Cationic Tungsten(VI) Penta-Methyl Complex: Synthesis, Characterization and its Application in Olefin Metathesis Reaction

— Tungsten-hexa-methyl readily reacts with B(C 6 F 5 ) 3 in dichloromethane and generates the corresponding well-de ﬁ ned cationic tungsten-penta-methyl complex which was identi ﬁ ed precisely by 1 H NMR, 13 C NMR, 1 H- 13 C NMR correlation spectroscopy. Unlike WMe 6 , this cationic complex has low energy barrier to form tungsten carbene intermediate, which was further supported by the fact that WMe 6 alone has no activity in ole ﬁ n metathesis reaction whereas the cationic complex shows catalytic activity for self-metathesis of 1-octene. Résumé réaction


INTRODUCTION
In 1973, Shortlan and Wilkinson described the first report on Tungsten hexamethyl [1], [2][3][4].After its synthesis, the structure was established in 1996 by Seppelt [5], [6,7] and Kaupp [8].The geometry was calculated by Demolliens et al. [9] and Kang et al. [10].However, until 2015 and to our knowledge there was no report on the catalytic activity of this homoleptic complex.WMe 6 is indeed difficult to prepare, extremely unstable, and very sensitive to even few ppm levels of oxygen or moisture.Handling this compound is challenging due to its explosive nature as reported by Shortlan and Wilkinson [1,2], Galyer et al. [3], Galyer and Wilkinson [4].In our previous communication, we demonstrated that grafting this unstable WMe 6 to the silica surface by SOMC (Surface OrganoMetallic Chemistry) strategy and methodology at À80°C can enhance its thermal stability due to the formation of the stable grafted complex, [SiO-W(Me) 5 ] which proved to be a nice precursor of alkane metathesis catalysts via the formation of surface monopodal tungsten carbyne [11].This carbyne is in equilibrium with a bis carbene upon addition of a donor ligand [12] (Scheme 1).Also, we demonstrated that WMe 6 on silica-alumina shows relatively higher reactivity than on silica because of the formation of very reactive cationic intermediate [13].Let us recall that Kress and Osborn were the first to report the synthesis of a cationic W-carbene very active in olefin metathesis [14].Based on the above fact we speculate that in order to prepare more reactive catalysts in terms of reactivity and selectivity a higher electrophilicity on tungsten center is essential.However, high oxidation state cationic complexes of tungsten are very rare, especially those which do not contain a coordinating ligand [15] and these complexes are expected to be more reactive than the corresponding neutral complexes [14].Cationic tungsten complex containing one or more stabilizing ligands e.g.oxygen, nitrogen, or oxygen, nitrogen, phosphorous containing ligand, cyclopentadienyl ligand are relatively stable and are described in the literature [14,[16][17][18][19][20].Very recently, Schowner et al. have reported the synthesis of first cationic tungsten-oxo-alkylidene-Nheterocyclic-carbene complexes where the cationic metal center was stabilized by N-heterocyclic carbene [20].
To the best of our knowledge, the only example for homoleptic cationic tungsten-alkyl complex was reported in the literature by Wilkinson et al.Even in this paper they mentioned the reaction of WMe 6 with AlCl 3 which would lead to the formation of WMe 5 + but without any proof [1][2][3][4].

EXPERIMENTAL
All experiments were carried out by using standard Schlenk and glovebox techniques under an inert nitrogen atmosphere.The syntheses and the treatments of the surface species were carried out using high-vacuum lines (<10 À5 mbar) and glovebox techniques.Pentane was distilled from a Na/K alloy under N 2 and dichloromethane from CaH 2 .Both solvents were degassed through freezepump-thaw cycles.SiO 2-700 was prepared by calcination at 300°C in the presence of air followed by dehydroxylation at 700°C under high vacuum (<10 À5 mbar) for 24 h.It contained 0.5-0.7 OH per nm 2 . 1 H NMR spectra were recorded at 600 MHz in CD 2 Cl 2 unless otherwise stated.Chemical shifts are reported in ppm with the solvent resonance (CD 2 Cl 2 : 5.32 ppm).Data are reported as follows: chemical shift, integration, and coupling constants (Hz). 13C NMR were recorded at 150 MHz in CD 2 Cl 2 unless otherwise stated with complete proton decoupling.Chemical shifts are reported in ppm from solvent peak as the standard (CD 2 Cl 2 : 53.84 ppm).

