The Formation of High‐Order Polybromides in a Room‐Temperature Ionic Liquid: From Monoanions ([Br₅]⁻ to [Br₁₁]⁻) to the Isolation of [PC₁₆H₃₆]₂[Br₂₄] as Determined by van der Waals Bonding Radii

Abstract

An unprecedented diversity of high-order bromine catenates (anionic polybromides) was generated in a tetraalkylphosphonium-based room temperature ionic liquid system. Raman spectroscopy was used to identify polybromide monoanions ranging from [Br5 ](-) to [Br11 ](-) in the bulk solution, while single-crystal X-ray diffraction identified extended networks of linked [Br11 ](-) units, forming a previously unknown polymeric [Br24 ](2-) dianion. This represents the largest polybromide species identified to date. In combination with recent work, this suggests that other, higher order molecular polybromide ions might be isolated.

Controlled Synthesis of Polyions of Heavy Main-Group Elements in Ionic Liquids

“…Intriguing examples for the utilization of halogens are the polybromides synthesized in ILs [43,69,70,71,72]. While polyiodides were known with up to 29 iodine atoms [73], the size of the corresponding polybromides was limited to a maximum of 10 atoms [74,75] before new approaches including ILs (Feldmann group) were developed [76].…”

“…Moreover, the reactivity of the lighter halogens necessitates a chemically stable environment, which can be provided by ILs due to their high redox stability [78]. The use of ILs lead to the isolation of several new polybromides: [P(Ph 3 )Br][Br 7 ] [69] ([P(Ph 3 )Br] = triphenylphosphine bromide), [P(Bz)(Ph) 3 ] 2 [Br 8 ] [69], [HMIm][Br 9 ] [70], [N(n-Bu) 3 Me] 2 [Br 20 ] [69], [C 4 MPyr] 2 [Br 20 ] [43,69], and [P 4444 ] 2 [Br 24 ] (Figure 1) [71]. The anion of [C 4 MPyr] 2 [Br 20 ] was regarded as the bromine-richest species (aside from the element itself) at its discovery by Wolff et al [43].…”

“…The anion of [C 4 MPyr] 2 [Br 20 ] was regarded as the bromine-richest species (aside from the element itself) at its discovery by Wolff et al [43]. This record has been excelled by the synthesis of [P 4444 ] 2 [Br 24 ] and its [Br 24 ] 2− anion by Easton et al [71]. …”

“…Visualization of the anion [Br 24 ] 2− according to [71]. Br–Br distances with d ≤ 320 are drawn as solid lines to emphasize the [Br 24 ] 2− unit.…”

“…Br–Br distances with d ≤ 320 are drawn as solid lines to emphasize the [Br 24 ] 2− unit. Additional Br–Br distances up to 370 pm (i.e., twice the van der Waals distance [71]; dashed broken-off bonds) indicate the network character.…”