CPPs and pillararenes.