Complexes of WOCl4 and WSCl4 with neutral N- and O- donor ligands: synthesis, spectroscopy and structures

The complexes [WOCl4(L)] and [WSCl4(L)] (L = OPPh3, OPMe3, pyridine, 2,2’-bipyridyl), [{WOCl4}2(μ-L-L)] and [{WSCl4}2(μ-L-L)] (L-L = Ph2P(O)(CH2)nP(O)Ph2 (n = 1, 2)) have been prepared from WOCl4 or WSCl4 and the ligands in anhydrous CH2Cl2 solution, and characterised by microanalysis, IR and NMR (1H, 31P{1H}) spectroscopy. X-Ray crystal structures are reported for [WOCl4(OPPh3)], [{WOCl4}2(μ-Ph2P(O)(CH2)P(O)Ph2)] and [{WSCl4}2(μ-Ph2P(O)(CH2)2P(O)Ph2)]. All, except those of 2,2’-bipyridyl, are six-coordinate with the neutral donor trans to W=O or W=S. Spectroscopic data suggest that the [WOCl4(2,2’bipy)] and [WSCl4(2,2’-bipy)] are seven-coordinate. Comparison of the structural and spectroscopic data for the two series of complexes indicate little difference in Lewis acidity between the two tungsten(VI) moieties. Decomposition of [WOCl4(OPMe3)] in solution gave the cyclic trimer [W3O3(μ-O)3Cl6(OPMe3)3], the structure of which revealed a six-membered W3O3 ring core with very asymmetric oxido-bridges. The structure of the tungsten(V) complex [WOCl3(2,2’-bipy)] is also reported.


Phosphine oxide complexes.
The WOCl4 and WSCl4 precursors were made by minor modifications of the literature route from WCl6 and O(SiMe3)2 or S(SiMe3)2, respectively, in CH2Cl2 [23]. The complexes were subsequently synthesised by reaction of the appropriate ligand with the WECl4 (E = O or S) in anhydrous CH2Cl2. Attempts to prepare the complexes by "one pot" reactions, i.e. by adding the O(SiMe3)2 or S(SiMe3)2 to WCl6 in CH2Cl2, followed by addition of the ligand, were less 3 successful and often gave impure products or mixtures. The complexes are very readily hydrolysed with formation of the robust [WO2Cl2(L)n] (and other products) and use of anhydrous ligands and solvents with Schlenk and glove box techniques is essential to obtain pure complexes. In several cases, adventitious hydrolysis led to the isolation of [WO2Cl2(L)n] complexes, as seen in other WOX4/WO2X2 systems [1,17].    [13,24]; our data are in agreement with the report of Behzadi et. al. [24]. The 31 P{ 1 H} NMR spectra show substantial high frequency shifts for the phosphoryl group upon coordination and for the complexes of

Complexes of N-heterocycles
To compare the complexes of the O-donor phosphine oxides with those of N-donor ligands, the complexes with pyridine and 2,2'-bipyridyl were examined. The yellow [WOCl4(py)] and brown [WSCl4(py)] complexes [7,19] were made by reacting the constituents in a 1:1 molar ratio in dry CH2Cl2 solution. The products are diamagnetic and the spectroscopic properties are unexceptional. Under reflux or with longer reaction times, reduction to W(V) species occurs [19]. The reaction of WOCl4 with 2,2'-bipyridyl has been reported before [7,25]  A complex [WOCl4(1,10-phen)] has been claimed [25], but with no reported data. Our attempts using similar reaction conditions to those used for [WOCl4(2,2'-bipy)] produced a mixture of products, one of which was identified by an X-ray structure determination as [WO2Cl2(1,10phen)]⋅CH2Cl2 by comparison of the unit cell parameters with the literature [27]. It is possible that the more rigid 1,10-phenanthroline is poorly matched to the seven-coordinate tungsten centre. It is notable that attempts to isolate [WOF4(1,10-phen)] from reaction of [WOF4(MeCN)] with 1,10-phen failed, with [WO2F2(1,10-phen)] being identified as one major product [17].
As indicated above, decomposition (probably hydrolysis) of some of the WOCl4 complexes to WO2Cl2 species was noted, whereas under more forcing condition (higher temperatures, excess ligand or extended reaction time) reduction to tungsten(V) species sometimes occurs [1,7,19,28]. During attempts to grow crystals from of the [WOCl4(OPR3)] by slow evaporation from CH2Cl2 solutions. a few white crystals were isolated, which proved to be cyclic {Wn(µ-O)n} species. Several batches of white crystals were obtained, most with the crystal quality too poor to merit report, but a few good crystals were obtained on one occasion from the OPMe3 system and the structure is shown in Figure 5.

Conclusions
The synthesis of a series of phosphine oxide and pyridyl ligand complexes of WOCl4 and WSCl4 has been described. Under the mild synthesis conditions used, reduction to lower oxidation states of tungsten is not significant, although all complexes are hydrolytically unstable, and must be manipulated and stored under strictly anhydrous conditions. Except for the 2,2'-bipy complexes, where the small ring chelate produces seven-coordination, the other complexes are six-coordinate. Comparison of the data from various spectroscopic probes, viz the IR ν(P=O) data, the coordination shift observed in the 31 P{ 1 H} NMR spectra of the phosphine oxide species, and the d(W−Op) bond lengths in the structures, show only small differences between the corresponding complexes of WOCl4 and WSCl4. The data suggest that WSCl4 may be a marginally less strong Lewis acid than WOCl4, but the differences in the various parameters are small. Future work will examine the corresponding complexes of these two hard W(VI) Lewis acids with much softer sulfur or phosphorus donor ligands, to establish if the trends are replicated.
Infrared spectra were recorded on a Perkin-Elmer Spectrum 100 spectrometer in the range 4000-200 cm −1 , with samples prepared as Nujol mulls between CsI plates. 1 H NMR spectra were recorded using a Bruker AV 400 spectrometer and referenced to the residual protioresonance of the solvent. 31 P{ 1 H} NMR spectra were obtained from CD2Cl2 solutions using a Bruker AV 400 spectrometer and referenced external 85% H3PO4. Microanalyses on new compounds were undertaken by London Metropolitan University or Medac.

X-ray experimental
Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073) rotating anode generator with VHF Varimax optics (70 micron focus) with the crystal held at 100 K (N2 cryostream). Crystallographic parameters are in Table 1. Structure solution and refinement were performed using SHELX(S/L)97, SHELX-2014/7 [30] and were mostly straightforward, some minor disorder within one PMe3 group is noted in the structure of [W3O3(µ-O)3Cl6(OPMe3)3]⋅2CH2Cl2, this was modelled satisfactorily using split C atom site occupancies. H atoms were added and refined with a riding model.

Conflicts
The authors have no conflicts to declare.