Synthesis and Reactivity of a Pentacoordinated Thiolate-Based Imido-Alkylidene W\(VI\) Complexes

Attempt to synthesize a bis-thiolate tungsten alkylidene catalysts with bulky arenethiolates unexpectedly yielded upon activation of DME a pentacoordinated complex of general formula [W(NAr) (CHCMe3)(SAr’)(j -O-CH2-CH2-OMe)], which is active towards self-metathesis of cis-4-nonene compared to the parent bis-benzenethiolate. Résumé— Synthèse et réactivité de complexes imido-alkylidène pentacoordinés du tungstène(VI) avec des ligands thiolates — Des essais dans le but de synthétiser un catalyseur bis-thiolate tungsten alkylidène comprenant des arènethiolates encombrés ont permis, via l’activation du DME, la synthèse inattendue d’un complexe de formule générale [W(NAr)(CHCMe3)(SAr’)(j -O-CH2-CH2-OMe)], qui s’avère être active vis-à-vis de l’autométhatèse du cis-4-nonène en comparaison avec l’homologue bisbenzènethiolate.


INTRODUCTION
Alkene metathesis plays a key role in petrochemical and polymer industries [1,2], and has also become widely used for organic synthesis.For these reasons, the development of homogeneous and heterogeneous metathesis catalysts still attracts major attention [3,4].Well-defined alkylidenes in high oxidation states, with electron-withdrawing ligands were identified as highly efficient metathesis catalysts [5][6][7], following the proposal by Chauvin that they were key intermediates [8,9].Recent progress in the understanding of the structure-activity relationship, involving surface chemistry [10][11][12][13] combined with computational studies [14][15][16][17], has highlighted trends for a more rational design of alkene metathesis catalysts and shown in particular that dissymmetry at the metal center can bring about high activity.In particular, d 0 -metal alkylidene sites with two ligands of very different electron donation abilities would favor low energy barriers for coordination of the olefin while also destabilizing the metallacyclobutane intermediates, leading to overall more active catalysts [18].MAP (MonoAlkoxide Pyrrolide) complexes illustrate perfectly this concept [2,[19][20][21][22][23].
The removal of one of the methyl group of the DME ligand most likely arises from the reaction of Ar'SK with the coordinated DME on the Lewis acidic W center, similarly to the demethylation of methylethers by thiolate in the presence of Lewis acids [29].
The two reaction products 1 and 2 were obtained as crystalline materials in 62% and 72% yield, respectively.They are easily separated as pure material as a result of their difference of solubility and their tendency to form crystalline materials.Both products were fully characterized by XRD, 1 H and 13 C NMR (Nuclear Magnetic Resonance) spectroscopy.The structure of 2 is given in supplementary information (The Figure S5 is in the supplementary information.CCDC 1409694 contains the supplementary crystallographic data).The single crystal XRD study of 1 reveals a pentacoordinated tungsten complex of formula W(NAr)(CHCMe 3 )(SAr')(j 2 -O-CH 2 -CH 2 -OMe) (Fig. 1).
The tungsten atom lies at 0.48 Å above the plane defined by C ene , O1 and S, thus resulting in a slightly distorted Trigonal BiPyramidal (TBP) molecular geometry, where the imido ligand and the O2-Me group from the activated DME occupy the axial positions.The tungsten center is more sterically hindered than in W(NAr)(CHCMe 3 )(SAr') 2 [26], giving a slightly longer W1-S1 bond distance (2.3930(5) Å versus 2.349(5) Å).The W-N bond length (1.7486(17) Å) is in the range of usual W-N bond, and so is the W-C ene one (1.915(2)Å).The much shorter W1-O1 bond distance (1.9212(14) Å, compared to the W1-O2 at 2.4404(15) Å), is consistent with the demethylation of DME, since the alkoxy group is a much stronger r-donor ligand than the parent ether ligand.It is noteworthy in that such a reaction was not observed when less bulky thiolate ligands such as the 2,6-diisopropylthiophenolate or 2,4,6-triisopropylthiophenolate were used [26,28], suggesting that the kinetics of bisthiolate complex formation is largely reduced when the steric hindrance increases, giving a notable advantage to the demethylation pathway.
The molecular complex 1 (1 mol%) is slightly active in the self-metathesis of cis-4-nonene to yield cis/trans-mixtures of 4-octenes, 4-nonenes and 5-decenes (The Figure S3 is in the supplementary information.)with an initial TOF of 3.10 À3 min À1 at 30°C.However, the catalyst deactivates, the conversion only reaching ca.14% after 78 h.It is noteworthy that this catalyst is Z-selective (81% after ca.1% conversion/ 250 minutes), yielding in fine 67% Z-selectivity after 78 h.

EXPERIMENTAL General
All experiments were carried out under dry and oxygen free argon atmosphere using either standard Schlenk or glovebox techniques for organometallic synthesis.For the syntheses, reactions were carried out using high vacuum lines (10 À5 mbar) and glove-box techniques.Pentane, toluene and diethyl ether were purified using double MBraun SPS (Solvent Purification System) alumina column, and were degassed using three freeze-pump-thaw cycles before being used.DME and THF were distilled from Na/Benzophenone.Et 3 N, Me 3 SiCl, 2,5-dimethyl-1H-pyrrole and 2,6diisopropylamine were dried on CaH 2 , distilled under reduced pressure and degassed using two freeze-pump-thaw cycles.Triflic acid was purified stirring it one day over triflic anhydride, followed by distillation under reduced pressure.All InfraRed (IR) spectra were recorded using a Bruker spectrometer placed in the glovebox, equipped with OPUS software.A typical experiment consisted in the measurement of transmission in 32 scans in the region from 4 000 to 400 cm À1 .The 1 H and 13 C-NMR spectra were obtained on Bruker DRX 200, DRX 250 or DRX 500 spectrometers.The solution spectra were recorded in C 6 D 6 at room temperature.The 1 H and 13 C chemical shifts are referenced relative to the residual solvent peak.W(NAr)(CHCMe 3 ) (DME)(OTf) 2 [28] and 1,3-dibromo-2-iodobenzene [30] were synthesized according to literature procedures.