1
1. Introduction
Olefin metathesis is a reaction that entails the redistribution of fragments of alkenes by the scission and regeneration of carbon-carbon double bonds. It was first observed in the 1950s by industrial chemists. In particular, in 1956, Herbert S.
Eleuterio, at DuPont's petrochemicals department, obtained a propylene-ethylene copolymer from a propylene feed passed over a molybdenum catalyst (MoO3-on- aluminum). Analysis showed that the output gas was a mixture of propylene, ethylene and 1-butene. Chemists at other petrochemical companies were getting similar results. These products could not be explained by reactions of olefins known at the time, but in 1967, Nissim Calderon and others at Goodyear Tire &
Rubber, figured out what was going on. The unexpected products are due to cleavage and reformation of the olefins' double bonds. One carbon of double bond of one olefin, along with everything attached to it, exchanges place with one carbon of the double bond of the other olefin, along with everything attached to it. The Goodyear researchers named the reaction "olefin metathesis"1.
Fig. 1 Metathesis “idea”, from Greek μετάθεσις, from μετατίθημι "I put in a different order"; Latin: trānspositiō.
1 Calderon N., Chen H.Y, Scott K.W, Tetrahedron Lett, 1967, 34, 3327-3329.
2 In the past 50 years, the alkene metathesis has become a key process2,3 in industrial, pharmaceuticals, polymers and basic chemistry, and its understanding and spread is especially due to three chemists: Yves Chauvin, Robert H. Grubbs and Richard R. Schrock who won the Nobel Prize for chemistry in 2005, “for the development of metathesis method in organic synthesis”. Yves Chauvin in fact, proposed a mechanism for the olefin metathesis (1971), 4 which now is the widely accepted mechanism of this transformation. It involves a [2+2] addition between an olefin and a metal-carbon double bond forming a metallacyclobutane intermediate, which then is cycloreverted into a metal alkylidene and a newly- formed olefin (Fig. 2).
Fig. 2 Metathesis reaction mechanism, proposed by Chauvin.
2 Schrock R. R., Chem. Rev., 2009, 109, 3211.
3 Grubbs R. H., Angew. Chem. Int. Ed., 2006, 45, 3760.
4 Herrison J.L., Chauvin Y., Makromol. Chem., 1971, 141, 161.
3 Chauvin hypothesis was the impetus that has moved the other two scientists to look for this kind of complexes that could provide the reaction.
The research activity of R. Schrock was almost entirely dedicated to synthesize complexes with metal-carbon multiple bonds with high oxidation state metal (especially Mo and W), compared to Fisher’s carbenes that contain metals in low oxidation state. They are known as metal alkylidens (M=CHR).
Grubbs’s studies instead involved metal-alkylidene complexes based on Ru(II) halides. These catalysts, contrary to Schrock’s ones, are not air or moisture sensitive, and not even sensitive to some important reactive groups such as alcohols and aldehydes. This insensitivity led to extend their application also to metathesis of functionalized alkenes.
Fig. 3 Grubbs and Schrock catalysts’ general structures.
While the petrochemical processes mainly rely on heterogeneous catalysts based on supported transition metal oxides5,6,7 in last decades use of homogeneous Mo,
5 Mol J.C.,J.Mol.Cata. A: Chem, 2004, 213, 39-45.
6 Ivin K. and Mol H., Olefin Metath. and Metathesis polim.
7 Ivin K.J. and Mol J.C., Academic Press, London, 1997.
4 W and Ru alkylidene catalysts has become common in fine chemical syntheses.
Early Mo- and W- based catalysts were prepared in a wide variety of ways from different starting materials in which the metal had different oxidation states (from 0 to VI)8,9,10. Typical syntheses involved reactions between WCl6 or WOCl4 and alkylating agents. Although these catalysts had been largely examined by different techniques, the oxidation state of the metal and the nature of the ligands were unclear. Therefore, they are known as “ill-defined” catalysts. Several complications were observed using these kind of molecules as catalysts for olefin metathesis, such as they are short-lived, often producing side products and they are deactivated by common Lewis basic functional groups11. For these reasons, the synthesis of “well-defined” catalysts became a priority in olefin metathesis research.
A “well-define catalysts”, following Schrock’s and Hoveyda’s definition is one which 1) is identical to the active species in terms of metal oxidation state and ligand coordination sphere, 2) reacts with olefins to yield observable new carbine complexes derived from those olefins and 3) is stable enough to be characterized through spectroscopic measures and preferably also X-ray structural analysis. So then, after the beginning with supported metal oxides, passing through the “ill- defined” catalysts, the last studies regard well-defined metal alkylidene complexes with different kind of ligands, to improve catalytic properties. To synthesize new
8 Ivin K. J., Mol J. C., Olefin Metathesis and Metathesis Polymerization, Academic Press, San Diego, 1997.
9. Ivin K. J, Olefin Metathesis, Academic, New York, 1983.
10 Dragutan V., Balaban A. T, Dimonie M., Olefin Metathesis and Ring-Opening Polymerization of Cyclo-Olefins, 2nd ed., Wiley, New York, 1985.
11 Schrock R. R., Hoveyda A. H., Angew. Chem. Int. Ed., 2003, 42, 45-92-4633.
5 molecules, which could work as catalysts, is useful to look at some of the characteristics a good catalyst should have:
- High activity for the target reactions. Catalytic activity can be measured by Turn Over Number, which is the number of moles of substrate that a mole of catalyst (for heterogeneous catalysis number of active sites) can convert before becoming inactivated. The term Turn Over Frequency (TOF) is used to refer to the turnover per time unit and is generally used to compared different catalysts.
- Selectivity, meaning the ability of increase the rate only of the target reaction. In this case only the metathesis reaction instead of possible olefin polymerization or collateral reactions.
- Stability toward the deactivation. In particular for metal- alkylidenes and alkilidynes as homogeneous catalysts, the major pathway of deactivation is dimerization (bimolecular decomposition reactions) involving the coupling of the alkylidene and alkylidyne ligands12.
Accordingly, in order to create good catalysts is also important to analyze which are the factors to play on, to get the characteristics described above.
