**4.1. Supported tungsten complexes with oxo and hydride ligands**

The first carbynic complex of tungsten(VI), [W(≡C*t*Bu)(CH<sup>2</sup> tBu)<sup>3</sup> ], was synthesized in 1978 by Clark and Schrock [62]. Later, various complexes of the same type were synthesized, for example, [W(≡C*t*Bu)X<sup>3</sup> ] (X = Cl, O*t*Bu, Ni Pr<sup>2</sup> ) [63, 64]. Some of these complexes (mainly those with a pronounced electrophilic character such as [W(≡C*t*Bu)(O*t*Bu)<sup>3</sup> ]) displayed a moderate activity in alkynes metathesis. Unfortunately, these systems deactivated rapidly by a bimolecular reaction leading to a dinuclear tungsten complex with a W≡W triple bond [65, 66]. In order to avoid this deactivation, these complexes were heterogeneized. [W(≡C*t*Bu)X<sup>3</sup> ] (X = Cl, O*t*Bu, CH<sup>2</sup> *t*Bu) was grafted on silica partially dehydroxylated at 500°C. Weiss et al. proposed that there was formation of the carbenic species [(≡SiO)W(═C*t*Bu)X<sup>3</sup> ] (X = Cl, *t*Bu) by addition of the Si─OH bond of silica on the carbyne bond [67]. The carbenic ligand was evidenced by its reactivity with acetone via a pseudo-Wittig reaction and indirectly by the catalytic activity in olefin metathesis. [W(≡C*t*Bu) (CH<sup>2</sup> *t*Bu)<sup>3</sup> ] was then grafted on silica, dehydroxylated at 200°C (SiO2–200) and at 700°C (SiO2–700). When silica was treated at 700°C the main reaction product was [(≡SiO)W(≡C*t*Bu)(CH<sup>2</sup> *t*Bu)<sup>2</sup> ] **18**, while [(≡SiO)<sup>2</sup> W(≡C*t*Bu)(CH<sup>2</sup> *t*Bu)] **19** was formed on silica treated at 200°C (**Figure 15**) [68].

**Figure 15.** Species formed during the reaction of [W(≡C*t*Bu)(CH<sup>2</sup> *t*Bu)<sup>3</sup> ] with SiO2–200 and SiO2–700.

• The metal complexes have a limited mobility on the surface, avoiding the bimolecular de-

• The catalysts can be characterized easily by use of spectroscopic methods, as all species are

• The good knowledge of the structure of the active site allows to propose a reasonable cata-

A lot of organometallic complexes of groups 4–8 were grafted on a variety of surfaces such as amorphous inorganic oxides [55], zeolites [59], or metals [60, 61]. This methodology led to numerous applications in fine chemistry and/or petrochemistry including reactions which were not known up to now. This is mainly due to a combination of organometallic synthesis and surface science. The catalytic efficiency of the materials prepared by this way depends on the coordination sphere around the metal, on the number, and the character (ionic or covalent) of the bonds with the support and on the nature of the oxide support (silica, alumina,

In the case of tungsten SOMC, the choices of the organometallic precursor and of the support are mainly dependent on the expected catalytic reaction and on the intermediates involved in the postulated catalytic cycle. The high oxidation state of tungsten (VI) allows the possibility of a number of ligands in the coordination sphere leading to both spectators and reactive species in the catalytic cycle. The reactive species will be hydrides, alkyl, carbenes, and carbynes. During the last few years, many studies were made with such surface complexes in olefin metathesis. We will review here only those containing the oxo ligand as they could be considered as models of the industrial heterogeneous catalysts. There are two principal methodologies which have been developed to achieve well-defined tungsten oxo species on oxide: (i) grafting of a reactive tungsten carbyne complex followed by transfer of oxygen from the

Clark and Schrock [62]. Later, various complexes of the same type were synthesized, for exam-

alkynes metathesis. Unfortunately, these systems deactivated rapidly by a bimolecular reaction leading to a dinuclear tungsten complex with a W≡W triple bond [65, 66]. In order to avoid this

grafted on silica partially dehydroxylated at 500°C. Weiss et al. proposed that there was formation

on the carbyne bond [67]. The carbenic ligand was evidenced by its reactivity with acetone via a pseudo-Wittig reaction and indirectly by the catalytic activity in olefin metathesis. [W(≡C*t*Bu)

When silica was treated at 700°C the main reaction product was [(≡SiO)W(≡C*t*Bu)(CH<sup>2</sup>

] was then grafted on silica, dehydroxylated at 200°C (SiO2–200) and at 700°C (SiO2–700).

*t*Bu)] **19** was formed on silica treated at 200°C (**Figure 15**) [68].

tBu)<sup>3</sup>

) [63, 64]. Some of these complexes (mainly those with a

] (X = Cl, *t*Bu) by addition of the Si─OH bond of silica

], was synthesized in 1978 by

*t*Bu) was

*t*Bu)<sup>2</sup> ] **18**,

]) displayed a moderate activity in

] (X = Cl, O*t*Bu, CH<sup>2</sup>

support and (ii) grafting of an organometallic complex bearing oxo ligand.

Pr<sup>2</sup>

**4.1. Supported tungsten complexes with oxo and hydride ligands**

The first carbynic complex of tungsten(VI), [W(≡C*t*Bu)(CH<sup>2</sup>

pronounced electrophilic character such as [W(≡C*t*Bu)(O*t*Bu)<sup>3</sup>

deactivation, these complexes were heterogeneized. [W(≡C*t*Bu)X<sup>3</sup>

] (X = Cl, O*t*Bu, Ni

of the carbenic species [(≡SiO)W(═C*t*Bu)X<sup>3</sup>

W(≡C*t*Bu)(CH<sup>2</sup>

composition reactions which are often observed in homogeneous catalysis [58].

lytic cycle and to determine how deactivation and regeneration will proceed.

identical.

