Triaminocyclopropenium Halide and Triiodide Salts: The Formation of Cyclopropenium Dimers

The crystal structures of tris(ethylmethylamino)cyclopropenium bromide and tris(allylmethylamino)cyclopropenium chloride are found to contain weak dimers of the cyclopropenium cation whereas the structure of the mixed salt of tris(dimethylamino)cyclopropenium chloride and dimethylammonium chloride reveals half of the cyclopropenium cations to have formed dimers and the other not to have formed dimers. These structures are compared to three related triiodide salts ([C3(NMeEt)3]I3, [C3(NEt2)3]I3 and [C3(NBu2)3]I3) as well as the previously-reported salts [C3(NEtMe)3]Cl, [C3(NH2)3]Cl, [C3(NH2)3]I and [C3(NEt2)3]I. The distances between the C3 centroids of the staggered dimers are at the short end of those normally found in π-stacked neutral arenes, let alone charged aromatic rings. Factors leading to the formation of dimers and the effect of dimer formation on the cyclopropenium structures are investigated. The crystal structures of triaminocyclopropenium halide salts are shown to exhibit dimer formation in some cases, but not in others, whereas corresponding triiodide salts only form cationic monomers in the solid state.