Extended further theoretical and experimental investigations on the reaction mechanism, which regarded Mo and W well defined alkylidene complexes (Fig. 4)13,14,15,13 , have highlighted that the key step of metathesis reaction is the coordination of the alkene to the metal center. This is associated with a distortion
12 Schrock R.R., Chem. Comm., 2005, 2773-2777.
13 Solans X., Copéret C., Eisenstein O., Organometallics, 2012, 31, 6812-6822.
14 Poater A., Solans X., Copéret C., Eisenstein O., JACS, 2007, 129, 8207-8216.
15 Rhers B., Salameh A., Copéret C., Eisenstein O., Schrock R., Organometallics, 2006, 25, 3554- 3557.
6 of the tetrahedral alkylidene complex, with no empty coordination site, into a trigonal pyramidal to generate a vacant site for the incoming olefin to form a Trigonal Bipyramid structure (TBP) with an axial olefin ligand. The TBP intermediate then, can interconvert into a generally more stable square-based pyramidal metallacyclobutane (SP). This last configuration correspond to a catalytic resting state, and it could actually determine deactivation process toward β-H transfer and formation of byproducts13.
Fig. 4 Improved Chauvin’s metathesis reaction mechanismfor metal alkylidenes, via metalacyclobutane intermediate.
DFT studies of this reaction mechanism14 have shown the efficiency of metathesis catalysts depends on the ability of the initial tetrahedral structure to distort to open a coordination site to accommodate the olefin and on the stability of the metallacyclobutane intermediate. An olefin metathesis catalyst having a geometry very close to that of the transition state associated with the coordination of an olefin
7 (low energy barrier), and generating a not too stable metallacyclobutane intermediate would be an efficient catalyst16.
Focusing on Shrock-type catalysts with the general formula [M(NR)(=CHR)(X)(Y)]
(M = Mo, W), is worth to note that latest studies highlighted that, the anionic X and Y substituents have important influence on the stability of the reaction intermediates17.
Fig. 5 Shrock-type catalysts with the general formula [M(NR)(=CHR)(X)(Y)] (M = Mo, W, E=NR, X,Y = anionic ligands).
The dissymmetry at the metal centre, in particular, the different electron donation ability of the X and Y ligands (one weaker and one stronger σ-donor ligand) affords a lower energy barrier for the coordination of the olefin and for that, increase the reactivity of the system18,19. This has been exemplified with monoalkoxy pyrrolyl alkylidene complexes (NR as E, MAP, Fig. 6), showing a high activity, selectivity and stability toward deactivation20,21,22.
Fig.6 MAP complex, general structure.
16 Solans-Monfort X., Clot E., Copéret C., Eisenstein O., JACS, 2005, 127, 14015-14025.
17 Solans X.,.Copéret C, Eisenstein O., JACS, 2012, 31, 6812-6822.
18 Poater A., Solans X., Copéret C., Dalton Trans., 2006, 3077-3087.
19 Solans X., Copéret C., Eisenstein O., J. Am. Chem. Soc., 2010, 132, 7750-7757.
20 Reithofer M.R. et al., Organometallics, 2013, 32, 2489-2492.
21 Gerber L. C. H., Schrock R. R. and Müller P., Organometallics, 2013, 32, 2373-2378.
22 Townsend M., Schrock R. R. and Hoveyda A. H., J. Am. Chem. Soc., 2012, 134, 11334-11337.
8 Also tungsten oxo alkylidene (O as E ligand) complexes have shown unprecedented activities and stabilities23,24.
Fig.7 Oxo alkylidene complex, general structure.
This improving in catalyst efficiency, substituting the N with O, had been hypothesized by DFT calculations25. Even though the oxo may not improve the activity of the catalyst, it can still lead to a more efficient one because it prevents deactivation pathway via -hydride transfer, thanks to the higher energy barrier associated with this pathway as a result of the stronger trans influence of the oxo ligand.
Recently, in order to improve also the prevention of the deactivation by dimerization of active species, Mo and W alkylidenes have been supported (grafted) on silica partially dehydroxylated at 700 °C. This kind of silica provides isolated silanols which act as alkoxide ligands but, at the same time, minimize the interaction between catalyst molecules that are already grafted on26,12. These supported catalysts displayed activities which in most cases, surpassed their molecular precursors.
23 Conley, M. P.; Mougel, V.; Peryshkov, D. V.; Forrest, W. P.; Gajan, D.; Lesage, A.; Emsley, L.;
Copéret, C.; Schrock, R. R. J. Am. Chem. Soc. 2013, 135, 19068-19070.
24 Conley, M. P.; Forrest, W. P.; Mougel, V.; Copéret, C.; Schrock, R. R. Angew. Chem. Int. Ed. 2014, 126, 51, 14445-14448.
25 Solans X., Copéret C., O. Eisenstein, J. Am. Chem. Soc., 2012, 6812-6822.
26 Mougel V., Copéret C., Chem. Sci., 2014, 5, 2475-2481.
9 In particular, the group in which this Thesis has been performed focused the work on silica-supported tungsten oxo alkylidene bearing phenolate ligands (X, Y), and discovered that these complexes catalyze the metathesis of internal olefins (cis-4- nonene as a prototypical substrate, Fig. 8) at very low catalyst loading. Is worthy to report the case of [(≡SiO)W(=O)(=CHtBu)(OHMT)2] and [(≡SiO)W(=O)(=CHtBu)(dAdPO)2] (OHMT = 2,6-dimesitylphenoxide, dAdPO = 2,6- diadamantyl-4-methylphenoxide)23,24.
Fig.8 Self-metathesis of cis-4-nonene.
Fig. 9 silica-supported tungsten oxo alkylidene bearing phenolate ligands.
10 They are, instead, surprisingly sluggish for 1-alkenes, because of the formation of very stable unsubstituted metallacycle generated by the ethylene byproduct (Fig.10).
Fig.10 Self-metathesis of 1-nonene.
However, in silica supported phenolate tungsten oxo alkylidene complexes, such as both complexes reported above, the calculated degree of -donation of the phenoxide and the surface siloxide ligands, respectively, are very close to each other.
We thus reasoned that thiolate analogues27(a,b,c) would be ideal targets since they would provide the same characteristics that have been demonstrating to be very important for having a good catalyst (strong trans-influence of O as E ligand, and X-Y with a different donation power). This add to a similar steric environment
compared to the corresponding phenolates28,29,30,31, and the advantage of
27 Note that thiolate ligands have shown improved Z-selectivities in Ru-based catalyst, see: (a) Occhipinti, G.; Hansen, F. R.; Törnroos, K. W.; Jensen, V. R. J. Am. Chem. Soc. 2013, 135, 3331.
(b) Khan, R. K. M.; Torker, S.; Hoveyda, A. H. J. Am. Chem. Soc. 2013, 135, 10258. (c) Koh, M. J.;
Khan, R. K. M.; Torker, S.; Hoveyda, A. H. Angewandte Chemie International Edition 2014, 53, 1968.