78 Alkenes

silica-alumina, etc.).

ple, [W(≡C*t*Bu)X<sup>3</sup>

(CH<sup>2</sup>

*t*Bu)<sup>3</sup>

while [(≡SiO)<sup>2</sup>

The structures of species **18** and **19** were confirmed by solid-state NMR (<sup>1</sup> H, <sup>13</sup>C, HETCOR, J-resolved). The interaction with the silica surface was studied by <sup>17</sup>O MAS NMR by using enriched silica [69]. This study showed the existence of interactions between protons of residual hydroxyl groups and the alkyl ligands of the supported species.

Species **18** shows a good activity in propene metathesis (initial TOF 5.5 min−1, TON = 11,000 after 40 h) [68]. Two mechanisms were proposed explaining the formation of the carbenic ligand. The first one is a *α*-H transfer from the alkyl ligand to the carbyne during the coordination of the olefin and formation of a bis-alkylidene complex [70]. The other possibility is to form directly the carbene by metathesis between the olefin and the carbyne: a metallacyclobutene is formed which decomposes into a carbene-alkenyl tungsten complex (**Figure 16**).

**Figure 16.** Possible mechanisms of formation of a carbene from the surface carbyne.

involves three steps: dehydrogenation of the alkane, olefin metathesis, and hydrogenation of the resulting olefin. This example shows that the support can have a nonnegligible effect on the catalysis. Treatment under hydrogen at 150°C of the complexes obtained on silica and alumina leads also to completely different species. On silica sintering is observed and TEM shows that tungsten particles (size 0.1–0.2 nm) are formed [71]. On alumina no sintering is observed and a hydride species is obtained [71, 73]. This hydride species is characterized by a small peak at 10.0 ppm in

H MAS NMR and by two infrared bands at 1903 and 1804 cm−1. The attribution of the two infrared bands was confirmed by isotopic exchange W─H/W─D upon addition of deuterium [71]. In addition, EXAFS showed the presence of a W═O double bond [74]. All these data combined to DFT calculations allowed to propose that the surface hydride is a tris-hydride oxo tungsten(VI) complex stabilized by coordination of the oxo ligand to one surface aluminum (**Figure 17**) [75]. The mechanism of formation of this surface species was elucidated by use of DFT calculations. These calculations suggested that the oxo species is formed by reaction of an unstable tungsten hydride species with one oxygen atom of the alumina surface. Such a phenomenon is prohibited on silica surfaces due to the stability of the Si─O bond. This oxo-hydride tungsten is more

to propene. For example, at 120°C the TON can reach 9000 and at 150°C 16,000 after 48 h. At 200°C the TON increases to 22,000 but a rapid deactivation of the catalyst is observed [76].

a good selectivity to propene [77], the tungsten hydride on alumina is very selective (more than 98%) even for ethylene/butene ratios lower than 1. From a mechanistic point of view, the initiation step occurs via the insertion of three ethylene molecules in the W─H bonds, leading to a tris-ethyl tungsten surface complex (the insertion of ethylene is more favorable from both thermodynamic and kinetic points of view than that of but-2-ene [78, 79]). The next step is the elimination of ethane (which can be detected by gas chromatography) by a *α*-H abstraction,

More interestingly, this system is also active for the direct conversion of ethylene to propene with a very good selectivity (more than 95%). The TON can reach 1120 after 120 h [80]. The

**Figure 18.** Formation of the active species from tungsten hydride and catalytic cycle for ethylene/but-2-ene cross-

catalyst for the cross-metathesis of ethylene and but-2-ene

/SiO<sup>2</sup>

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81

catalyst to achieve

1

metathesis.

active than the industrial WO<sup>3</sup>

/SiO<sup>2</sup>

While an excess of ethylene is needed in the case of the industrial WO<sup>3</sup>

leading to the active ethyl-ethylidene oxo tungsten complex (**Figure 18**) [76].

**Figure 17.** Mechanism proposed for the formation of the oxo hydride tungsten species on alumina.

The grafting reaction of [W(≡C*t*Bu)(CH<sup>2</sup> *t*Bu)<sup>3</sup> ] was also studied on alumina dehydroxylated at 500°C. Alumina is a complex support as aluminum can be tetra-, penta-, or hexacoordinated and its surface hydroxyl groups can be bound to one, two, or three aluminum atoms. As a result, there is a great variety of hydroxyl groups with variable acidity. The determination of the surface complexes obtained after grafting needed the use of a variety of experimental (DRIFT, solid-state NMR, EXAFS) and theoretical (DFT calculations) methods [71]. The evolved gas (one neopentane per grafted tungsten) and microanalysis were in agreement with the formation of a complex having only one covalent bond with the surface, [(Als O) W(≡C*t*Bu)(CH<sup>2</sup> *t*Bu)<sup>2</sup> ] **20**. The infrared study showed that there is a partial consumption of the hydroxyl groups and that only those linked to one tetrahedral aluminum have reacted [72]. The <sup>13</sup>C CP-MAS NMR spectrum of [(Als O)W(≡C*t*Bu)(CH<sup>2</sup> *t*Bu)<sup>2</sup> ] shows only a broad signal between 50 and 110 ppm for the W─CH<sup>2</sup> ─carbon atoms. DFT calculations show that this broadening is due to an interaction between these methylene groups and the residual surface aluminum groups.

