Pentamethylantimony

Pentamethylantimony or pentamethylstiborane is an organometalllic compound containing five methyl groups bound to an antimony atom with formula Sb(CH₃)₅. It is an example of a hypervalent compound. The molecular shape is trigonal bipyramid. Some other antimony(V) organometallic compounds include pentapropynylantimony (Sb(CCCH₃)₅) and pentaphenyl antimony (Sb(C₆H₅)₅). Other known pentamethyl-pnictides include pentamethylbismuth and pentamethylarsenic.

Production

Pentamethylantimony can be made by reacting Sb(CH₃)₃Br₂ with two equivalents of methyl lithium. Another production route is to convert trimethylstibine to the trimethyl antimony dichloride, and then replace the chlorine with methyl groups with methyl lithium.

:Sb(CH₃)₃ + Cl₂ → Sb(CH₃)₃Cl₂
:Sb(CH₃)₃Cl₂ + 2LiCH₃ → Sb(CH₃)₅ + 2LiCl

Properties

Pentamethylantimony is colourless. At -143 °C it crystallizes in the orthorhombic system with space group ''Ccmm''. Unit cell dimensions are a=6.630 Å b=11.004 Å c=11.090 Å. There are four formula per unit cell. Unit cell volume is 809.1 Å³. The trigonal bipyramid shape has three equatorial positions for carbon, and two axial positions at the peaks of the pyramids. The length of the antimony-carbon bond is around 214 pm for equatorial methyl groups and 222 pm for the axial positions. The bond angles are 120° for ∠C-Sb-C across the equator, and 90° for ∠C-Sb-C between equator and axis. The molecules rapidly change carbon atom position, so that in NMR spectrum as low as −100 °C, there is only one kind of hydrogen position.

Pentamethylantimony is more stable than pentamethylbismuth, because in lower energy trimethylbismuth, the non-bonding pair of electrons is more shielded due to the f-electrons and the lanthanoid contraction. Trimethylantimony is higher in energy, and thus less is released in a decomposition of pentamethylantimony. Pentamethylantimony can be stored as a liquid in clean glass at room temperature.

Pentamethylantimony melts at -19 °C. Although it decomposes when boiling is attempted and can explode, it has a high vapour pressure at 8 mmHg at 25 °C.

There are two absorption bands in the ultraviolet at 2380 and 2500 Å.

Reactions

Pentamethylantimony reacts with methyl lithium to yield a colourless lithium hexamethylantimonate in tetrahydrofuran.

:Sb(CH₃)₅ + LiCH₃ → Li(thf)Sb(CH₃)₆

Pentamethylantimony reacts with silsesquioxanes to yield tetramethylstibonium silsesquioxanes. eg (''cyclo''-C₆H₁₁)₇Si₇O₉(OH)₃ yields (''cyclo''-C₆H₁₁)₇Si₇O₉(OSb(CH₃)₄)₃. The reaction happens quickly when there are more than two OH groups.

Phosphonic acids and phosphinic acids combine with pentamethylantimony to yield compounds like (CH₃)₄SbOP(O)Ph₂, (CH₃)₄SbOP(O)(OH)Ph and (CH₃)₄SbOP(O)(OH)₃, eliminating methane.

Stannocene Sn(C₅H₅)₂ combines with pentamethylantimony to produce bis(tetramethylstibonium)tetracyclopentadienylstannate ( CH₃)₄Sbsub>2Sn(C₅H₅)₄).

Pentamethylantimony reacts with many very weak acids to form a tetramethylstibonium salt or tetramethylstibonium derivative with the acid. Such acids include water (H₂O), alcohols, thiols, phenol, carboxylic acids, hydrogen fluoride, thiocyanic acid, hydrazoic acid, difluorophosphoric acid, thiophosphinic acids, and alkylsilols.

With halogens, pentamethylantimony has one or two methyl groups replaced by the halogen atoms. Lewis acids also react to form tetramethyl stibonium salts, including CH₃)₄SblBr₄, CH₃)₄SbCH₃SbCl₅],

Pentamethylantimony reacts with the surface of silica to coat it with Si-O-Sb(CH₃)₄ groups. Over 250 °C this decomposes to Sb(CH₃) and leaves methyl groups attached to the silica surface.

References

{{antimony compounds

Organoantimony compounds
Hypervalent molecules
Methyl complexes