28 Townsend, E. M.; Hyvl, J.; Forrest, W. P.; Schrock, R. R.; Müller, P.; Hoveyda, A. H.
Organometallics 2014, 33, 5334.
29 Coperet, C.; Mougel, V.; Frater, G. E.; Varga J.; Hegedus, C. Immobilized Tungsten Catalysts and Use Thereof in Olefin Metathesis. Eur. Pat. Appl. 13003540 2013.
30 Coperet, C.; Mougel, V.; Frater, G. E.; Varga J.; Hegedus, C. ́ 253 Robe, E. Use of Immobilized Molybdenum and Tungsten containing 254 Catalysts in Olefin Cross metathesis. Eur. Pat. Appl.
109914P771.
31 Florian Allouche, Master thesis, ETH Zürich, 2013.
11 enhancing the dissymmetry at the metal center. Moreover, we hypothesized that sulfur, being less electronegative, more polarizable and bigger than oxygen, would provide longer W-S bonds, so we could have less steric bulk and an easier approach of the olefin during the metathesis reaction.
We therefore decided to investigate the reactivity of [W(=O)(=CHtBu)(SHMT)2] (see figure 11), thiolate analogue to [(≡SiO)W(=O)(=CHtBu)(OHMT)2]. Herein is reported the synthesis, the grafting on partially dehydroxylated silica at 700 °C (SiO2-(700)) together with the catalytic performances of oxo alkylidene complexes with pendant thiolate ligands.
Fig. 11 a) [(≡SiO)W(=O)(=CHtBu)(OHMT)2], OHMT = 2,6-dimesitylphenoxide, b) target product [W(=O)(=CHtBu)(SHMT)2] SHMT = 2,6-dimesitylthiophenoxide.
12
2. Results and discussion
2.1 Synthesis of bis-thiolate tungsten complex
The thiolate complex 1 was synthesized by salt metathesis of [W(=O)(=CHtBu)Cl2(PMe2Ph)2]32,33 with two equivalents of potassium 2,6- dimesitylthiophenolate, obtained by deprotonation of the parent thiolate using KH (Fig. 12). The product was isolated as a crystalline yellow material in 70% yield, by recrystallization in pentane. A similar synthesis was independently described by Schrock and Hoveyda during the course of our studies28.
The product was characterized by NMR, IR, elemental analysis and single crystal XRD. Crystals suitable for XRD measurement were obtained from recrystallization in pentane.
Fig. 12 Synthesis of WO(SHMT)2(PPhMe2)(CHCMe3), 1.
32 Peryshkov, D. V.; Schrock, R. R. Organometallics 2012, 31, 7278.
33 Peryshkov, D. V.; Schrock, R. R.; Takase, M. K.; Muller, P.; Hoveyda, A. H. J. Am. Chem. Soc.
2011, 133, 20754.
13 The IR spectrum of the compound shows the absorptions typical34,35,36 of the aromatic ligands and of the W=O bond which appears at 950 cm-1.
NMR spectra (in C6D6) are in agreement with the molecular structure obtained from X-Ray diffraction experiments. In particular, the alkylidene fragment shows resonance at 8.94 ppm and 284.40 ppm in 1H and 13C spectra respectively. These values can be compared to OHMT analogue32 (1H NMR, C6D6 δ = 7.34 and 13C NMR, C6D6 δ = 253.6). Worth to note is the signal at 1.09 in 1H NMR, assignable to the two methyl groups of the phosphine. The 31P NMR spectrum shows one signal (-1.2 ppm) has been assigned to the metal-coordinated phosphine coordinated to the metal, and another one at - 46.46 (normally attributed to the free phosphine37) that can be assigned to a phosphine in rapid exchange with the metal.
X-Ray diffraction analysis shows that complex 1 adopts a distorted trigonal bipyramid geometry (Figure 13), similar to the related phenoxide complex [W(=O)(=CHtBu)(OHMT)2]38, OHMT = 2,6-dimesitylphenoxide, in which the oxo ligand, C1 and C2 of the syn neopentylidene ligand and the sulfur lie in the equatorial plane while the phosphine and the second thiolate ligand are facing trans to each other in axial position. Worthy of note is the significant difference
between the axial W-S1 bond distance (2.388(2) Å) and the equatorial W-S2 (2.450(3) Å), highlighting the stronger trans influence of the oxo over the phosphine ligand.
34 Dolci S.,Marchetti F., PampaloniG. and Zacchini S., Dalton Trans., 2013, 42, 5635.
35 Mirza Z. A. et al, J. Chem. Pharm. Res., 2014, 6, 706-710.
36 Wengrovius J. H. and Schrock R. R., Organometallics, 1982, 1, 148-155.
37 Kagan, Gerald; Li, Weibin; Hopson, Russell; Williard, Paul G. Organic Letters, 2009 , 11, 21, 4818 – 4821.
38 O'Donoghue, M. B.; Schrock, R. R.; LaPointe, A. M.; Davis, W. M. Organometallics 1996, 15, 1334.
14
Figure 13 Molecular structure of [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)], 1.
Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms were omitted for clarity.
Table 1 Selected bonds for [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)] (distances are given in Å)
Structural parameters [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)]
W1 – S2 2.450(3)
W1 – P1 2.579(3)
W1 – S1 2.388(2)
W1 – O1 1.715(8)
W1 – C1 1.894(10)
15
Table 2 Selected angles for [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)] (given in degrees).
Structural parameters [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)]
S2 – W1 – P1 74.63 (8)
S1 – W1 – S2 99.61 (8)
O1 – W1 – P1 171.82 (9)
O1 – W1 – S2 135.8 (3)
O1 – W1 – P1 78.8 (2)
O1 – W1 – S1 102.3 (2)
All spectroscopic data related to 1 show that one phosphine, as it was present in the precursor, is still coordinated to the metal, contrary to what happens for the oxo analogue [W(=O)(=CHtBu)(OHMT)2]. This is probably due to the long W-S bond, which supplies the possibility for tungsten to host the phosphine ligand (Fig.14).
Fig.14 a) OHMT complex, oxo analogue to the target product, b) target complex, c) complex 1, d) precursor.