In contrast to complexes **18** and **19** formed on silica, complex **20** has a good activity in propane metathesis at 150°C with an initial TOF equal to 1.8 h−1 [73]. The mechanism of this reaction involves three steps: dehydrogenation of the alkane, olefin metathesis, and hydrogenation of the resulting olefin. This example shows that the support can have a nonnegligible effect on the catalysis. Treatment under hydrogen at 150°C of the complexes obtained on silica and alumina leads also to completely different species. On silica sintering is observed and TEM shows that tungsten particles (size 0.1–0.2 nm) are formed [71]. On alumina no sintering is observed and a hydride species is obtained [71, 73]. This hydride species is characterized by a small peak at 10.0 ppm in 1 H MAS NMR and by two infrared bands at 1903 and 1804 cm−1. The attribution of the two infrared bands was confirmed by isotopic exchange W─H/W─D upon addition of deuterium [71]. In addition, EXAFS showed the presence of a W═O double bond [74]. All these data combined to DFT calculations allowed to propose that the surface hydride is a tris-hydride oxo tungsten(VI) complex stabilized by coordination of the oxo ligand to one surface aluminum (**Figure 17**) [75].

The mechanism of formation of this surface species was elucidated by use of DFT calculations. These calculations suggested that the oxo species is formed by reaction of an unstable tungsten hydride species with one oxygen atom of the alumina surface. Such a phenomenon is prohibited on silica surfaces due to the stability of the Si─O bond. This oxo-hydride tungsten is more active than the industrial WO<sup>3</sup> /SiO<sup>2</sup> catalyst for the cross-metathesis of ethylene and but-2-ene to propene. For example, at 120°C the TON can reach 9000 and at 150°C 16,000 after 48 h. At 200°C the TON increases to 22,000 but a rapid deactivation of the catalyst is observed [76].

While an excess of ethylene is needed in the case of the industrial WO<sup>3</sup> /SiO<sup>2</sup> catalyst to achieve a good selectivity to propene [77], the tungsten hydride on alumina is very selective (more than 98%) even for ethylene/butene ratios lower than 1. From a mechanistic point of view, the initiation step occurs via the insertion of three ethylene molecules in the W─H bonds, leading to a tris-ethyl tungsten surface complex (the insertion of ethylene is more favorable from both thermodynamic and kinetic points of view than that of but-2-ene [78, 79]). The next step is the elimination of ethane (which can be detected by gas chromatography) by a *α*-H abstraction, leading to the active ethyl-ethylidene oxo tungsten complex (**Figure 18**) [76].

More interestingly, this system is also active for the direct conversion of ethylene to propene with a very good selectivity (more than 95%). The TON can reach 1120 after 120 h [80]. The

The grafting reaction of [W(≡C*t*Bu)(CH<sup>2</sup>

*t*Bu)<sup>2</sup>

The <sup>13</sup>C CP-MAS NMR spectrum of [(Als

between 50 and 110 ppm for the W─CH<sup>2</sup>

W(≡C*t*Bu)(CH<sup>2</sup>

80 Alkenes

aluminum groups.

*t*Bu)<sup>3</sup>

**Figure 17.** Mechanism proposed for the formation of the oxo hydride tungsten species on alumina.

500°C. Alumina is a complex support as aluminum can be tetra-, penta-, or hexacoordinated and its surface hydroxyl groups can be bound to one, two, or three aluminum atoms. As a result, there is a great variety of hydroxyl groups with variable acidity. The determination of the surface complexes obtained after grafting needed the use of a variety of experimental (DRIFT, solid-state NMR, EXAFS) and theoretical (DFT calculations) methods [71]. The evolved gas (one neopentane per grafted tungsten) and microanalysis were in agreement with the formation of a complex having only one covalent bond with the surface, [(Als

hydroxyl groups and that only those linked to one tetrahedral aluminum have reacted [72].

broadening is due to an interaction between these methylene groups and the residual surface

In contrast to complexes **18** and **19** formed on silica, complex **20** has a good activity in propane metathesis at 150°C with an initial TOF equal to 1.8 h−1 [73]. The mechanism of this reaction

O)W(≡C*t*Bu)(CH<sup>2</sup>

] **20**. The infrared study showed that there is a partial consumption of the

*t*Bu)<sup>2</sup>

─carbon atoms. DFT calculations show that this

] was also studied on alumina dehydroxylated at

O)

] shows only a broad signal

**Figure 18.** Formation of the active species from tungsten hydride and catalytic cycle for ethylene/but-2-ene crossmetathesis.

three neopentyl groups are located in equatorial position (**Figure 20**). These results were also

Complex **22** was used in propene metathesis at 80°C. The evolution of one neopentane molecule per grafted complex and the detection of the olefin metathesis products (ethylene and but-2-ene) are in agreement with the formation of the [(≡SiO)(W═O)(CH<sup>2</sup>

(═CH*t*Bu)] surface complex (**Figure 21**). This species is formed *in situ* by coordination of propene to tungsten (which is electro-deficient), leading to a hexacoordinated complex with a great steric hindrance which will favor the *α*-H abstraction. DFT calculations show that the formation of the carbene species is thermodynamically favored, the energy barrier

The catalytic performances of **22** were compared to those of the supported imido-carbene

Complex **23** deactivates rapidly (**Figure 22a**) while complex **22** remains stable. As a result, after 95 h the TON is 22,000 for **22** and ca. 2,500 for **23** [83]. In terms of selectivity, complex **22** is stable with equimolar amounts of ethylene and but-2-ene while in the case of compound **23**

In the case of the imido complex, the deactivation mechanism has been determined: It is due to the decomposition of the 2-methyltungstocyclobutane by *β*-H transfer, leading to a

−1.min−1 for **22**. The main difference is the evolution with time of stream:

*t*Bu)(═CH*t*Bu)] **23** in propene metathesis. The ini-

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H3 )(CH<sup>2</sup>

**Figure 20.** Grafting reaction of complex **21** on dehydroxylated silica.

**Figure 21.** Mechanism of formation of the surface oxo-carbene species.

tial activities are quite similar for the two surface species: 4.5 molC3H6.molW

isobutene and higher olefins (pentenes and hexenes) are also formed (**Figure 22b**).