Preparation of Hexamethyltungsten, WMe 6
The molecular precursor WMe 6 was prepared from WCl 6 and (CH 3 ) 2 Zn, following the literature procedure [1,6].To a mixture of WCl 6 (1.80 g, 4.5 mmol) in dichloromethane (25 mL) was added (CH 3 ) 2 Zn (13.6 mmol, 1.0 M in heptane) at À80 °C, and after addition, the reaction mixture was warmed to À35 °C and stirred at this temperature for another 30 min.After successive filtrations with pentane and removal of the solvent, the red solid corresponding to WMe 6 was obtained (0.16 g, 12%).
The 13 C-enriched W( 13 CH 3 ) 6 was synthesised as described below: 13 C-enriched ( 13 CH 3 ) 2 Zn was prepared from a suspension of 13 CH 3 Li and ZnCl 2 (2/1), with subsequent synthetic steps being analogous to those provided above.

Preparation of [WMe 5
+ B(C 6 F 5 ) 3 Me À ] A cold solution (À20 °C) of B(C 6 F 5 ) 3 (100 mg) in dichloromethane was added drop wise to the cold (À20 °C) solution of hexamethyltungsten (55 mg, 1 equiv.with respect to B (C 6 F 5 ) 3 ) in dichloromethane.The mixture was stirred for 15 min at À20 °C.Color of the solution intensified to reddish indicating the formation of ionic complex.At temperature below À60 °C, this ionic complex is precipitated out from the solution however this 10e À compound is highly unstable and decompose into black tungsten power while drying.Caution!This 10e À compound is highly unstable and is prone to violent decomposition.

Grafting of [WMe 5
+ B(C 6 F 5 ) 3 Me À ] on Silica 700 A solution of [WMe 5 + B(C 6 F 5 ) 3 Me À ] (85 mg, 1.2 equiv.with respect to WMe 6 and with respect to the amount of surface-accessible silanols) in dichlomethane (15 mL) was mixed with silica (SiO 2-700 ; 1.0 g) at À50 °C for one hour, allowed to warm to À30 °C, and then stirred for an additional 2 h.At the end of the reaction, the resulting brown solid was washed with pentane (3920 mL) and dried under dynamic vacuum (1 mPa, 1 h).

