Structural isomerism in tris-tolyl halo-phosphonium and halo-arsonium tri-halides, [(CH 3C 6H 4) 3EX][X 3], (E = P, As; X = Br, I)

The group 15 ligands (o-CH 3C 6H 4) 3P, (m-CH 3C 6H 4) 3P, (p-CH 3C 6H 4) 3P, Ph 3As, (o-CH 3C 6H 4) 3As and (p-CH 3C 6H 4) 3As have been reacted with two equivalents of di-iodine or di-bromine to yield complexes of formula R 3EX 4 (E = P, As; X = I, Br). These halogenated group 15 compounds are ionic, [R 3EX][X 3] consisting of halo-phosphonium or halo-arsonium cations and trihalide anions. These adducts exhibit structural isomerism and may exist either as simple 1:1 ion pairs, [R 3EX][X 3], isomer (A), which display a weak X⋯X interaction between cation and anion, or as a 2:1 complex, which consists of a [{R 3EX} 2X 3] + cationic species made up of two [R 3EX] + cations interacting with one [X 3] - anion. The overall charge is balanced by a second [X 3] - anion. These 2:1 species also exhibit structural isomerism due to subtle differences in the connectivity of the [{R 3EX} 2X 3] + fragment, as the {R 3EX} + units may either interact at the same end of the [X 3] - ion, to give a Y-shaped motif, isomer (B), or at opposite ends, giving a Z-shaped motif, isomer (C). The type of structural isomer formed is related to the way in which [Ar 3EX] + cations pack together via aryl embraces. Isomer (A) and (C) structures form chains of side-to-side, anti-parallel embracing cations. In (A) and (C) structures a square-like stacking motif of cations is observed. In contrast, isomer (B) structures feature side-to-side, parallel embracing cations, and do not exhibit the square motif. © 2012 The Royal Society of Chemistry.