Hexamethyltungsten WMe6 (M= 274.05, chemical formula C6H18W) is a red crystalline solid having meting point ~30°C and is extremely volatile: it sublimes readily at -30° C/ 10-2 Torr. Hexamethyltungsten has been characterized by elemental analysis, spectral properties and chemical reactivity. WMe6 is soluble in organic solvent solvents (light petroleum fractions, aromatic hydrocarbons, ethers, CS2 and CCl4).
Mass-spectroscopy of WM6: The highest mass number observed was W(CH3)5+ ion with normal isotope distribution; lower mass peaks resulted from subsequent loss of further Me groups ranging from W(CH3)4+ to W+.
Infrared spectroscopy: Solutions of WMe6 had absorption at 482 cm-1 assigned to W-C stretching frequency, as well as the peaks expected for CH3 vibrations, viz. C-H stretching at 2980 and 2870 cm-1, C-H deformation at 1395 and 1090 cm-1 and CH3 rocking at 800 cm-1.
Electronic spectrum: Absorption rising into the UV range was observed (presumably due to an intense charge transfer band accounting for the deep red color of the compound.
NMR spectroscopy: the 1H spectrum in deuterotoluene showed a single sharp signal at τ 8.38 with satellites due to 183W coupling, J (183W-H)=3.0 Hz; the peak remained sharp on cooling to -90° C. The 13C spectrum in C6D6 showed a single peak 1000 Hz upfield from the solvent peaks, with satellites J(188W-3C)=400 Hz.
Chemical reactivity: WMe6 is spontaneously flammable in air and must be handled in rigorously degassed and air-free systems and solvents. WMe6 in light petroleum solution is rapidly reduced by H2 to an unstable blue species, presumably containing W(V).
Molecular structure of hexamethyltungsten WMe6, has been determined by single-crystal X-ray diffraction at - 163 °C. The molecule of WMe6 has a strongly distorted trigonal prismatic structure with C3v symmetry. This irregular structure is not a result of intermolecular forces, but rather represents its true molecular structure. [[i][PS1] ]
Ab initio and density functional calculations confirmed that the equilibrium structure of WMe6 is a distorted trigonal prism of C3 symmetry (with local C3v symmetry for the WC6 skeleton), whereas a regular prismatic D3 structure (with D3h skeleton) is ca. 20 kJ/mol higher in energy. Electron correlations must be taken into account during the calculations, e.g. for the description of hyperconjugative “agostic” C−H → W interactions which are pronounced in WMe6. [[ii][PS2] ]
The observed single-line 13C and 1H NMR spectra of WMe6 were explained by dynamic motions due to the low D3 inversion and methyl rotation barriers.
The single crystal XRD results extended gas-phase electron diffraction study which determined that the coordination geometry of gaseous WMe6 is not octahedral but favored a regular prismatic structure but could not rule out a distortion to C3v. [[iii][PS3] ]
Synthesis of WMe6: WCl6 in Et2O under air-free /anhydrous conditions with about 3 eq. LiMe at a in the ~0°C to ~30° C temperature (the reaction temperature should not exceed 30°C (to avoid thermal decomposition of the product), but at temperatures < 0° C the reaction is very slow and incomplete). The relative proportions of WCl6 and LiMe are critical to the yield of WMe6 (best yields (up to ~50% based on W) were obtained for WCl6:LiMe mole ratio of 1:3; if only 2 eq. of LiMe were used, unstable Cl-containing products were obtained; whereas if >4 eq. are used, no WMe6 was isolated from the products). The use of Et2O as the reaction medium was essential to the success of this reaction (using THF or a light petroleum as solvent no isolatable WMe6 species were obtained).
The substantially pure product WMe6 was then obtained by removing Et2O by vacuum evaporation at 0° C and then subliming WMe6 at RT condensed on a cold surface (e.g. at dry ice temperature, -78°C) as a red oil containing traces of Et2O. In order to obtain crystals of improved purity the evaporated Et2O is first replaced by a light petroleum fraction, filter and then remove the solvent from the filtered solution by vacuum evacuation at -20°C. The residue is then sublimed at room temperature and collected as a pure crystalline red solid on a surface cooled to about -10°C. The solid WMe6 can be stored under purified N2 or in vacuum at -40° C without any significant decomposition; thermal decomposition of solid WMe6 is very slow even at room temperature. Dilute solutions in the organic solvents are quite stable and can readily handled at RT in the absence of air. [[iv],[v][PS4] ]
An improved synthesis using trimethylaluminium in conjunction with trimethylamine, instead of methyllithium was reported [[vi]]:
WCl6 + 6 AlMe3 → WMe6 + 6 Al(CH3)2Cl
Alternatively, dimethylzinc can be used as the alkylating agent[[vii]]
WX6 + 3 ZnMe2 → WMe6 + 3 ZnX2 (X = F, Cl)
[i] V. Pfennig, K. Seppelt, Science, 1996, Vol. 271, Iss.5249, pp. 626-628, « Crystal and Molecular Structures of Hexamethyltungsten and Hexamethylrhenium », DOI: 10.1126/science.271.5249.626, https://science.sciencemag.org/content/271/5249/626.abstract
[ii] M. Kaupp, J. Am. Chem. Soc. 1996, 118, 12, 3018–3024, https://doi.org/10.1021/ja952231p, « The Structure of Hexamethyltungsten, W(CH3)6: Distorted Trigonal Prismatic with C3 Symmetry »
[iii] A. Haaland, A. Hammel, K. Rypdal, Hans V. Volden, J. Am. Chem. Soc. 1990, 112, 11, 4547–4549, https://doi.org/10.1021/ja00167a065 , « The coordination geometry of gaseous hexamethyltungsten is not octahedral »
[iv] A.J. Shortland, G. Wilkinson, "Preparation and properties of hexamethyltungsten", J. Chem. Soc., Dalton Trans. 1973, (8): 872–876. doi:10.1039/DT9730000872.
[v] G. Wilkinson, US3816491A, United States, “Hexamethyltungsten”, https://patents.google.com/patent/US3816491A/en
[vi] G. Wilkinson, A. L. Galyer, "New synthesis of hexamethyltungsten(VI). The octamethyltungstate-(VI) lon", J. Chem. Soc., Dalton Trans. 1976, 2): 2235. doi:10.1039/DT9760002235.
[vii] S. Kleinhenz, V. Pfennig, K. Seppelt,. "Preparation and Structures of [W(CH3)6], [Re(CH3)6], [Nb(CH3)6]−, and [Ta(CH3)6]−", Chem. Eur. J. 1998) 4 (9): 1687. doi:10.1002/(SICI)1521-3765(19980904)4:9
In the absence of air WMe6 decomposes only very slowly at RT, but at 50°C decomposes rapidly yielding metallic W. WM6 is therefore useful as a metallizing agent for materials which are sensitive to higher metallizing temperatures. For example, when a small amount of WMe6 was allowed to volatilize into an evacuated glass tube containing an polyethylene part heated to about 50°C, the polyethylene was rapidly coated with a decorative black layer consisting of metallic tungsten W. [i]
The patent application has been submitted for the use of WMe6 for chemical vapor deposition of W thin films in the manufacturing of semiconductor devices [[ii]] , however, to date WMe6 has not been used for this purpose.
[i] G. Wilkinson, US3816491A, United States, “Hexamethyltungsten”, https://patents.google.com/patent/US3816491A/en
[ii] S. Matsumoto, O. Ikeda, K. Ohmi, (Canon K. K., Japan) Eur. Pat. Appl. (1991).