Crystal packing interpretation of the association of chiral threefold propeller ions: TRISPHAT anion with a triarylcarbenium cation

Abstract

Experiments attempting to use the chiral [tris(tetrachlorobenzenediolato)phosphate(V)]− anion (TRISPHAT, 1) to induce a preferred configuration upon the [tris(4-dimethylaminobenzene)carbenium] cation of crystal-violet (2) via ion pairing with co-threefold-axial embraces of the propeller-shaped ions were not successful; no evidence was found for an asymmetric induction in solution. The salt that crystallises, [tris(4-dimethylaminobenzene)carbenium][tris(tetrachlorobenzenediolato)phosphate(V)] dichloromethane diethyl ether solvate (3), provides some understanding of this result through analysis of its crystal packing, in which there are no threefold coaxial cation·anion embraces, but there is a centrosymmetric (racemic), flattened, sixfold phenyl embrace (6PE) between cations 2. A dominant feature of the crystal packing is a centrosymmetric association of two anions and two cations. We attribute the lack of threefold hetero-ion embraces to incompatible canting of the aryl rings (propeller blades) in 1 and 2, and thereby can interpret the lack of transfer of chiral information between them. Modelling shows that there is compatible canting of the aryl rings of [Fe(phen)3] cations and 1, allowing the formation of excellent hetero-ion homochiral (OFF)3 embraces. Some calculated intermolecular energies are reported.