In consideration of the fact that complex b (Fig 14) is expected to be more active as metathesis catalysts in view of the less coordinated tungsten centre, we decided to find a way to remove the phosphine ligand from compound c (Fig 14) and different approaches were attempted. The first two attempts included the use of a metal compound: PdCl2(MeCN)2 and CuCl39. Unfortunately, in both cases the 1H
39 L. K. Johnson, S. C. Virgil, R. H. Grubbs, J. W. Ziller, J. Am. Chem. SOC. 1990, 112, 5384.
16 NMR spectra presented a large and unclear amount of signals indicating the formation of new species. Another attempt was made by using a Merrifield resin as a phosphine scavenger40, but formation of new species and possible de- coordination of the ligand form the metal was observed by NMR. Since NMR evidences appeared to indicate the occurrence of exchange process of the phosphine ligand, a further attempt was to dissolve the complex in toluene and evaporate the solvent under vacuum. This last method allowed to scavenge only a few percentage of the phosphine.
The only approach revealed to be successful was Schrock’s one41 using the strong Lewis acid tris(pentafluorophenyl)borane42. Complex 2 was then prepared by addition of tris(pentafluorophenyl)borane to a solution of 1 in toluene and was isolated as a crystalline yellow material by recrystallization in pentane in 85% yield.
Fig. 15 Synthesis of WO(SHMT)2(CHCMe3), 2.
40 B. H. Lipshutz and P. A. Blomgren, Organic Letters, 2001, vol. 3, 12, 1869-1871.
41 D. Peryshkov, R. Shrock et al., J. Am. Chem. Soc., 2011, 133, 2754-20757.
42 H. Jacobsen, H. Berke et al., Organometallics, 1999, 18, 1724-1735.
17 Compound 2 was characterized by NMR, IR, elemental analysis and single crystal X-Ray diffraction. Crystals suitable for XRD measurement were obtained from recrystallization in pentane. The IR spectrum of the compound shows the absorptions typical34,35,36 of the aromatic ligands and of the W=O bond which appears at 985 cm-1.
NMR spectra (in C6D6) are in agreement with the molecular structure obtained from the XRD. In particular, it has been possible to assign, as in complex 1, the 1H NMR singlet at 8.90 ppm to the akylidene proton, whereas the related carbon resonates at 284.28 in the 13C NMR spectrum. No signals belonging to the phosphine were found, not even in the 31P NMR spectrum. The phosphine removal was confirmed by XRD measurement. The X-ray structure of 2 consists of a distorted tetrahedron with the alkylidene in syn configuration, as presented in Figure 16. The W=O and W=C bond lengths of 1.664(8) Å and 1.875(13) Å are typical of tungsten oxo alkylidene complexes, for example 1.690(1) and 1.895(2) respectively in [W(=O)(=CHtBu)(OHMT)2]38. The W-S1 bond length (2.345(2) Å) is essentially the same as the W-S2 one (2.352(2) Å). Comparing the tungsten bond’s length with all the coordinated ligands, is possible to notice that in complex 1 the values are higher, according to the presence of the phosphine ligand. In such complexes, the opening of each face of the tetrahedron, defined by three “basal” ligands and excluding the fourth “pivotal ligand”, has been used to characterize the ease of access to the metal center43. In this complex the most open face is trans to the alkylidene (339.65° vs 328.4° for an ideal tetrahedron) and approach of olefin on
43 Solans-Monfort, X.; Coperet, C.; Eisenstein, O. J. Am. Chem. Soc., 2010, 132, 7750.
18 that face would not lead to the formation of the metallacycle intermediate, as suspected for such a symmetric complex44.
Figure 16 Molecular structure of 2. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms were omitted for clarity.
Table 3 Selected bonds for (distances are given in Å).
Structural parameters [W(=O)(=CHtBu)(SHMT)2]
W1 – S1 2.345(2)
W1 – S2 2.352(3)
W1 – C1 1.875(13)
W1 – O1 1.664(8)
44 Solans-Monfort, X.; Copéret, C.; Eisenstein, O. Organometallics, 2012, 31, 6812.
19
Table 4 Selected angles for [W(=O)(=CHtBu)(SHMT)2] (given in degrees).
Structural parameters [W(=O)(=CHtBu)(SHMT)2]
S1 – W1 – S2 117.45 (12)
C1 – W1 – S1 107.8 (4)
C1 – W1 – S2 104.6 (4)
O1 – W1 – S1 109.0 (3)
O1 – W1 – S2 112.9 (3)
O1 – W1 – C1 104.1 (5)
2.2 Grafting on SiO
2-700surface
Silica partially dehydroxylated at 700 °C (SiO2-700) contains mainly isolated silanols with a density of ca. 0.8 OH.nm-2. Grafting 1 and 2 on SiO2-(700) afforded the corresponding materials 1@SiO2 and 2@SiO2 (Fig. 17).
Scheme 17 Grafting of 1 and 2 on SiO2-(700) yielding [(≡SiO)W(=O)(=CHtBu)(SHMT)(PMe2Ph)], 1@SiO2 and [(≡SiO)W(=O)(=CHtBu)(SHMT)], 2@SiO2.
Monitoring the grafting step by IR spectroscopy (Fig. 18, 19 and 20) showed a significant decrease of the isolated silanol band at 3747 cm-1, indicating partial consumption of the silanol groups, similarly to what observed for the OHMT
20 complex [(≡SiO)W(=O)(=CHtBu)(OHMT)]45. The broad band observed for both materials around 3650 cm-1 indicated that some surface silanols were interacting with the aromatic ligand of the grafted complex as reported in the OHMT work. The bands observed from 3100 to 2800 and from 1500 to 1350 were associated with C-H stretching and C-H bending respectively. Mass balance analysis by NMR of the released thiol indicated that around 50% and 40% of the surface silanols reacted with 1 and 2 respectively, in agreement with the W content of 1.98% for 1 and 1.92% for 2 determined by elemental analysis. The composition of
[(≡SiO)W(=O)(=CHtBu)(SHMT)(PMe2Ph)], 1@SiO2 and
[(≡SiO)W(=O)(=CHtBu)(SHMT)], 2@SiO2, were confirmed by elemental analysis of C, H, S and P.
Figure 18 FTIR transmission spectra of 1@SiO2.
45 Conley, M. P.; Mougel, V.; Peryshkov, D. V.; Forrest, W. P.; Gajan, D.; Lesage, A.; Emsley, L.;
Copéret, C.; Schrock, R. R. J. Am. Chem. Soc. 2013, 135, 19068.
1500 2000
2500 3000
3500
Wavenumbers (cm-1)
21
Figure 19 FTIR transmission spectra of 2@SiO2.
Figure 20 FTIR transmission spectra of 1@SiO2, 2@SiO2 and SiO2-(700).
