Silicon tetrabromide

Comparison of SiX4

The properties of the tetrahalosilanes, all of which are tetrahedral, are significantly affected by nature of the halide. These trends apply also to the mixed halides. Melting points, boiling points, and bond lengths increase with the atomic mass of the halide. The opposite trend is observed for the Si-X bond energies.

Lewis acidity

Covalently saturated silicon complexes like SiBr₄, along with tetrahalides of germanium (Ge) and tin (Sn), are Lewis acids. Although silicon tetrahalides obey the octet rule, they add Lewis basic ligands to give adducts with the formula SiBr₄L and SiBr₄L₂ (where L is a Lewis base). The Lewis acidic properties of the tetrahalides tend to increase as follows: SiI₄ < SiBr₄ < SiCl₄ < SiF₄. This trend is attributed to the relative electronegativities of the halogens.

The strength of the Si-X bonds decrease in the order: Si-F > Si-Cl > Si-Br > Si-I.

Synthesis

Silicon tetrabromide is synthesized by the reaction of silicon with hydrogen bromide at 600 °C.

Si + 4 HBr → SiBr₄ + 2 H₂

Side products include dibromosilane (SiH₂Br₂) and tribromosilane (SiHBr₃).

Si + 2 HBr → SiH₂Br₂ Si + 3 HBr → SiHBr₃ + H₂

Reactivity

Like other halosilanes, SiBr₄ can be converted to hydrides, alkoxides, amides, and alkyls, i.e., products with the following functional groups: Si-H, Si-OR, Si-NR₂, Si-R, and Si-X bonds respectively.

Silicon tetrabromide can be readily reduced by hydrides or complex hydrides.

4 R₂AlH + SiBr₄ → SiH₄ + 4 R₂AlBr

Reactions with alcohols and amines proceeed as follows:

SiBr₄ + 4 ROH → Si(OR)₄ + 4 HBr SiBr₄ + 8 HNR₂ → Si(NR₂)₄ + 4 HNR₂HBr

Grignard reactions with metal alkyl halides are particularly important reactions due to their production of organosilicon compounds which can be converted to silicones.

SiBr₄ + n RMgX → R_nSiBr_4−n + n MgXBr

Redistribution reactions occur between two different silicon tetrahalides (as well as halogenated polysilanes) when heated to 100 ˚C, resulting in various mixed halosilanes. The melting points and boiling points of these mixed halosilanes generally increase as their molecular weights increase. (Can occur with X= H, F, Cl, Br, and I)

2 SiBr₄ + 2 SiCl₄ → SiBr₃Cl + 2 SiBr₂Cl₂ + SiBrCl₃ Si₂Cl₆ + Si₂Br₆ → Si₂Cl_nBr_6−n

Silicon tetrabromide hydrolyzes readily when exposed to air causing it to fume:

SiBr₄ + 2 H₂O → SiO₂ + 4 HBr

Silicon tetrabromide is stable in the presence of oxygen at room temperature, but bromosiloxanes form at 670-695 ˚C .

2 SiBr₄ + 1⁄2 O₂ → Br₃SiOSiBr₃ + Br₂

Uses

Due to its close similarity to silicon tetrachloride, there are few applications unique to SiBr₄. The pyrolysis of SiBr₄ does have the advantage of depositing silicon at faster rates than that of SiCl₄, however SiCl₄ is usually preferred due to its availability in high purity. Pyrolysis of SiBr₄ followed by treatment with ammonia yields silicon nitride (Si₃N₄) coatings, a hard compound used for ceramics, sealants, and the production of many cutting tools.