Stepwise Reduction of Azapentabenzocorannulene

Mono- and dianions of 2-tert-butyl-3a²-azapentabenzo[bc,ef,hi,kl,no]corannulene (1 a) were synthesized by chemical reduction with sodium and cesium metals, and crystallized as the corresponding salts in the presence of 18-crown-6 ether. X-ray diffraction analysis of the sodium salt, [{Na⁺(18-crown-6)(THF)₂}₃{Na⁺(18-crown-6)(THF)}(1 a²⁻)₂], revealed the presence of a naked dianion. In contrast, controlled reaction of 1 a with Cs allowed the isolation of singly and doubly reduced forms of 1 a, both forming π-complexes with cesium ions in the solid state. In [{Cs⁺(18-crown-6)}(1 a⁻)]⋅THF, asymmetric binding of the Cs⁺ ion to the concave surface of 1 a⁻ is observed, whereas in [{Cs⁺(18-crown-6)}₂(1 a²⁻)], two Cs⁺ ions bind to both the concave and convex surfaces of the dianion. The present study provides the first successful isolation and characterization of the reduced products of heteroatom-containing buckybowl molecules.