*t*Bu)

83

−1.min−1 for **23** and

comforted by DFT calculations.

being Δ*E* = −29 kJ.mol−1 [83].

4.9 molC3H6.molW

complex [(≡SiO)(W═N(2,6-*i*PrC6

**Figure 19.** Proposed mechanism for the direct conversion of ethylene to propene.

mechanism of this reaction passes probably via the same ethyl ethylidene oxo tungsten complex than above as ethane is also detected at the first stages of the reaction. This complex can then convert ethylene to propene via three successive reactions: (i) dimerization of ethylene to but-1-ene; (ii) isomerization of but-1-ene to but-2-ene; and (iii) cross-metathesis between ethylene and but-2-ene (**Figure 19**).

This hydride species displays also a good activity in the metathesis of isobutene into 2,3-dimethylbutenes with a relative selectivity reaching 92% [81, 82]. It is the first example of use of a supported tungsten complex for this reaction, which is very difficult, due to the high steric hindrance in the gem-tetra-substituted metallocyclobutane intermediate. All these works show the beneficial effect of the oxo ligand (even if it is only spectator) in the coordination sphere of tungsten for metathesis of olefins. In this case, the oxo ligand was formed by extraction from the surface of alumina toward the xophilic tungsten center after its reduction under hydrogen.

### **4.2. Supported oxo-alkyl and oxo-carbene tungsten complexes**

As shown above, the industrial WO<sup>3</sup> /SiO<sup>2</sup> catalyst contains only a very small amount of really active sites, rendering their characterization by spectroscopic methods very difficult. However, it has been proposed by several authors that the active site is a tungsten(VI) complex with the formula [(≡Si─O)<sup>2</sup> W(═O)(═CHR] and containing two siloxy ligands, one oxo ligand and a carbene. However, oxo alkylidene tungsten complexes are very unstable and there are only few reports on them. The first supported tungsten oxo alkyl species active in olefin metathesis was achieved by grafting reaction of the complexes synthesized by Osborn et al. (**Figure 6**) [26]. Upon *α*-H abstraction, these complexes can lead to supported oxo-alkylidene species. The first oxo-alkyl complex of tungsten supported on silica was prepared by reaction of W(═O)(CH<sup>2</sup> *t*Bu)<sup>4</sup> with silica dehydroxylated at 700°C [83]. The reaction occurs via breaking of a W─C bond and leads selectively to [(≡SiO)(W═O)(CH<sup>2</sup> *t*Bu)<sup>3</sup> ] **22** (**Figure 20**). This complex has been fully characterized by DRIFT, Raman spectroscopy, solid-state NMR, and EXAFS. The Raman spectrum displays a band at 935 cm−1 which is characteristic of the W═O bond. EXAFS indicates that the supported complex has a bipyramid trigonal structure with the oxo (d(W═O) = 0.171 nm) and siloxy ligands (d(W─O) = 0.197 nm) in axial position while the three neopentyl groups are located in equatorial position (**Figure 20**). These results were also comforted by DFT calculations.

Complex **22** was used in propene metathesis at 80°C. The evolution of one neopentane molecule per grafted complex and the detection of the olefin metathesis products (ethylene and but-2-ene) are in agreement with the formation of the [(≡SiO)(W═O)(CH<sup>2</sup> *t*Bu) (═CH*t*Bu)] surface complex (**Figure 21**). This species is formed *in situ* by coordination of propene to tungsten (which is electro-deficient), leading to a hexacoordinated complex with a great steric hindrance which will favor the *α*-H abstraction. DFT calculations show that the formation of the carbene species is thermodynamically favored, the energy barrier being Δ*E* = −29 kJ.mol−1 [83].

The catalytic performances of **22** were compared to those of the supported imido-carbene complex [(≡SiO)(W═N(2,6-*i*PrC6 H3 )(CH<sup>2</sup> *t*Bu)(═CH*t*Bu)] **23** in propene metathesis. The initial activities are quite similar for the two surface species: 4.5 molC3H6.molW −1.min−1 for **23** and 4.9 molC3H6.molW −1.min−1 for **22**. The main difference is the evolution with time of stream: Complex **23** deactivates rapidly (**Figure 22a**) while complex **22** remains stable. As a result, after 95 h the TON is 22,000 for **22** and ca. 2,500 for **23** [83]. In terms of selectivity, complex **22** is stable with equimolar amounts of ethylene and but-2-ene while in the case of compound **23** isobutene and higher olefins (pentenes and hexenes) are also formed (**Figure 22b**).

