TUNGSTEN (VI) ARYLOXIDES

Tungsten hexakis(phenoxide) W(OPh)6

Fig. Molecular structure of W(OPh)6

Tungsten hexaphenoxide (Tungsten hexakis(phenoxide)) [W(OPh)6] (M=) is dark red needles, soluble in toluene, methanol or acetonitrile. 1H NMR (CDCl3): δ 6.86 (18H, m, CH); 7.21 (12H, m, CH). 13C-{1H} NMR (CDCl3): δ 120.4 (CH); 123.4 (CH); 128.8 (CH); 161.8 (CO).

W(OPh)6 has been crystallographically characterised: it is monomeric in the solid state, with the tungsten centre surrounded by six O atoms in an almost regular octahedral array. (Fig.):  the O-W-O bond angles vary between 87.5(2) and 92.6(2)°, and the W-O bond lengths vary between 1.883(4) and 1.917(4) Å; the W-O-C angles range from 136.8(8) to 151.0(5)°.

 Most metal alkoxides and aryloxides are hydrolysed by atmospheric moisture (with reactivity arising from the polarised nature of the metal oxygen bond (Mδ+—Oδ-— C) and the presence of vacant orbitals that can accommodate electrons, thus the metal centre being highly susceptible to nucleophilic attack). However this is not the case for tungsten hexaphenoxide, which is air stable and does not undergo hydrolysis (f.e., during the synthesis of [W(OPh)6] the product is washed with aqueous NaOH and recrystallised from methanol). However, [W(OPh)6] is hydrolysed under acid conditions.

Tungsten hexaphenoxide is so stable due to a lack of vacant coordination sites as well as due to the steric hindrance of approaching nucleophiles by bulky phenyl groups. This stability of [W(OPh)6] is very beneficial when using it as a precursor in CVD (facilitates precursor handling).

Synthesis of [W(OPh)6]

Fig. Preparation of [W(OPh)6] from tungsten tris (ethylene glycolate) and phenylacetate

[W(OPh)6] can be synthesized by two routes [[i]]:

1 Route: Alcoholysis of a metal halide (WCl6 or WOCl4)  - a facile way to produce homoleptic aryloxides of tungsten(VI), [W(OAr)6], which were first reported as early as 1937 by Funk and Baumann.

1. WCl6 + 6 PhOH →[ W(OPh)6] + 6 H Cl
2. WOCl4 + 6 P h O H → [W(OPh)6] + 4 HCl + H2O

Adding phenol PhOH to a toluene solution of WCl6 results in the solution changing colour from black to dark red. However, the homoleptic phenoxide is not yet formed (even though PhOH is sufficiently acidic to eliminate HCl from the reaction): for the reaction to be driven to completion the mixture must be heated at reflux for several hours. Afterwards, the solvent is removed in vacuo, and the crude product heated at 80-90 °C (in vacuo) to remove any remaining phenol. Tungsten chloride-phenoxide (by-product that have not undergone complete reaction is removed by washing with a 10% NaOH (aq.). Relatively pure [W(OPh)6] is obtained, that can be further purified by recrystallisation from a MeOH or MeCN solution: dark red needles of [W(OPh)6] are obtained in ca. 47-70 % yield.[]

2 route: [W(OPh)6 can be prepared by the trans-esterification reaction between tungsten tris(ethyleneglycolate), [W(O-CH2CH2-O)3] (or W(eg)3) and phenylacetate PhOAc. This method has an advantage of involving no anaerobic chemistry, since W(eg)3 is air stable and is readily prepared from tungstic acid and ethylene glycol:

[i] W. B. Cross, PhD thesis,  University College London, 2002,  « Chemical Vapour Deposition of Tungsten Oxide Thin Films from Single-Source Precursors », https://discovery.ucl.ac.uk/id/eprint/10102009/1/out.pdf

[W(OPh)6] for WO3 by AA-CVD

[W(OPh)6] has been applied as precursor for AA-CVD growth of WO3-based   nanostructures (intrinsic or metal nanoparticle-doped (for example Au-, Cu- or Pt-doped)) (other precursor for the metal nanoparticles used were [HAuCl4·3H2O], [Cu(acac)2] and [H2PtCl6·xH2O]). These nanostructures were deposited on glass substrates at 350 °C These metal-decorated low-dimensional nanostructures were grown directly on alumina or silicon-based carriers. Gas sensors based on WO3 nanoneedles (undoped or Au-doped) demonstrated high selectivity towards detecting NO2 [[i], [ii]]

[i] T. Stoycheva, S. Vallejos, J. Caldererc, I. Parkin, C. Blackman, X. Correiga, Procedia Eng., Vol. 5, 2010, p.131-134 (Proc. Eurosensors XXIV, 2010), “Characterization and gas sensing properties of intrinsic and Au-doped WO3 nanostuctures deposited by AACVD technique”, https://doi.org/10.1016/j.proeng.2010.09.065,

[ii] F.E. Annanouch, S. Vallejos, T. Stoycheva, C. Blackman, E. Llobet, Thin Solid Films, 2013, Vol. 548, 2, p. 703–709,  Aerosol assisted chemical vapour deposition of gas-sensitive nanomaterials

    http://www.sciencedirect.com/science/article/pii/S0040609013007268

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