1500 2000
2500 3000
3500
Wavenumber (cm-1)
1500 2000
2500 3000
3500
Wavenumber (cm-1) Abs (a.u.)Abs (a.u.)Abs (a.u.)
SiO2-(700)
tBu)(PPhMe2)]
tBu)]
0
0
0
22 The 1H Magic Angle Spinning (MAS) NMR spectra of 1@SiO2 and 2@SiO2 showed distinct resonances at 10.6 ppm and 10.3 ppm respectively, assigned to the alkylidene W=CHtBu signal.
Figure 21 1H MAS NMR spectrum of 1@SiO2.
Figure 22 1H MAS NMR spectrum) of 2@SiO2.
23 At variance to the OHMT complex [(≡SiO)W(=O)(=CHtBu)(OHMT)], where the proton chemical shift of the alkylidene was very similar to its molecular precursor [W(=O)(=CHtBu)(OHMT)2], indicative of similar electronic properties of OHMT and surface siloxy ligands, the alkilidene protons of 1@SiO2 and of 2@SiO2 are significantly shifted of about 1.9 ppm and 1.4 ppm, respectively, with respect to their molecular precursors. This is consistent with a stronger desymmetrization of the complex set upon grafting for the SHMT complexes with respect to their phenoxide analogues.
The 31P MAS NMR spectra of 1@SiO2 revealed a major signal at 8 ppm and a minor (5%) at 22 ppm (Figure 23). While the major signal matches to the signal of the coordinated phosphine in the molecular precursor, the minor signal likely originate from the interaction of the phosphine with surface silanols to afford a P(V) species46.
Figure 23 31P MAS NMR spectrum of 1@SiO2.
46 Samantaray, M. K.; Alauzun, J.; Gajan, D.; Kavitake, S.; Mehdi, A.; Veyre, L.; Lelli, M.; Lesage, A.; Emsley, L.; Copéret, C.; Thieuleux, C. J. Am. Chem. Soc. 2013, 135, 3193.
200 100 0 - 100
31P Chemical Shift (ppm)
24 The 13C Cross Polarization (CP) MAS NMR spectrum of [(≡SiO)W(=O)(=CHtBu)(SHMT)(PMe2Ph)], and [(≡SiO)W(=O)(=CHtBu)(SHMT)], contained signals from SHMT and phosphine (for 1@SiO2). As generally observed for supported tungsten alkylidene complexes at natural abundance, the alkylidene signals were not observed in 13C spectra. However, proton signals of the alkylidene groups combined to the signals at 47 and 31 ppm for 1@SiO2 and 43 and 30 ppm for 2@SiO2 assignable to the β and γ carbon atoms of the neopentylidene group, are clear indications of the presence of the alkylidene moieties in the grafted compounds.
Figure 24 13 C CP MAS NMR spectrum of 1@SiO2. (*: spinning side bonds).
250 200 150 100 50 0
13C Chemical Shift (ppm)
* *
25
Figure 25 13C CP MAS NMR spectrum of 2@SiO2.
2.3 Catalytic tests
To test the catalytic activity of the synthesized complexes, three kind of olefins were selected: internal, terminal olefin and one with an ester group. The activity was evaluated as percentage of conversion vs time. For each kind of olefin the catalytic study of the metathesis reaction gives a different feature. Even if all of them are self-metathesis, the one of cis-4-nonene cannot reach 100% of conversion, but only a dynamic equilibrium, with 50% of conversion (Fig. 26), as well as the ethyl oleate (Fig. 28). Self-metathesis of 1-nonene, instead, can undergo complete conversion into the products because one of them is ethylene, which can be easily removed from the reaction environment to shift the equilibrium to the right (Fig. 27).
13C Chemical Shift (ppm)
26
Fig.26 Self-metathesis of cis-4-nonene.
Fig.27 Self-metathesis of 1-nonene.
Fig.28 Self-metathesis of ethyl oleate.
27 Each self-metathesis reaction has its proper mechanism and products, but in particular 1-nonene and ethyl oleate metathesis are worthy to note because, contrary to cis-4-nonene, they can lead more easily to the catalyst deactivation.
Since 1-nonene homocoupling produces ethylene, this can react with the tungsten complex forming very stable unsubstitutes metallacycle complexes that would be inactive with respect to metathesis reaction. Deactivation from ethyl oleate originates from a different cause: coordination or reaction of the functional groups of ethyl oleate itself with the tungsten complex can cause the decomposition of the tungsten catalyst.
To perform the catalytic tests, at t=0 the solutions of the alkene in toluene containing the internal standard were added to the catalyst introduced in a conical base vial containing a wing shaped magnetic stirred, and the reaction mixture was stirred and kept at 30 °C using an aluminum heating block. Aliquots of the solution were sampled, diluted with pure toluene and quenched by the addition of wet ethyl acetate. The resulting solution was analyzed by GC/FID. Conversion and EZ selectivity are shown by [eq.1,2,3].
𝑃𝑟𝑜𝑑𝑢𝑐𝑡 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛𝑡 = ∑[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]𝑡 [𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒]𝑖𝑛𝑖
(1)
𝑍 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 = Σ[𝑍 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]𝑡 Σ[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]𝑡
(2)
𝐸 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 = Σ[𝐸 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]𝑡 Σ[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]𝑡
(3)
28 The other important parameter, with which the catalytic activity is evaluated, is the Turn Over Number (TON). It is the number of moles of substrate that a mole of catalyst (for heterogeneous catalysis number of active sites) can convert before becoming inactivated. The term Turn Over Frequency (TOF) is used to refer to the turnover per time unit, which is generally used for comparing different catalysts activity.
2.3.1 Cis-4-nonene test
The catalytic activity in alkene metathesis of the molecular and grafted complexes was evaluated with cis-4-nonene olefin (used as a prototypical substrate), (Table 6). The two molecular catalysts show very low activity in the conditions tested:
compound 1 was inactive47,28 and only 6% conversion was observed after 24 h with 2 (Fig. 29). Dramatic differences were observed with the grafted catalysts. While 1@SiO2 showed little activity with only 34% conversion in 24 h, 2@SiO2 reached equilibrium conversion in less than 10 minutes, with an initial Turn Over Frequency (TOF, determined at 3 min.) of 138 min-1. At lower catalyst loading (0.02 mol%) equilibrium conversion is reached in about 2 h.
47 In agreement with Schrock and coworkers who reported very low conversion of 18% after 24h at 1 mol% loading in 1-octene metathesis (see reference 33 for details).