In the case of the imido complex, the deactivation mechanism has been determined: It is due to the decomposition of the 2-methyltungstocyclobutane by *β*-H transfer, leading to a

**Figure 20.** Grafting reaction of complex **21** on dehydroxylated silica.

mechanism of this reaction passes probably via the same ethyl ethylidene oxo tungsten complex than above as ethane is also detected at the first stages of the reaction. This complex can then convert ethylene to propene via three successive reactions: (i) dimerization of ethylene to but-1-ene; (ii) isomerization of but-1-ene to but-2-ene; and (iii) cross-metathesis between

This hydride species displays also a good activity in the metathesis of isobutene into 2,3-dimethylbutenes with a relative selectivity reaching 92% [81, 82]. It is the first example of use of a supported tungsten complex for this reaction, which is very difficult, due to the high steric hindrance in the gem-tetra-substituted metallocyclobutane intermediate. All these works show the beneficial effect of the oxo ligand (even if it is only spectator) in the coordination sphere of tungsten for metathesis of olefins. In this case, the oxo ligand was formed by extraction from the surface of alumina toward the xophilic tungsten center after its reduction

**4.2. Supported oxo-alkyl and oxo-carbene tungsten complexes**

**Figure 19.** Proposed mechanism for the direct conversion of ethylene to propene.

of a W─C bond and leads selectively to [(≡SiO)(W═O)(CH<sup>2</sup>

/SiO<sup>2</sup>

really active sites, rendering their characterization by spectroscopic methods very difficult. However, it has been proposed by several authors that the active site is a tungsten(VI) complex

and a carbene. However, oxo alkylidene tungsten complexes are very unstable and there are only few reports on them. The first supported tungsten oxo alkyl species active in olefin metathesis was achieved by grafting reaction of the complexes synthesized by Osborn et al. (**Figure 6**) [26]. Upon *α*-H abstraction, these complexes can lead to supported oxo-alkylidene species. The first oxo-alkyl complex of tungsten supported on silica was prepared by reaction

has been fully characterized by DRIFT, Raman spectroscopy, solid-state NMR, and EXAFS. The Raman spectrum displays a band at 935 cm−1 which is characteristic of the W═O bond. EXAFS indicates that the supported complex has a bipyramid trigonal structure with the oxo (d(W═O) = 0.171 nm) and siloxy ligands (d(W─O) = 0.197 nm) in axial position while the

catalyst contains only a very small amount of

] **22** (**Figure 20**). This complex

W(═O)(═CHR] and containing two siloxy ligands, one oxo ligand

*t*Bu)<sup>3</sup>

with silica dehydroxylated at 700°C [83]. The reaction occurs via breaking

ethylene and but-2-ene (**Figure 19**).

As shown above, the industrial WO<sup>3</sup>

with the formula [(≡Si─O)<sup>2</sup>

*t*Bu)<sup>4</sup>

under hydrogen.

82 Alkenes

of W(═O)(CH<sup>2</sup>

**Figure 21.** Mechanism of formation of the surface oxo-carbene species.

**Figure 22.** Metathesis of propene catalyzed by complexes **22** and **23**: (a) Conversion as a function of time; (b) isobutene selectivity as a function of time for complex **23**.

meta-allyl-tungsten hydride. This hydride is then converted into an inactive tungsten(IV) species with the evolution of isobutene by reductive elimination (**Figure 23**) [83].

The variation of the isobutene selectivity is quite the same than that of the conversion (**Figure 22a** and **b**) in agreement with a deactivation mechanism implying isobutene. This result shows also that the oxo ligand has a nonnegligible effect on the activity and stability of the tungsten catalysts. Recently, Eisenstein et al. performed DFT calculations on these compounds and found that replacement of the imido ligand by an oxo one increases the energy barrier of the *β*-H transfer in the mechanism of **Figure 23** and so stabilizes the catalyst [84]. [(≡SiO)W(═O)(CH*t*Bu)<sup>3</sup> ] was the first reported model of the WO<sup>3</sup> /SiO<sup>2</sup> industrial catalyst. The latter surface compound is monopodally anchored to the surface, which is different from the proposed active site in the industrial WO<sup>3</sup> /SiO<sup>2</sup> catalyst precursor. However, the real model for the industrial catalyst (being a bipodal tungsten oxo carbene species) was not achieved with the former organometallic complex grafted on SiO2–200 (support that frequently yields bipodal species). The expected W─C silanolysis step does not occur (**Figure 24**), even after thermal treatment. Such reactivity is reminiscent of tungsten aqueous organometallic chemistry described by Schrock and Lippard [85, 86]. Indeed, the coordination environment of **22**

is very similar to that of the trisoxo alkyl dinuclear complex [O(WONp<sup>3</sup>

and W(═O)Cl(CH<sup>2</sup>

SiMe<sup>3</sup> )3

)(CH<sup>2</sup>

] by hydrolysis at −78°C with 2 eq. of H<sup>2</sup>

at 200°C yields the well-defined bipodal species [(≡SiO)<sup>2</sup>

SiCl/HCl. Thus, grafting of [WO(CH<sup>2</sup>

SiMe<sup>3</sup> )3

D and 2D <sup>1</sup>

recently studied the reactivity of [W(≡CSiMe<sup>3</sup>

*t*Bu)<sup>4</sup>

ent behavior compared to [W(≡CtBu)Np<sup>3</sup>

SiMe<sup>3</sup> )3 ].

