Structural Perspective on Aggregation of Alkali Metal Ions with Charged Planar and Curved Carbon π-Surfaces

This work overviews the coordination properties and aggregation patterns of charged π-carbon surfaces with the Group 1 metal ions. A detailed account of works on single- and multielectron reduction of planar and curved polyarenes is given based on structural considerations. First, a wealth of structurally characterized examples of alkali metal complexes with planar polyaromatic hydrocarbons having fused six-membered rings only and a mix of six- and five-membered ring systems is presented. This follows by the discussion of coordination preferences of negatively charged geodesic polyarenes that include fullerene fragments (also referred to as buckybowls or π-bowls) and fullerenes. Only recently, the first anionic products of buckybowls with alkali metal counterions have been isolated in the solid state and crystallographically characterized. This allowed to reveal the coordination preferences of mono-, di-, and tetrareduced bowl-shaped anions and to evaluate geometric perturbations of their core upon multielectron acquisition. Only a few structurally confirmed examples of fullerene anions isolated from solutions with alkali metal counterions have been so far reported. Their charges range from 1- to 3- only with the highly charged anions of C₆₀ still awaiting their isolation and structural characterization. These recent findings and their implications for our understanding of alkali metal intercalation relevant to the design and advancement of novel carbon-based electrode materials are highlighted.