Tuning Binding of Rubidium Ions to Planar and Curved Negatively Charged π Surfaces

Two new rubidium salts of the bowl-shaped
corannulene monoanion
radical C20H10•– (1•–), [Rb­(15-crown-5)2+]­[C20H10–] (2) and [Rb­(dicyclohexano-18-crown-6)+]­[C20H10–] (3), have been synthesized
and structurally characterized in
this work. An excess of 15-crown-5 ether provided full encapsulation
of the rubidium ion in [Rb­(15-crown-5)2]+, which
precluded metal-directed π interactions with the anionic surface
of C20H10•– in the
solid-state structure of 2. The use of more sterically
demanding dicyclohexano-18-crown-6 ether prevented the formation of
polymeric chains previously seen with 18-crown-6 ether and favored
the isolation of the discrete contact ion pair 3. Complex 3 is built on a convex η6 coordination of
Rb+ to C20H10•–, reaffirming the exo-binding preference of rubidium
ions toward the singly charged corannulene bowl. A rubidium salt of
the planar coronene monoanion radical C24H12•– (4•–), [Rb­(18-crown-6)+]­[C24H12–] (5), has also been prepared and structurally
characterized to reveal a new binding mode of C24H12•–. The analysis of the X-ray structure
of 5, augmented by DFT calculations, confirmed the existence
of strong C–H···π interactions between
the crown ether and charged surface of C24H12•–. These interactions result in a good
alignment of matching in size crown ether and coronene moieties, thereby
being responsible for the formation of a hub-bound coronene product.
DFT calculations were also used to follow the spin density distribution
in the [Rb­(18-crown-6)]­[C24H12] system as well
as in original fragments.