29
Catalyst mol% TOF3mina Time to equilibrium conversion
1 0.1 0 No conversion after 24h
2 0.1 < 0.1 6% conversion after 24h 1@SiO2 0.1 < 1 34% conversion after 24h 2@SiO2 0.1 138 (41%) 10 min.
2@SiO2 0.02 85 (5%) 2 h
[a] TOF at 3 min, given in min-1 with the corresponding conversions given in brackets.
Table 6. Homocoupling of cis-4-nonene (toluene, 30 °C).
Substrate mol% TOF3mina Time to equilibrium
conversion (SiO)W(=O)(=CHCMe2Ph)(dAdPO) 0.02 325 (22%) 10 min
0.005 356 (5%) 5 h (≡SiO)W(=O)(=CHtBu)(OHMT)2 0.1 170b 3 min (500)
0.02 280 (17%) <60 min (2500)
[a] Turnover frequency (TOF) at 3 min, given in min1 with the corresponding conversions given in brackets. [b] calculated at equilibrium
Tab. 7 Catalytic activity for 4-cis-nonene homocoupling, of 2@SiO2 oxo analogues.
Figure 29 Metathesis of cis-4-nonene by the molecular precursors [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)]
(black squares), [W(=O)(=CHtBu)(SHMT)2] (blue diamonds).
0%
5%
10%
15%
20%
25%
30%
0 50 100 150 200 250 300
Conversion of non-4-ene/ %
Time / min
30
Figure 30 Metathesis of cis-4-nonene by complexes 1@SiO2 (blue diamonds), 2@SiO2 (black squares) 0.1 mol%, 30 °C.
Figure 31 E/Z selectivity vs. time plot for the self-metathesis of cis-4-nonene by 2@SiO2 (1000 equivalents/W). E (white circles), Z (black diamonds).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0 5 10 15 20 25 30 35
EZ Selectivity %
Min
31
Figure 32 Metathesis of cis-4-nonene by complex 2@SiO2 (0.02%, 30 °C).
Cromatogram of Cis-4-nonene test of 2@SiO2at 0.1% mol, at 3 min.
Analyte Retention time
Heptane 2.66
Trans-4-octene 5.27
Cis-4-octene 5.38
Trans-4-nonene 10.50
Cis-4-nonene 10.58
Cis-5-decene 18.18
Trans-5-decene 18.26
0%
10%
20%
30%
40%
50%
0 50 100 150 200 250
Conversion of non-4-ene/ %
Time / min
32 Cromatograms of Cis-4-nonene test of 2@SiO2at 0.02% mol, at 3 min and equilibrium.
Analyte Retention time
Heptane 2.66
Trans-4-octene 5.27
Cis-4-octene 5.38
Trans-4-nonene 10.50
Cis-4-nonene 10.58
Cis-5-decene 18.18
Trans-5-decene 18.26
33 2.3.2 Ethyl oleate and 1-nonene test
The complex 2@SiO2 also metathesizes ethyl oleate, at 0.1 mol% catalyst loading with initial TOF of 2 min-1, reaching equilibrium conversion in 36 h (Table 2). Worthy of note, 2@SiO2 quantitatively converts 1-nonene in less than 5 h with initial TOF of about 27 min-1, in contrast to [(≡SiO)W(=O)(=CHtBu)(OHMT)] which deactivated very fast (max. conv. = 66% after 24 h) and presented surprisingly low initial TOF of 13 min-1 under the same reaction conditions. This high activity and stability allowed us to decrease catalyst loading to 0.02 mol%, reaching full conversion in about 48 h without the need to continuously remove ethylene.
Substrate mol% TOF3mina Time to equilibrium conversion
ethyl oleate 0.1 2 (0.6%) 36 h
1-nonene 0.1 27 (9%) 5 h (>93% conversionb)
1-nonene 0.02 37 (4%) 48 h (>93% conversionb)
[a] TOF at 3 min, given in min-1 with the corresponding conversions given in brackets. [b] Under the condition employed (closed vial only opened for sampling), full conversion could not be observed.
Table 8. Catalytic activity of 2@SiO2 in toluene, 30 °C.
Substrate mol
%
TOF3mina Time to equilibrium conversionc
(≡SiO)W(=O)(=CHtBu)(OHMT)2 0.1 13 (4%) 66% after 8h (660)
(SiO)W(=O)(=CHCMe2Ph)(dAdPO) 0.1 5 (2%) 42% conversion after 24h
[a] Turnover frequency (TOF) at 3 min, given in min1 with the corresponding conversions given in brackets. [c] Ton given in parenthesis.
Table 9: Catalytic activity for 1-nonene homocoupling, of 2@SiO2 oxo analogues.
34
Substrate mol
%
TOF3mina Time to equilibrium conversionb
(≡SiO)W(=O)(=CHtBu)(OHMT)2 0.2 44 (29%) <3h (250)
0.05 143 (19%) 46% after 4h (920) (SiO)W(=O)(=CHCMe2Ph)(dAdPO) 0.05 133 (15%) ca. 6h
[a] Turnover frequency (TOF) at 3 min, given in min1 with the corresponding conversions given in brackets. [b] Ton given in parenthesis.
Table 10: Catalytic activity for ethyl oleate homocoupling, of 2@SiO2 oxo analogues.
Figure 33 Metathesis of 1-nonene by complex 2@SiO2 (0.1 mol %, 30 °C).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 100 200 300 400 500
Conversion of 1-nonene/ %
Time / min
35
Figure 34 E/Z selectivity vs. time plot for the self-metathesis of 1-nonene by 2@SiO2 (1000 equivalents/W).
E (white circles), Z (black diamonds).
Figure 35 Metathesis of 1-nonene by complex 2@SiO2 (0.02 mol %, 30 °C).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0 100 200 300 400 500
EZ selectivity %
Min
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 1000 2000 3000 4000 5000
Conversion of 1-nonene/ %
Time / min
36 Cromatograms of 1-nonene test for 2@SiO2 at 0.1% mol, at 3 min, 50% of conversion and equilibrium.
Analyte Retention time (min)
Heptane 2.66
1-nonene 10.18
Cis-8-hexadecene 28.38
Trans-8-hexadecene 28.44
37 Cromatograms of 1-Nonene test for 2@SiO2 at 0.02% mol, at 3 min, 50% of conversion and equilibrium.
Analyte Retention time
Heptane 2.66
1-nonene 10.18
Cis-8-hexadecene 28.38
Trans-8-hexadecene 28.44
38
Figure 36 E/Z selectivity vs. time plot for the self-metathesis of 1-nonene by 2@SiO2 (5000 equivalents/W).
