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

J Am Chem Soc. 2015 Dec 16;137(49):15528-32. doi: 10.1021/jacs.5b10388. Epub 2015 Dec 1.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't