RESULTS AND DISCUSSION
As a part of our continuing program to explore the novel applications of tungsten hexamethyl, we report here the synthesis of the cationic tungsten-penta-methyl complex [WMe 5 ] + [MeB(C 6 F 5 ) 3 À ], starting from WMe 6 and its application in olefin metathesis.The experimental procedure is very simple.Simple mixing of bulky and noncoordinating Lewis acid, B(C 6 F 5 ) 3 with WMe 6 at very low temperature (À20 °C) generates quantitatively the cationic tungsten-penta-methyl complex [WMe 5 ] + [MeB(C 6 F 5 ) 3 À ].
In a preliminary study, we tried to synthesise WMe 5 + according to literature reference using AlCl 3 as a de-methylating reagent but after several attempts we were unable to get the desired product [1][2][3][4].Subsequently, we switched to tris(pentafluorophenyl)-boron which is known to form a non-nucleophilic anion after de-methylation reaction.When the reaction was monitored by NMR at À20 °C, it was found that the reaction between WMe 6 and B(C 6 F 5 ) 3 was very fast and the peak corresponding to WMe 6 at 1.75 ppm in 1 H-NMR almost completely disappeared in less than 15 minutes and two new peaks at 2.68 ppm and 0.48 ppm were formed (Fig. 1).The product generated by the above reaction has a fair stability below À40 °C in the absence of light.However, upon warming this reaction mixture to 0 °C from À40 °C, a very first decomposition of the cationic complex was observed with release of gaseous methane.Based on the above experimental facts, one can assume the two possibilities; where Path I (Scheme 2) is the formation of neutral tungsten complex by the ligand metathetical exchange and Path II (Scheme 2) is de-methylation from WMe 6 , formation of cationic tungsten complex.
The 1 H-NMR chemical shift at +0.5 ppm corresponds to [CH 3 -B(C 6 F 5 ) 3 ] À which is well documented in the literature [21,22] and as expected the resonance for the methyl proton of WMe 6 at 1.75 ppm is shifted downfield to 2.7 ppm, and ascribed to [WMe 5 ] + .Similarly the 13 C-NMR spectra also clearly shows that the peak at 83 ppm of WMe 6 is also completely replaced by two peaks at 10.8 ppm [CH 3 -B(C 6 F 5 ) 3 ] À and 103.2 ppm corresponding to [W(CH 3 ) 5 ] + which is good agreement with the literature value reported for [CH 3 -B(C 6 F 5 ) 3 ] À (Fig. 1).We also found in the 1 H- 13 C correlation spectra that the peak at 0.5 ppm in 1 H-NMR was correlating with the peak at 10.8 ppm in 13 C-NMR and the peak at 2.7 ppm in 1 H-NMR was correlating with the peak at 103.2 ppm in 13 C-NMR.These spectroscopic data strongly support the formation of [WMe 5 ] + [MeB(C 6 F 5 ) 3 ] À .In separate experiments, we synthesised 13 C label W(C 13 H 3 ) 6 and upon treatment with B(C 6 F 5 ) 3 we have thus identified incorporation of the labeled methyl in the final [(C 13 H 3 )B(C 6 F 5 ) 3 ] À anion.The above experiment clearly indicates that in [MeB(C 6 F 5 ) 3 ] À , 'CH 3 ' came from WMe 6 and strongly supports Path II (Scheme 2).
We then tried to graft cationic complex on the surface of silica-700 , in order to get a cationic tungsten catalyst onto the heterogeneous support.However, after several attempts we were unable to identify any well define heterogeneous cationic tungsten complex except a mixture of neutral tungsten-methyl complex (Scheme 3).] onto the silica 700 support.
Tungsten cationic complex was found to be very effective for the metathesis of 1-octene and ROMP (Ring Opening Metathesis Polymerization) for cyclooctene.Cyclooctene was polymerized to polyoctenamer by ROMP at even À40 °C by this cationic tungsten complex.However, this complex was inactive for metathesis of trans 2-octene (Scheme 4).The detailed mechanism and the precise nature of the active catalytic species are still unknown for this catalytic system.However from the control experiment, we found that WMe 6 does not metathetise cyclooctene nor 1octene.From the above experiments, we can assume that the formation of a tungsten carbene intermediate is more favoured when starting from the cationic complex rather than neutral WMe 6 .A very similar observation was reported by Kress and Osborn, where they showed that cationic complexes are more active towards olefin metathesis than the corresponding neutral complexes [14].Based on this, we speculate that a tetra coordinated cationic [WMe 3 (=CH 2 )] + [-MeB(C 6 F 5 ) 3 ] À intermediate is formed from [WMe 5 ] + [MeB (C 6 F 5 ) 3 ] À by a-H transfer into neighbouring methyl groups.However, we have never seen any carbene intermediates by NMR during the decomposition of [WMe 5 ] + [MeB(C 6 F 5 ) 3 ] À while the experiments were carried out inside the NMR instrument for long time.

CONCLUSION
In summary, we have developed a straight forward strategy for the synthesis of extremely unstable high oxidation state cationic tungsten(VI)-methyl complex in a precise way, starting from WMe 6 .We fully characterised this unstable ionic species by 1 H-NMR, 13 C-NMR, 1 H- 13 C correlation, etc., and also, we showed that this cationic complex is reactive in olefin metathesis reaction.To the best of our knowledge, this is the first example of a well defined cationic tungsten complex.Further studies are in progress to assess the reactivity of this tungsten complex and its application in other catalysis.Metathesis of olefin by cationic tungsten-methyl complex.BARF À = B(C 6 F 5 ) 3