Heterogeneous bromination of alkenes using Bi(III) polybromide complexes as {Br₂} source

Abstract

A new polybromide Bi(III) complex (PyH)₃{[Bi₂Br₉](Br₂)} was synthesized and characterized by XRD and other methods. This compound is able to act as a selective bromination agent towards various types of substituted alkenes.

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The chemistry of inorganic polybromide coordination compounds has a very long, but rather sporadic history. The first reliably characterized examples of this class were reported several decades ago: in 1972, Hubbard et al. presented the X-ray structure for an antimony(III) complex, where {Sb₂Br₉} units were connected into an infinite chain by dibromine linkers.¹ Four years earlier, an example of {Br₂}-interconnected hexanuclear clusters of W, obtained by high-temperature synthesis, was reported by von Schnering et al.² Then, relevant studies appeared for the complexes of Au,^3,4 Cu⁵ and Pt⁶ (in the latter case, the number of {Br₂} units was especially high). However, in all of these cases only single examples were described and no further development followed, although fascinating nature of these compounds was noted by the authors.

Very recently we have reported an emergent family of Bi(III) polybromide complexes which demonstrated a great diversity of structural types and molar content of the “captured” dibromine.^7,8 These complexes reveal a great stability towards both heating and vacuumization.⁷ Our preliminary experiments showed that the polybromide unit remains chemically active, preserving its ability to brominate alkenes (1-octene was used as a test reagent).⁷ This observation encouraged us to study whether other Bi(III) polybromides would demonstrate similar properties and can be used as mild and selective bromination agents for a wide range of organic substrates.

In this paper, we introduce a new member of Bi(III) polybromide family, (PyH)₃{[Bi₂Br₉](Br₂)} (1). In addition, we summarize the data on selectivity of bromination demonstrated by 1 towards various alkenes.

The synthetic procedure for 1 followed our previously developed strategy:^7,8 “bromide of certain cation + [BiBr₆]³⁻ + Br₂ dissolved in HBr”. This approach seemed to be generally valid, and in the case of pyridinium bromide it led to the desired product 1. However, our attempts to expand the scope of cations involved in this chemistry showed that the formation of polybromides did not occur in many cases. For example, the reaction with H₂(4,4′-bipy)²⁺ resulted in binuclear (H₂(4,4′-bipy))₂[Bi₂Br₁₀] of which the structure was reported earlier⁹ (the product was identified by PXRD). The colour of the precipitates forming in reactions of this type may serve as a primitive, but still reliable, test for formation/non-formation of {Br_x(Br₂)_y}^x− units (at least when the cationic part does not significantly absorb visible light): the bromide complexes are light yellow to yellow while the polybromides are deep orange to red. Besides, the presence of symmetric {Br₂} in solid state can be sometimes detected by Raman spectroscopy, giving intense bands in the 160–290 cm⁻¹ range (depending on the shape of polybromide fragment),^10–15 although it can overlap with the Bi–X bands. Finally, the simple iodine–starch test may be successfully applied for detection of polybromide (see ESI†). Reliability of this method seem to be high (we have never found that it gives false positive results when conducted with bromide complexes). Results of experiments with a selection of cations are summarized in ESI.† Apparently, there is no straightforward correlation between the nature of cation and product, although we cannot rule out that it will be found later if more representative experiments are performed.

The crystal structure of 1 (see ESI† for experimental details) contains [Bi₂(μ-Br)₃Br₆]³⁻ anions, pyridinium cations and {Br₂} fragments (Fig. 1). The anionic part is closely related to those found by us earlier in the polybromides formed in presence of N-methylpyridinium⁷ and 4-methylpicolinium⁸ cations, being, in fact, the third representative of this structural type. The Bi–Br (terminal) and Br (bridging) distances (2.7064(5)–2.7776(5) Å and 3.0015(5)–3.0444(5) Å, respectively) are typical for the [Bi₂Br₉]³⁻.^16–21 The Br–Br bond distance in {Br₂} is 2.3190(9) Å; each [Bi₂(μ-Br)₃Br₆]³⁻ anion has specific Br⋯Br contacts with two {Br₂} fragments (the Br⋯Br distance is 3.1385(7) Å, see Table S2 in ESI† for other bond lengths and angles). Therefore, {Br₂} and {Bi₂(μ-Br)₃Br₆}³⁻ units are interconnected into zigzag chains extending along c axis (Fig. 2). The organic cations are situated in the plane of zigzag chain between {Br₂} units.

