Crystal structure of bis(3-bromomesityl)(quinolin-1-ium-8-yl)boron(III) tribromide

The structure of bis(3-bromomesityl)8-quinolyliniumboron(III) tribromide is reported: the refinement indicates that a degree of ‘over-bromination’ of the cation has occurred.


Chemical context
We recently prepared the preorganized unimolecular frustrated Lewis pair molecule 8-quinolyldimesitylborane (Son et al., 2010) and hypothesized that it could participate in the heterolytic cleavage of molecular bromine. Halogen addition to a frustrated Lewis pair was recently reported in the literature (Frö mel et al., 2012). The combination of 8-quinolyldimesitylborane with three equivalents of Br 2 in hexanes led to precipitation of the title compound. Features of the structure suggest heterolytic cleavage of Br 2 occurred at the frustrated Lewis pair site. The bromination of the mesityl groups is likely due to electrophilic aromatic substitution from a brominium ion that yields HBr, manifest as a proton on the quinoline nitrogen atom and bromide bound to molecular bromine to form the tribromide ion. Alternatively, radical bromination of the solvent (hexane) yields HBr; however, a radical mechanism is not likely for the bromination of mesityl groups. Typically bromination of aromatics is performed with a Lewis acid catalyst and occurs through an electrophilic aromatic substitution mechanism.

Structural commentary
The title compound crystallizes in the space group P1, and contains one cation and two half tribromide ions (completed by inversion symmetry) in the asymmetric unit. The cation ISSN 2056-9890 ( Fig. 1) features a planar three-coordinate triarylborane with two 3-bromomesityl groups and an 8-quinolyl group. The nitrogen atom is protonated and the positive charge is balanced by the presence of a tribromide anion, Br 3 À . The tribromide anions are shared between asymmetric units of the crystal, such that each unit contains two halves of an anion (Br5 and Br7 lie on crystallographic inversion centers). The Br5-Br6 distance is 2.5427 (11) Å and the Br7-Br8 distance is 2.546 (2) Å . Other bond distances and angles are given on Table 1. The mesityl groups are brominated at the meta positions such that one position is nearly completely brominated while the other meta position on the same ring is brominated to a much lesser extent. The best solution was found with refined bromine occupancy at the meta positions (C10 ring: Br1 = 0.95, Br4 = 0.09 for a total Br count of 1.04 on the ring; C19 ring: Br2 = 0.89, Br3 = 0.24 for a total Br count of 1.13 on the ring). The balance of electron density at the positions is accounted by partial hydrogen atoms at a reciprocal value of the bromine occupancy to give an overall formulation for the cation of C 27 H 26.82 BBr 2.18 N + .

Supramolecular features
The cations are arranged in rows that propagate along the aaxis direction wherein each cation is in the same orientation due to translation along the row. Inversion centers are located on the dimesitylboryl side of the row, just beyond the brominated mesityl groups, and the packing of the cations in the crystal results in interdigitated parallel quinolinium rings; these symmetrically sandwich a tribromide anion, such that the central atom of the anion is located at an inversion center. A packing diagram is shown in Fig. 2 The molecular structure of the title compound. Hydrogen atoms are omitted for clarity. Displacement ellipsoids are shown at the 30% probability level. [Symmetry codes: (i) 1 À x,1 À y,1 À z; (ii) 1 + x, y, z.] Table 1 Selected geometric parameters (Å , ).

Database survey
A search in the Cambridge Structural Database (Groom & Allen, 2014) for structures with the tribromide anion revealed 162 hits while a search for structures with the dimesitylboryl fragment revealed 539 hits. Among these are several structures of planar organic aromatic cations as tribromide salts.

Synthesis and crystallization
Reactions were performed using Schlenk and glovebox techniques under an atmosphere of N 2 using dried and distilled solvents. Dimesityl(8-quinolyl)borane was prepared according to the literature (Son et al., 2010). A round-bottom air-free flask was charged with 110 mg (0.29 mmol) dimesityl(8quinolyl)borane and 20 ml hexanes. In a separate flask, 2 ml of a solution of 5% Br 2 in CCl 4 (1 mmol Br 2 ) was added to 10 ml hexanes and subjected to one freeze-pump-thaw cycle. The Br 2 solution was transferred to the borane solution via a cannula at room temperature with stirring, and immediately a light-yellow precipitate formed. The solvent was removed in vacuo. Dichloromethane was added to the solid reside into which the title compound was dissolved; remaining insolubles were filtered off. Pale-yellow prisms of the title compound were grown by vapor diffusion of pentane into the methylene chloride solution.