E (white circles), Z (black diamonds).
Figure 37 Metathesis of Ethyl Oleate by complex 2@SiO2 (0.1 mol %, 30 °C).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0 100 200 300 400 500
EZ selectivity %
Min
0%
10%
20%
30%
40%
50%
60%
0 500 1000 1500 2000 2500 3000
Conversion/ %
Time / min
39 Cromatograms for Ethyl oleate test of 2@SiO2 at 0.1% mol at 3 min and equilibrium.
Analyte Retention time
Octadecane 6.739
Octadecene 6.818
Trans-9-ehtyloctadecenoate 12.210
Ethyloleate 12.280
Trans-9-diethyloctadecendioate 15.789
Cis-9-diethyloctadecendioate 15.861
40 Reaction with 13C labeled Ethylene
The high activity of 2@SiO2 towards 1-alkene lead us to investigate its reaction with ethylene. Following the procedure developed in the group where this Thesis has been performed24,48,49, the aim was to explore the formation of the metallacyclobutane. This kind of study is relevant in order to understand the unprecedented activity of a tungsten oxo alkilydene complex with terminal olefins.
Therefore, exposing a catalyst to 13C-labeled would allow to monitor the formation of the metallacyclobutane complex with characterization by solid state NMR and study of its catalytic activity. To perform the reaction, the catalyst 2@SiO2 was loaded in a glass reactor and evacuated under high vacuum (10-5 mbar) and 15 equivalents of 13C-labeled ethylene were vacuum transferred on the grafted complex at -196 °C. The volatiles were vacuum transferred and collected for analysis by GC-FID.
The analysis indicated the formation of trace amounts of 3,3-dimethylbutene. The solid was analyzed by magic angle spinning solid-state proton and carbon NMR within one day after exposure to ethylene, showing same spectrum than the starting material 2@SiO2. Therefore, in contrast to all of the supported tungsten neopentylidene metathesis catalysts investigated so far, no mono-13C-labeled 3,3- dimethylpropene was released when 2@SiO2 was contacted with 15 equivalents of 13C-labeled)ethylene, and the solid state NMR of the catalyst after exposure was identical to the parent compound. It is noteworthy that no signals from methylidene and/or metallacyclobutane species were observable.
48 Blanc, F.; Berthoud, R.; Copéret, C.; Lesage, A.; Emsley, L.; Singh, R.; Kreickmann, T.; Schrock, R. R. Proc. Nat. Acad. Sci. U.S.A. 2008, 105, 12123.
49 Mougel, V.; Coperet, C. Chem. Sci. 2014, 5, 2475.
41 In addition, the catalytic activity of this compound in cis-4-nonene metathesis was unchanged to prior to exposure to ethylene (similar initial TOF and time to equilibrium, Table 11). Under the same conditions, phenoxide analogues [(≡SiO)W(=O)(=CHtBu)(OAr)] (OAr = OHMT but also dAdPO24), studied in the group where this Thesis has been performed, quantitatively reacted with 13C ethylene to afford a SP–metallacycle as the major surface species and the compound formed by exposure to ethylene does significantly reduced activity towards cis-4-nonene. This observation allows us to conclude that the formation of unsubstituted metallacycles in presence of ethylene is much less favored for 2@SiO2 than for its phenoxide analogue, and could explain its highest activity with 1-alkenes.
Fig. 38 Structures of 2@SiO2 and its phenoxide analogues.
42
Figure 39 Metathesis of cis-4-nonene before (blue diamonds) and after (white squares) contact with 15 equivalents of 13C labeled ethylene on complex 2@SiO2 according to the procedure described above (0.1
mol%, 30 °C).
OR TOF (min-1) Time to equilibrium
2@SiO2 138 10 min
2@SiO2 after ethylene exposure 129 10 min
[(≡SiO)W(=O)(=CHtBu)(OHMT)] 13 66% after 8h
Table 11 Metathesis of cis-4-nonene before and after contact with 15 equivalents of 13C labeled ethylene on complexes according to the procedure described above (0.1 mol%, 30 °C). OHMT = 2,6-dimesitylphenoxide,
dAdPO = 2,6-diadamantyl-4-methylphenoxide.
43
3. Conclusions
Tungsten oxo alkylidene complexes bearing bulky thiophenoxide ligands [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)] and [W(=O)(=CHtBu)(SHMT)2] (SHMT = 2,6- dimesitylthiophenoxide) were synthesized and grafted on partially dehydroxylated silica. While the molecular precursors show no significant catalytic activity, the grafted analogues are efficient catalysts for alkene metathesis of internal olefin.
The grafted and phosphine free surface complex [(≡SiO)W(=O)(=CHtBu)(SHMT)]
showed unprecedented activity in the metathesis of terminal olefins. This reaction was proved challenging even with state of the art supported tungsten oxo alkylidene catalysts due to their propensity to form stable unsubstituted metallacycle in presence of ethylene. This high activity could be attributed to the lower stability of unsubstituted metallacycles formed with ethylene.
Desymmetrization of the ligand set of tungsten oxo alkylidene catalysts has proven effective for generating efficient 1-alkene metathesis catalysts, probably because it disfavors the formation of very stable metallacycles. These results highlight the fact that a fine tuning of the structure and electronic properties of supported alkene metathesis catalysts allows tailoring the catalyst to its potential application, as exemplified here for 1-alkene metathesis.
This thesis work led to a publication entitled “Strongly σ Donating Thiophenoxide in Silica-Supported Tungsten Oxo Catalysts for Improved 1‑Alkene Metathesis Efficiency” Organometallics 2015, 34, 551−554. (Victor Mougel, † Margherita Pucino,† and Christophe Copéret*).
44
4. Experimental part
4.1 General considerations
32,50All experiments were carried out under dry and oxygen free argon atmosphere using either standard Schlenk or glove-box techniques for molecular precursors.
For the supported complexes, reactions were carried out using high vacuum lines (10-5 mBar) and glove-box techniques. Pentane and toluene were purified using double MBraun SPS alumina column, and were degassed by three freeze-pump- thaw cycles before being used. Benzene, heptane and benzene-[D]6 were distilled from Na/Benzophenone and degassed by three freeze-pump-thaw cycles.