)(CH<sup>2</sup>

**Figure 24.** Reaction of W(═O)(CH<sup>2</sup>

IR, advanced solid-state NMR (<sup>1</sup>

grafting selectivity of [WO(CH<sup>2</sup>

Then, the new complex [WOCl(CH<sup>2</sup>

the unstable intermediate [WO(OSiMe<sup>3</sup>

[WO(OSiMe<sup>3</sup>

SiMe<sup>3</sup> ) 3

with 1 eq. of Me<sup>3</sup>

HCl and SiMe<sup>4</sup>

(CH<sup>2</sup>

toward excess water. In order to push forward the second protonolysis step, the neopentyl ligand needs to be replaced by a more reactive fragment. Interestingly, Xue et al. have

SiMe<sup>3</sup> )3

water, the authors observed mainly two products: tungsten bis-oxo bis-neosilyl trimer and

SiMe<sup>3</sup> )3

DFT calculations allowed understanding and rationalizing the experimental results regarding

More recently, Schrock et al. synthesized new oxo tungsten complexes bearing various ligands and studied their grafting on silica dehydroxylated at 700°C. Ligands such as 2,6-mesitylphenoxy, 2,6-diadamantyl-methylphenoxy, thio-2,6-masitylphenoxy, or tris(tert-butoxy)siloxy

were used and the corresponding carbenes were synthesized (**Figure 25**) [34, 89–91].

)(CH<sup>2</sup>

SiMe<sup>3</sup> ) 3

with SiO2–200.

SiMe<sup>3</sup> )3

release. This was demonstrated by mass balance analysis, elemental analysis,

WO(CH<sup>2</sup>

Cl] compared to its neopentyl counterpart [88].

] [87]. When [W(≡CSiMe<sup>3</sup>

] with H<sup>2</sup>

)(CH<sup>2</sup>

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] was obtained in 70% yield from [W(≡CSiMe<sup>3</sup>

SiMe<sup>3</sup> ) 2

H, <sup>13</sup>C, 29Si and <sup>17</sup>O), and EXAFS. Furthermore,

)2

SiMe<sup>3</sup> ) 3

O in THF (resulting in the formation of

Cl] onto silica dehydroxylated

] **24** via consecutive

] as reported by Xue) followed by reaction

] which is stable

] reacts with

)

O and found a differ-

**Figure 23.** Mechanism of deactivation of the tungsten catalyst.

**Figure 24.** Reaction of W(═O)(CH<sup>2</sup> *t*Bu)<sup>4</sup> and W(═O)Cl(CH<sup>2</sup> SiMe<sup>3</sup> )3 with SiO2–200.

meta-allyl-tungsten hydride. This hydride is then converted into an inactive tungsten(IV)

**Figure 22.** Metathesis of propene catalyzed by complexes **22** and **23**: (a) Conversion as a function of time; (b) isobutene

The variation of the isobutene selectivity is quite the same than that of the conversion (**Figure 22a** and **b**) in agreement with a deactivation mechanism implying isobutene. This result shows also that the oxo ligand has a nonnegligible effect on the activity and stability of the tungsten catalysts. Recently, Eisenstein et al. performed DFT calculations on these compounds and found that replacement of the imido ligand by an oxo one increases the energy barrier of the *β*-H transfer in the mechanism of **Figure 23** and so stabilizes the catalyst [84].

] was the first reported model of the WO<sup>3</sup>

latter surface compound is monopodally anchored to the surface, which is different from the

for the industrial catalyst (being a bipodal tungsten oxo carbene species) was not achieved with the former organometallic complex grafted on SiO2–200 (support that frequently yields bipodal species). The expected W─C silanolysis step does not occur (**Figure 24**), even after thermal treatment. Such reactivity is reminiscent of tungsten aqueous organometallic chemistry described by Schrock and Lippard [85, 86]. Indeed, the coordination environment of **22**

/SiO<sup>2</sup>

/SiO<sup>2</sup>

catalyst precursor. However, the real model

industrial catalyst. The

species with the evolution of isobutene by reductive elimination (**Figure 23**) [83].

[(≡SiO)W(═O)(CH*t*Bu)<sup>3</sup>

84 Alkenes

proposed active site in the industrial WO<sup>3</sup>

selectivity as a function of time for complex **23**.

**Figure 23.** Mechanism of deactivation of the tungsten catalyst.

is very similar to that of the trisoxo alkyl dinuclear complex [O(WONp<sup>3</sup> )2 ] which is stable toward excess water. In order to push forward the second protonolysis step, the neopentyl ligand needs to be replaced by a more reactive fragment. Interestingly, Xue et al. have recently studied the reactivity of [W(≡CSiMe<sup>3</sup> )(CH<sup>2</sup> SiMe<sup>3</sup> ) 3 ] with H<sup>2</sup> O and found a different behavior compared to [W(≡CtBu)Np<sup>3</sup> ] [87]. When [W(≡CSiMe<sup>3</sup> )(CH<sup>2</sup> SiMe<sup>3</sup> ) 3 ] reacts with water, the authors observed mainly two products: tungsten bis-oxo bis-neosilyl trimer and [WO(OSiMe<sup>3</sup> )(CH<sup>2</sup> SiMe<sup>3</sup> )3 ].

Then, the new complex [WOCl(CH<sup>2</sup> SiMe<sup>3</sup> ) 3 ] was obtained in 70% yield from [W(≡CSiMe<sup>3</sup> ) (CH<sup>2</sup> SiMe<sup>3</sup> )3 ] by hydrolysis at −78°C with 2 eq. of H<sup>2</sup> O in THF (resulting in the formation of the unstable intermediate [WO(OSiMe<sup>3</sup> )(CH<sup>2</sup> SiMe<sup>3</sup> )3 ] as reported by Xue) followed by reaction with 1 eq. of Me<sup>3</sup> SiCl/HCl. Thus, grafting of [WO(CH<sup>2</sup> SiMe<sup>3</sup> )3 Cl] onto silica dehydroxylated at 200°C yields the well-defined bipodal species [(≡SiO)<sup>2</sup> WO(CH<sup>2</sup> SiMe<sup>3</sup> ) 2 ] **24** via consecutive HCl and SiMe<sup>4</sup> release. This was demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (<sup>1</sup> D and 2D <sup>1</sup> H, <sup>13</sup>C, 29Si and <sup>17</sup>O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity of [WO(CH<sup>2</sup> SiMe<sup>3</sup> )3 Cl] compared to its neopentyl counterpart [88].