Fig. 1 Fragment of the crystal structure of 1 (ellipsoids of 50% probability). X = 1/2N(2) + 1/2C(21). Symmetry transformations used to generate equivalent atoms: (i) x, 1/2 − y, z; (ii) −x, 1 − y, −z.

Fig. 2 Crystal packing of zigzag chain in the structure 1 (projection of bc plane; organic cations are omitted).

The Raman spectrum of 1 contains the bands which are characteristic for polybromides (160–290 cm⁻¹, see above). There is a strong (approximately 285 cm⁻¹) and two weak bands (approx. 185 and 150 cm⁻¹), and the spectrum is very similar to that of (N-MePy)₃{[Bi₂Br₉](Br₂)}⁷ (Fig. 3). This observation stands in a good agreement with the close structural relationship between the anionic parts of these complexes.

Fig. 3 Raman spectra of 1 (black) and (N-MePy)₃{[Bi₂Br₉](Br₂)} (red).

While organic tribromides are often used for selective alkene bromination,²² examples of analogous application of inorganic polybromides are scarce. By applying ¹H NMR for rapid screening, we found that the rate of alkene bromination by 1 decreased in the following row: butyl-vinyl ether > allyl alcohol ≈ 3-buten-1-ol ≈ 3-methyl-3-buten-ol ≈ glycerol allyl ether > cyclooctene ≈ styrene ≈ tert-butylethylene > 1-heptene > allyl bromide (Table 1). This order is similar to that observed for bromination with molecular bromine in diluted solutions.²³ However, the alkenes containing hydroxo group react with 1 much faster than other substrates. This is strikingly different from homogeneous bromination by Br₂, in which allyl alcohol reacts slower than 1-hexene. Probably, in our case, the hydroxy-substituted alkenes are absorbed from the non-polar CCl₄ solution on the surface of the polybromide 1 due to the hydrogen bond interactions of OH groups, and this absorption strongly accelerates otherwise heterogeneous bromination. Slow bromination of allyl bromide supports this hypothesis.

Table 1 Bromination of various alkenes by 1

Pair of alkenes (initial molar ratio 1 : 1)	Ratio of bromination products
Butyl-vinyl ether/allyl alcohol	>10 : 1
Allyl alcohol/cyclooctene	>10 : 1
Allyl alcohol/3-buten-1-ol	≈1 : 1
3-Buten-1-ol/cyclooctene	>10 : 1
3-Methyl-3-buten-1-ol/cyclooctene	>10 : 1
Glycerol allyl ether/cyclooctene	>10 : 1
Cyclooctene/styrene	≈1 : 1
Styrene/tert-butylethylene	≈1 : 1
tert-Butylethylene/1-heptene	3 : 1
1-Heptene/allyl bromide	>10 : 1

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It must be noted that 1 reacts with alkenes much slower than Br₂ (typically the process completes in 0.1–10 h), providing significant selectivity (from 3 : 1 to 10 : 1 for each pair of alkenes following the order of the row) even at rather high alkene concentration employed (0.02 M). The reactions with Br₂ under these conditions were fast (complete in several seconds) and non-selective.

Conclusions

From the point of view of the preparation of new polybromide complexes, we have shown that the straightforward strategy applied by us earlier^7,8 may be used for preparation of further members of this family. On the other hand, we found that, in presence of some cations, only “classical” Bi(III) bromide complexes result even when Br₂ is added. The correlation between the nature of cation and formation/non-formation of polybromide units remains to be established.

The brominating activity of 1 is, in general, comparable with that of Br₂ in dilute solutions. However, hydroxy-substituted alkenes react with 1 much faster than other substrates, and this feature may be exploitable for chemoselective bromination of corresponding compounds. Relevant experiments are underway.

Acknowledgements

This work has been supported by Russian Science Foundation (Grant No. 14-23-00013). S. A. Adonin thanks the Grant Council at the President of Russian Federation for a research fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Procedures of synthesis and bromination experiments, details of crystallographic experiment. CCDC 1474709. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra10109g

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