Octadecane was distilled under partial pressure. 1-nonene and cis-4-nonene were distilled from Na, degassed by three freeze-pump-thaw cycles and stored 5 hours over activated Selexsorb CD®. Ethyl Oleate was purified by distillation followed by 5 hours contact over activated Selexsorb CD®. Silica (Aerosil Degussa, 200 m2g-1) was compacted with distilled water, calcined at 500 °C under air for 4 h and treated under vacuum (10-5 mbar) at 500 °C for 12 h and then at 700 °C for 12 h (support referred to as SiO2-(700)) and contained 0.26 mmol of OH per g as measured by titration with MeMgCl51. All infrared (IR) spectra were recorded using a Bruker FT- IR Alpha spectrometer placed in the glovebox, equipped with OPUS software.
Spectral range 275-7500, resolution < 2cm-1, RockSolid interferometer, DTGS (triglycine sulfate) detector, SiC globar source, solid samples were investigated in a magnetic pellet holder. A typical experiment consisted in the measurement of transmission in 32 scans in the region from 4000 to 400 cm-1. The solutions 1H and
50 J. Elison, K. Ruhlandt-Senge, P. Power, Angew. Chem. Int. Ed., 1994, 33, 1178.
51 A. J. Rossini, A. Zagdoun, M. Lelli, D. Gajan, C. Copéret, A. Lesage, L. Emsley et al., Chem.
Sci., 2012, 3, 108.
45
13C-NMR spectra were obtained on Bruker DRX 300, DRX 250 or DRX 500 spectrometers. The solution spectra were recorded in benzene-[D]6 at room temperature. The 1H and 13C chemical shifts are referenced relative to the residual solvent peak. Solid-state NMR spectra were recorded under MAS on Bruker Advance III 400 and 700 MHz spectrometers with a conventional triple resonance 4 mm CP-MAS probe. The samples were introduced in zirconia rotors in the glovebox and tightly closed. Compounds [W(=O)(=CHtBu)Cl2(PMe2Ph)2]1, and 2,4,6-triisopropylthiophenol (SHMT)2 were synthesized according to literature procedures.
46 4.2 Synthesis of 2,6-dimesitylthiophenolate KSHMT.
A solution of the 2,6-dimesitylphenilthiol (HSHMT) (0.823 mg, 2.37 mmol, 1 equiv.) in toluene (20 mL) was added dropwise to a suspension of KH (93 mg, 2.33 mmol, 0,67 mmol, 0.98 equiv.) in cold toluene (70 mL, -78 °C), releasing H2. The reaction mixture was stirred for 2 h, during which time white precipitate formed. The solid was filtered and washed with cold toluene (2*10 mL, -40 °C). The filtrate was concentrated and stored at -40 °C overnight to yield a second crop which was collected and washed similarly (260 g, 0.67mmol, 98% combined yield). 1H NMR (300MHz, C6D6) δ (ppm): 2.11 (s, 4H), 2.16 (s, 6H), 2.23 (s, 8H) orto and para CH3, 6.75 (overlapping, m, aryl), 7.55 (s, 1H).
4.3 Synthesis of [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)] ,1.
A yellow solution of [W(=O)(=CHtBu)Cl2(PMe2Ph)2] (0.424 g, 0.68 mmol) in cold toluene (25 mL, -30 °C) was added to a cold suspension (-30 °C) of potassium 2,6-dimesitylthiophenolate (0.635 g, 1.65 mmol, 2.4 equiv.) in toluene (25 ml). The mixture was allowed to warm at room temperature and stirred for 3.5 h, to afford a yellow solution. The solution was taken to dryness in vacuo and the yellow solid obtained was extracted in pentane (6*4 ml) to afford after filtration on Celite® a dark yellow solution. After concentration to 1/3 of its volume and cooling at -40°, 527 mg of yellow crystals were collected in two crops (70% yield). 1H NMR (300MHz, C6D6) δ (ppm): 8.94 (s, 1H, CHCMe3, JWH = 4Hz), 7.34 (br, 2H, aryl), 7.13 (m, 2H, aryl), 6.85 (m, 14H, aryl) 2.27 (s, 12H, HMT para CH3), 2.08 (br s, 24H, HMT orto CH3), 1.09 (s, 6H, PMe2), 1.02 (s, 9H, CHCMe3). 13C NMR (300MHz, C6D6) δ (ppm):
284.40, (CHCMe3), 144.37, 138.94, 136.73, 45.15, 30.37, 21.60, 14.53. 31P NMR (300MHz, RT, C6D6) δ (ppm): 8.19, -1.2, - 46.46. Elemental Analysis Found
47 (calculated for - formula C H P S W) W 16.3 % (expected 16.7 %), C 66.33 % (expected 66.66 %), H 6.55 % (expected 6.51 %), S 5.39 % (expected 5.83 %), P 2.69 % (expected 2.82 %).
4.4 Synthesis of [W(=O)(=CHtBu)(SHMT)2], 2.
B(C6F5) (63.2 mg, 0.123 mmol) was added portionwise to a solution of [W(=O)(=CHtBu)(SHMT)2(PMe2Ph)] in 10 ml of toluene (135.6 mg, 0.123 mmol, 1 equiv.). The mixture was stirred for 1.5 h, during which time the color gradually darkened. The solution was taken to dryness to afford a brown oil. Upon pentane addition (12 mL) an off-white precipitate formed. The mixture was filtered through Celite® leaving a transparent dark yellow solution. The solution left standing at -40
°C overnight, resulting in the formation of precipitate and few colorless crystals of ((C6F5)B.P(PhMe2)). Cold filtration of this suspension afforded a yellow solution.
After concentration of the solution to 1/4 and storage at -40 °C, large amount of yellow crystals formed. These crystals were filtered, washed with cold pentane and recrystallized following same procedure to afford 101 mg of the title compound as dark yellow crystals suitable for XRD analysis (0,105 mmol, 85 % yield). 1H NMR (300MHz, C6D6) δ (ppm): 8.90 (s, 1, CHCMe3, JWH = 4 Hz), 6.85 (m, 14 H, Aryl), 2.25 (s, 12 H, para CMe3), 2.18 (s, 3H), 2.12 ( s, 12, para HMT CH3), 2.10 (s 12 H, orto HMT CH3), 1.05 (s, 9, CHCMe3). 13C NMR (500MHz, C6D6) δ (ppm): 284.28 (CHCMe3), 144.26, 138.79, 137.0, 136.64, 135.77, 45.24, 30.53, 21.75, 21.56, 21.29, 20.24 (CMe3). Despite analytically pure by NMR, no satisfactory elemental analysis of this compound could be obtained, likely due to the thermal sensitivity of the complex.