More recently, Schrock et al. synthesized new oxo tungsten complexes bearing various ligands and studied their grafting on silica dehydroxylated at 700°C. Ligands such as 2,6-mesitylphenoxy, 2,6-diadamantyl-methylphenoxy, thio-2,6-masitylphenoxy, or tris(tert-butoxy)siloxy were used and the corresponding carbenes were synthesized (**Figure 25**) [34, 89–91].

processes of fine chemistry, polymerization, or petrochemistry. This reaction can be catalyzed

The homogeneous systems are well-described and structure-activity relationships were made allowing defining the best ligands and oxidation state of the metal for example. The heterogeneous systems, in contrast, are ill-defined even if they are preferred by industrials, due to their

/SiO<sup>2</sup>

ture of the active species, which is in a small proportion, remains unknown even if it is generally

eled by grafting organometallic complexes of tungsten containing an oxo ligand and able to give carbene species on the surface. These systems can be considered as models of the active site

of such catalysts were reported, preventing the attainability of structure-activity relationships.

[30], 117

Ph)(OR)<sup>2</sup> Wengrovius and Schrock [29], 118; Wengrovius et al. [30]

117

117

<sup>+</sup> Schowner et al. [35], 123

)<sup>3</sup> Mougel and Coperet [34], 122

] Casey et al. [20], 31

)X M = Mo, W; X = Cl, Br, ONp, Np Kress et al. [26], 111

Pyr =

]

[(≡SiO)W(═O)(═CH*t*Bu)(O-2,6-mesitylphenoxide)] Conley et al. [90], 156

**Table 1.** Some representative metathesis catalysts listed in this review.

] (X = Cl, *t*Bu) Weiss and Lössel [67], 61

*t*Bu)] Le Roux, 2005 [68], 62

] Le Roux, 2005 [68], 62

] Le Roux, 2005 [71], 68

*t*Bu)(═CH*t*Bu)] Mazoyer, 2010 [74], 94

] Mazoyer, 2010 [74], 94

] Grekov et al. [88], 162

but their synthesis remains very complicated and up to now only few examples

] M = Mo, W Malcolmson et al. [21], 30; Schrock and Czekelius [22],

107; Marinescu et al. [23], 108; Schrock [24], 109

Wengrovius and Schrock [29], 118; Wengrovius et al. [30]

Ph Wengrovius and Schrock [29], 118; Wengrovius et al.

is often used by industry but the exact struc-

Olefin Metathesis by Group VI (Mo, W) Metal Compounds

http://dx.doi.org/10.5772/intechopen.69320

87

W(═O)(═CHR)]. This model was then mod-

by homogeneous or heterogeneous systems.

easy separation and recycling. Actually, WO<sup>3</sup>

of WO<sup>3</sup>

[(CO)<sup>5</sup>

/SiO<sup>2</sup>

Homogeneous catalysts

C(CH<sup>3</sup> )3

Ph)(═N─Ar)(OR′)<sup>2</sup>

L(═CH─*t*Bu) L = PMe<sup>3</sup>

Ph)-(Me<sup>2</sup>

)(OSi(OtBu)<sup>3</sup>

*t*Bu)<sup>2</sup>

*t*Bu)<sup>2</sup>

H3 )(CH<sup>2</sup>

*t*Bu)<sup>3</sup>

SiMe<sup>3</sup> )2

Ph)(Mes)(OAr)(NCMe)<sup>2</sup>

2,5-dimethylpyrrolide, OAr = aryloxide)

W═C(C6 H5 )2

[M(═CHCMe<sup>2</sup>

M(═O)(CH<sup>2</sup>

W(═O)Cl<sup>2</sup>

W(═O)(═CHCMe<sup>2</sup>

W(═O)(═CHCMe<sup>2</sup>

W(═O)(═CHCMe<sup>3</sup>

[W(═O)(═CHCMe<sup>2</sup>

Heterogeneous catalysts [(≡SiO)W(═C*t*Bu)X<sup>3</sup>

[(≡SiO)W(≡C*t*Bu)(CH<sup>2</sup>

[(≡SiO)(W═O)(CH<sup>2</sup>

[(≡SiO)(W═N(2,6-*i*PrC6

WO(CH<sup>2</sup>

W(≡C*t*Bu)(CH<sup>2</sup>

O)W(≡C*t*Bu)(CH<sup>2</sup>

[(≡SiO)<sup>2</sup>

[(≡SiO)<sup>2</sup>

[(Als

accepted that it should be a species like [(≡Si─O)<sup>2</sup>

**Catalyst Reference**

, PEt<sup>3</sup> , PMe<sup>2</sup>

Pyr)(OAr) (Me<sup>2</sup>

**Table 1** lists all well-defined systems reviewed in this chapter.

**Figure 25.** Grafting reaction of oxo-carbene tungsten complexes on silica.

The characterization of the grafted materials by infrared spectroscopy, chemical analysis, and solid-state NMR shows that the presence of these sterically encumbered ligands leads to a nonselective grafting reaction. For example, [W(═O)(═CH*t*Bu)(O-2,6-mesitylphenoxide)<sup>2</sup> ] leads mainly (ca. 80%) to [(≡SiO)W(═O)(═CH*t*Bu)(O-2,6-mesitylphenoxide)], formed by *σ* metathesis between the silanol group and the W─Ar ligand. The minor product, [(≡SiO) W(═O)(CH<sup>2</sup> *t*Bu)(O-2,6-mesitylphenoxide)<sup>2</sup> ], is formed by protonation of the carbene moiety by the silanol group [90].

