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.[1] Some other antimony(V) organometallic compounds include pentapropynylantimony (Sb(CCCH3)5) and pentaphenyl antimony (Sb(C6H5)5).[2] Other known pentamethyl-pnictides include pentamethylbismuth and pentamethylarsenic.

Pentamethylantimony
Names
Systematic IUPAC name
pentamethyl-λ5-stibane
Identifiers
3D model (JSmol)
ChemSpider
EC Number
  • 239-173-4239-173-4
Properties
C5H15Sb
Molar mass 196.935 g·mol−1
Appearance Colourless liquid
Melting point −19 °C (−2 °F; 254 K)
Boiling point 160 °C (320 °F; 433 K)
Related compounds
Related compounds
 Trimethylstibine
Pentamethylarsenic
Pentamethylbismuth
Pentamethyltantalum
Pentaphenylantimony
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N (what is YN ?)
Infobox references

Production

Pentamethylantimony can be made by reacting Sb(CH3)3Br2 with two equivalents of methyl lithium.[3] Another production route is to convert trimethylstibine to the trimethyl antimony dichloride, and then replace the chlorine with methyl groups with methyl lithium.[2]

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


Properties

Pentamethylantimony is colourless.[3] At -143°V 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.[3] 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.[3] 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.[2]

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.[3] Pentamethylantimony can be stored as a liquid in clean glass at room temperature.[4]

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.[4]

There are two absorption bands in the ultraviolet at 2380 and 2500 Å.[4]

Reactions

Pentamethylantimony reacts with methyl lithium to yield a colourless lithium hexamethylantimonate in tetrahydrofuran.[3]

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.[5]

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.[6]

Stannocene Sn(C5H5)2 combines with pentamethylantimony to produce bis(tetramethylstibonium)tetracyclopentadienylstannate ([(CH3)4Sb]2Sn(C5H5)4).[7]

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.[8]

With halogens, pentamethylantimony has one or two methyl groups replaced by the halogen atoms.[8] Lewis acids also react to form tetramethyl stibonium salts, including [(CH3)4Sb]TlBr4, [(CH3)4Sb][CH3SbCl5], [8]

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.[9]

References
  1. Greene, Tim M.; Downs, Anthony J.; Pulham, Colin R.; Haaland, Arne; Verne, Hans Peter; Volden, Hans Vidar; Timofeeva, Tatjana V. (November 1998). "Molecular Structures of Pentamethylarsenic(V) and Trimethyldichloroarsenic(V) by Gas Electron Diffraction and ab Initio Calculations:? Molecular Mechanics Calculations on Pentamethylarsenic(V), Pentaphenylarsenic(V), and Related Compounds". Organometallics. 17 (24): 5287–5293. doi:10.1021/om980520r.
  2. Haaland, Arne; Hammel, Andreas; Rypdal, Kristin; Swang, Ole; Brunvoll, Jon; Gropen, Odd; Greune, Michael; Weidlein, Johann; Nasiri, Ahmad; Okada, Yoshito (1993). "Molecular Structure of Pentamethylantimony by Gas Electron Diffraction; Structure and Bonding in Sb(CH3)5 and Bi(CH3)5 Studied by Ab Initio MO Calculations". Acta Chemica Scandinavica. 47: 368–373. doi:10.3891/acta.chem.scand.47-0368.
  3. Wallenhauer, Stephan; Seppelt, Konrad (January 1995). "Antimony(V) and Bismuth(V) Methyl Compounds: A Structural Comparison". Inorganic Chemistry. 34 (1): 116–119. doi:10.1021/ic00105a021.
  4. Downs, A. J.; Schmutzler, R.; Steer, I. A. (1966). "The vibrational spectrum and structure of pentamethylantimony". Chemical Communications (London) (8): 221. doi:10.1039/C19660000221.
  5. Feher, Frank J.; Budzichowski, Theodore A.; Rahimian, Kamyar; Ziller, Joseph W. (May 1992). "Reactions of incompletely-condensed silsesquioxanes with pentamethylantimony: a new synthesis of metallasilsesquioxanes with important implications for the chemistry of silica surfaces". Journal of the American Chemical Society. 114 (10): 3859–3866. doi:10.1021/ja00036a038.
  6. Graves, Guy E.; Van Wazer, John R. (May 1978). "Methyl group replacement on pentamethylantimony with organophosphorus substituents". Journal of Organometallic Chemistry. 150 (2): 233–237. doi:10.1016/S0022-328X(00)84725-7.
  7. Bos, Klaas D.; Bulten, Eric J.; Meinema, Harry A.; Noltes, Jan G. (20 March 1979). "Synthesis of bis(tetramethylstibonium)tetracyclopentadienylstannate a novel type of organotin(II) compound". Journal of Organometallic Chemistry. 168 (2): 159–162. doi:10.1016/s0022-328x(00)83270-2. hdl:1874/25359.
  8. Hubert Schmidbaur (1976). Advances in Organometallic Chemistry. Academic Press. ISBN 9780080580159.
  9. Wang, Y.; Morrow, B. A. (January 1996). "Infrared Study of the Chemisorption of Pentamethylantimony on Silica". Langmuir. 12 (17): 4153–4157. doi:10.1021/la951514s.
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