Structural isomerism in tris-tolyl halo-phosphonium and halo-arsonium tri-halides, [(CH3C6H4)3EX][X3], (E = P, As; X = Br, I)

Abstract

The group 15 ligands (o-CH₃C₆H₄)₃P, (m-CH₃C₆H₄)₃P, (p-CH₃C₆H₄)₃P, Ph₃As, (o-CH₃C₆H₄)₃As and (p-CH₃C₆H₄)₃As have been reacted with two equivalents of di-iodine or di-bromine to yield complexes of formula R₃EX₄ (E = P, As; X = I, Br). These halogenated group 15 compounds are ionic, [R₃EX][X₃] 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, [R₃EX][X₃], isomer (A), which display a weak X⋯X interaction between cation and anion, or as a 2 : 1 complex, which consists of a [{R₃EX}₂X₃]⁺ cationic species made up of two [R₃EX]⁺ cations interacting with one [X₃]⁻ anion. The overall charge is balanced by a second [X₃]⁻ anion. These 2 : 1 species also exhibit structural isomerism due to subtle differences in the connectivity of the [{R₃EX}₂X₃]⁺ fragment, as the {R₃EX}⁺ units may either interact at the same end of the [X₃]⁻ 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 [Ar₃EX]⁺ 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.