The decades-old mystery of bis­­(di­ethyl ether)tungsten(IV) chloride solved

A hexa­coordinated metal centre with an octa­hedral coordination environment is a very common motif in transition metal chemistry. This can involve neutral, as well as anionic, ligands or a combination of both. The latter can, inter alia, be found in many Group 6 complexes, often with donor solvent mol­ecules acting as neutral ligands and saturating the coordination sphere of the metal(IV) centre. In particular, with regard to tungsten, many complexes of the type WX₄(solvent)₂ are known in the literature and are even found in textbooks nowadays, as, for instance, WCl₄(py)₂ (Brenčič et al., 1979) or WCl₄(MeCN)₂ (Manteghetti et al., 1999), which have even been structurally characterized by single-crystal X-ray diffraction. A common, yet ambiguous, example of this is bis­(diethyl ether)tungsten(IV) chloride, WCl₄(Et₂O)₂. Although this compound has been `floating around' in the literature for many decades, surprisingly little analytical data concerning it have been reported, begging the question if it even truly exists.

Taking a look at the history of the compound, it starts in 1975 when Thiele et al. (1975) studied different benzyl­tungsten compounds, tetra­benzyl­tungsten amongst others, which they obtained from the reaction of di­benzyl­magnesium with tungsten(IV) chloride bis­(tetra­hydro­furan), WCl₄(thf)₂. This, in turn, was synthesized from tungsten(VI) chloride, WCl₆, by treatment with di­ethyl­zinc in diethyl ether, whereas the authors proposed that the tungsten(IV) chloride bis­(diethyl ether) adduct, WCl₄(Et₂O)₂, was formed initially, and was then transformed into the isolatable bis­(tetra­hydro­furan) adduct. Unfortunately, no analytical data for the bis­(diethyl ether) complex was provided. At roughly the same time, Chisholm, Cotton and co-workers investigated the use of WCl₄(Et₂O)₂ as a possible precursor in the preparation of tungsten–tungsten triple-bond complexes (Chisholm et al., 1976a,b). While WCl₄(Et₂O)₂ was explicitly mentioned as a bis­(diethyl etherate), it is worth pointing out that it was prepared in situ (by the same route as described above), with a 100% yield assumed, and was used without characterization. Therefore, although WCl₄(Et₂O)_x had already proven to be an inter­esting synthon, the exact nature and structure of the complex remained unclear and its composition, suggested to be a bis­(diethyl ether) adduct, was based simply upon chemical intuition and the logical assumption of an octa­hedral complex with hexa­coordinated tungsten (Fig. 1).

Figure 1 Tungsten tetra­chloride bis­(di­ethyl etherate), WCl4(Et2O)2.

Almost a decade later, in 1985, Castellani & Gallazzi (1985) revisited the preparation of diethyl ether complexes of molybdenum and tungsten tetra­chlorides, and made an attempt to characterize WCl₄(Et₂O)_x by elemental analysis to elucidate its exact composition. In contrast to previous articles, they reported that the mono(diethyl etherate), WCl₄(Et₂O), was obtained from diethyl ether solutions of WCl₆, which was based on elemental analysis suggesting not two but one equivalent of C₄H₁₀O [WCl₄(Et₂O)₂: C 20.3, H 4.3%; WCl₄(Et₂O): C 12.0, H 2.5%; reported by Castellani & Gallazzi (1985): C 11.5, H 2.4%]. First, it is worth mentioning that Castellani & Gallazzi seem to be the first to actually attempt to isolate the complex as a pure solid, rather than just synthesize and use it in situ. Furthermore, they reported that an orange reaction mixture, along with a yellow solid, formed initially, but that ultimately black crystalline needles were obtained and isolated. It seems odd that a compound could be obtained in the form of black crystals from an orange reaction mixture and, taking into account that a yellow precipitate was observed initially, this might point towards an equilibrium between a mono- and bis­(diethyl etherate), due to the relatively weak complexation of the diethyl ether ligands. In fact, Castellani & Gallazzi pointed out correctly that other complexes, such as WCl₄(thf)₂ and WCl₄(py)₂, always feature two complexed solvent mol­ecules.

It is puzzling that for decades this mystery remained unsolved, even though this complex was frequently used as a starting material in tungsten chemistry. Finally, in 2021, almost half a century after the complex first appeared in the literature, Jurca and co-workers were able to obtain a pure crystalline sample of the bis­(diethyl ether) complex, WCl₄(Et₂O)₂, and conduct a complete structural characterization by single-crystal X-ray diffraction and NMR spectroscopy (Shaw et al., 2021). Different from previous reports, the authors used tungsten tetra­chloride, WCl₄, as the starting material, dissolving it in diethyl ether. In a striking similarity to the report of Castellani & Gallazzi from 1985, they observed a yellow solution and were able to obtain not a black but a yellow crystal from it. Structural characterization of this crystal by X-ray diffraction confirmed it to be tungsten tetra­chloride bis­(diethyl etherate), WCl₄(Et₂O)₂, with trans-coordinated diethyl ether mol­ecules in the axial positions (Fig. 1)! With this, the quest for the elucidation and structural characterization of the long-discussed WCl₄(Et₂O)₂ finally comes to an end, closing this gap in tungsten tetra­chloride bis­(solvent) complexes.

The question remains, did Castellani & Gallazzi actually obtain the mono(diethyl etherate), WCl₄(Et₂O), in 1985, as suggested? It seems not unlikely given the fact that diethyl ether is a rather weakly bonded ligand, and they reported black crystals instead of yellow ones and the elemental analysis was very much in line with a mono(diethyl etherate). Two inter­esting details are given by Jurca and co-workers here as well, namely, that `prolonged exposure to ambient conditions led to the appearance of dark spots on the crystal surface' and that `the compound is unstable under vacuum' (Shaw et al., 2021). This clearly suggests the loss of one equivalent of diethyl ether due to rather weak complexation, which would explain Castellani & Gallazzi's elemental analysis results.