In order to reveal the stability and robustness of the real model of industrial catalyst **24** (being a supported bipodal tungsten oxo species active in olefin metathesis) with respect to the isoelectronic monopodal tungsten oxo aryloxide species, **25**, both materials were exposed to a continuous flow of propylene with a molar flow rate of 60 molC3H6.molW −1 min−1 (20 mLC3H6 min−1; 80°C, 1 bar). Both materials gave only metathesis products (equimolar amounts of ethylene and 2-butenes). **25** presents a very fast deactivation, affording a TON = 3,000 after 25 h on stream. Conversely, **24** efficiently performs propene metathesis with sustained activity over 24 h operating time (TON = 24,000). Although **24** (after activation with the olefinic substrate) and **25** have the same coordination environment and both catalysts are active in propylene metathesis, there is a remarkable difference in stability. The local structure of **24** closely resembles the proposed industrial WO<sup>3</sup> /SiO<sup>2</sup> catalyst and showed a fairly stable catalytic activity with time on stream of propylene. On the other hand, **25** belongs to "model of model" rather than a true and robust catalyst, deactivated rapidly with time on stream. The huge different in the catalytic performance may be due to the importance of the bipodal nature of **24**, or the presence of a bulky organic ligand in **25**, which can gradually undergo intramolecular C─H activation with time and loss of the active tungsten alkylidene sites.

## **5. Conclusion**

As shown above, there is a great interest to the study of olefin metathesis, not only by academics but also by industry. Indeed, this reaction can be considered as a key step in many processes of fine chemistry, polymerization, or petrochemistry. This reaction can be catalyzed by homogeneous or heterogeneous systems.

**Table 1** lists all well-defined systems reviewed in this chapter.

The characterization of the grafted materials by infrared spectroscopy, chemical analysis, and solid-state NMR shows that the presence of these sterically encumbered ligands leads to a nonselective grafting reaction. For example, [W(═O)(═CH*t*Bu)(O-2,6-mesitylphenoxide)<sup>2</sup>

leads mainly (ca. 80%) to [(≡SiO)W(═O)(═CH*t*Bu)(O-2,6-mesitylphenoxide)], formed by *σ* metathesis between the silanol group and the W─Ar ligand. The minor product, [(≡SiO)

In order to reveal the stability and robustness of the real model of industrial catalyst **24** (being a supported bipodal tungsten oxo species active in olefin metathesis) with respect to the isoelectronic monopodal tungsten oxo aryloxide species, **25**, both materials were exposed to a

80°C, 1 bar). Both materials gave only metathesis products (equimolar amounts of ethylene and 2-butenes). **25** presents a very fast deactivation, affording a TON = 3,000 after 25 h on stream. Conversely, **24** efficiently performs propene metathesis with sustained activity over 24 h operating time (TON = 24,000). Although **24** (after activation with the olefinic substrate) and **25** have the same coordination environment and both catalysts are active in propylene metathesis, there is a remarkable difference in stability. The local structure

stable catalytic activity with time on stream of propylene. On the other hand, **25** belongs to "model of model" rather than a true and robust catalyst, deactivated rapidly with time on stream. The huge different in the catalytic performance may be due to the importance of the bipodal nature of **24**, or the presence of a bulky organic ligand in **25**, which can gradually undergo intramolecular C─H activation with time and loss of the active tung-

As shown above, there is a great interest to the study of olefin metathesis, not only by academics but also by industry. Indeed, this reaction can be considered as a key step in many

/SiO<sup>2</sup>

], is formed by protonation of the carbene moiety

*t*Bu)(O-2,6-mesitylphenoxide)<sup>2</sup>

**Figure 25.** Grafting reaction of oxo-carbene tungsten complexes on silica.

of **24** closely resembles the proposed industrial WO<sup>3</sup>

continuous flow of propylene with a molar flow rate of 60 molC3H6.molW

W(═O)(CH<sup>2</sup>

86 Alkenes

by the silanol group [90].

sten alkylidene sites.

**5. Conclusion**

]

−1 min−1 (20 mLC3H6 min−1;

catalyst and showed a fairly

The homogeneous systems are well-described and structure-activity relationships were made allowing defining the best ligands and oxidation state of the metal for example. The heterogeneous systems, in contrast, are ill-defined even if they are preferred by industrials, due to their easy separation and recycling. Actually, WO<sup>3</sup> /SiO<sup>2</sup> is often used by industry but the exact structure of the active species, which is in a small proportion, remains unknown even if it is generally accepted that it should be a species like [(≡Si─O)<sup>2</sup> W(═O)(═CHR)]. This model was then modeled by grafting organometallic complexes of tungsten containing an oxo ligand and able to give carbene species on the surface. These systems can be considered as models of the active site of WO<sup>3</sup> /SiO<sup>2</sup> but their synthesis remains very complicated and up to now only few examples of such catalysts were reported, preventing the attainability of structure-activity relationships.


**Table 1.** Some representative metathesis catalysts listed in this review.
