“Naked” Mono- and Dianions of Corannulene with Lithium Counterions

Controlled reduction of corannulene (C20H10, 1) with lithium metal has been investigated
in several O-donor solvent media (THF, DME, and diglyme). In contrast
to the prior reduction studies performed in situ,
the isolation of solid products of singly and doubly charged corannulene
in the form of crystalline lithium salts has been accomplished. The
single-crystal X-ray diffraction studies of two products containing
monoreduced corannulene, [Li­(DME)3+]­[1–] (2) and [{Li­(15-crown-5)+}2]­[1–]2 (3), revealed the presence of naked bowl-shaped monoanions.
The corannulene dianions have been crystallized from two different
media as corresponding salts, [{Li­(DME)+}­{Li­(DME)1.5+}]2[12–]2 (4) and [Li­(diglyme)2+]2[12–] (5), illustrating the effect of solvents on the alkali metal coordination
environment and binding to the curved π-carbon surfaces. The
adduct 4 has Li+ ions bound to the five- and
six-membered rings of the doubly reduced corannulene convex face.
In contrast to the above contact ion pair obtained from the DME solution,
crystallization of the C20H102– anion from the strongly chelating diglyme leads to the isolation
of free 12–, in which the geometrical
parameters of the corannulene core are not affected by direct coordination
of alkali metal ions. Notably, the latter represents the first example
of the “naked” corannulene dianion and allows us to
examine how charging the C20H10 bowl with two
electrons affects its core geometry. The single-crystal X-ray structural
data are complemented by full characterization of products 2–5 using NMR, ESR, and UV–vis spectroscopies.