Cyclopropenium Cations Break the Rules of Attraction to Form Closely Bound Dimers

The crystal structures
of tris­(ethylmethylamino)-cyclopropenium
chloride and tris­(diethylamino)-cyclopropenium iodide reveal the presence
of closely bound dicationic dimers formed from two closed-shell monomer
units. The distances between the C3 centroids of the staggered
monomers are at the short end of those normally found in π-stacked
neutral arenes, let alone charged aromatic rings. Computational analysis
reveals that short-range interactions are dominated by strong dispersion
forces, enabling metastable dicationic dimers to form without covalent
intermolecular bonding. Surrounding counterions then provide a background
source of charge balance, imparting strong thermodynamic stability
to the system. Additionally, these counterions form a weak but attractive
electrostatic bridge between the monomer units, contributing to the
surprisingly short observed intermolecular C3–C3 centroid distance.