**2. Group VI complexes used in homogeneous catalysis**

The first homogeneous catalytic systems using group VI metals (W or Mo) were ill-defined Ziegler-Natta type compounds, formed *in situ* by contacting a precatalyst and a cocatalyst [10]. The first example of olefin metathesis was described in 1955 by Anderson and Merckling who observed the formation of polynorbornene during the reaction of norbornene and TiCl<sup>4</sup> / LiAl(heptyl)<sup>4</sup> [11]. Two other catalytic systems based on quite the same elements (WCl6 /AlEt<sup>3</sup>

and MoCl<sup>5</sup> /AlEt<sup>3</sup> ) were described by Natta et al. in 1964 [12]. Depending on the catalytic system used, trans- or cis-polymers were obtained by polymerization of cyclopentene and tungsten leading to the trans compound. Later, various catalysts based on tungsten combined to an alkylating agent were described such as WCl6 /EtOH/EtAlCl<sup>2</sup> , WCl6 /SnBu<sup>4</sup> , or (ArO)<sup>4</sup> WCl<sup>2</sup> /SnMe<sup>4</sup> and are active for the metathesis of internal olefins such as cis-pent-2-ene [13–15]. Systems based on the combination of nitrosyl complexes of molybdenum or tungsten with alkylaluminum species were developed for the metathesis of terminal olefins (pent-1-ene, oct-1-ene) [16–19]. In parallel, many academic researchers tried to synthesize metal alkylidene species which should be active for olefin metathesis as proposed by Chauvin. Casey et al. isolated in 1973 the first electrophilic metal alkylidene, [(CO)<sup>5</sup> W═C(C6 H5 )2 ], which is of the Fisher type and is active in olefin metathesis at room temperature without any cocatalyst [20]. After 1980, Schrock has prepared all a family of molybdenum and tungsten complexes with nucleophilic carbene functions. The general formula of these complexes, which are highly active in olefin metathesis, is [M(═CHCMe<sup>2</sup> Ph)(═N─Ar)(ORʹ)<sup>2</sup> ] where Rʹ and Ar are sterically encumbered groups. The structure of some of these complexes is depicted in **Figure 4**.

ligand, is able to perform the cross-metathesis of methyl oleate with ethylene (**Figure 5**) with a good yield (Turnover number (TON) = 4750) [23]. The reaction products, dec-1-ene and methyl decen-9-oate, are important intermediates in the chemical industry and are used for the preparation of polyolefins, surfactants, and lubricating compounds. The TON can be con-

Osborn et al. have developed another family of complexes where the imido group is replaced by an oxo ligand. For that purpose, they prepared oxo alkyl complexes of molybdenum and tungsten. The idea was that these species were not stable, due to the small energy of the metal-carbon bond, and should lead to the formation of the carbenic complex upon *α*-H abstraction. Oxo complexes of molybdenum and tungsten with neopentyl and/or neosilyl ligands were prepared (**Figure 6**) [26] and are active in the metathesis of

Later, Schrock et al. have synthesized pentacoordinated complexes of tungsten(VI) with oxo and alkylidene groups stabilized by phosphine ligands (**Figure 7**) [29, 30]. These systems are active in

Complex **9**, which contains two chloro ligands, has been used as a precursor for the preparation of numerous oxo carbenic complexes where chlorine was replaced by other ligands such as phenoxide, pyrolidyl, thiophenoxide, or siloxide. By this way, Schrock et al. have studied the effect of the modification of the coordination sphere of this complex on its activity and selectivity in metathesis of terminal olefins and ROMP of norbornene derivatives. For exam-

**13** is active in ROMP of 2,3-dicarbomethoxynorbornadiene (DCMNBD) and leads selectively

Complexes with thiophenoxide ligands were also prepared and their activity in metathesis of oct-1-ene and polymerization of DCMNBD was compared to that of the corresponding

linear, cyclic, and functional olefins, but in presence of Lewis acids such as AlCl<sup>3</sup>

metathesis of terminal and internal olefins in presence of a Lewis acid such as AlCl<sup>3</sup>

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

[25].

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

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

, SnCl<sup>4</sup> , 71

[31].

Pyr = 2,5-dimethylpyrrolide, OAr = aryloxide)

F5 )3

Ph)(OR)<sup>2</sup> **10** leads selectively

leads to a consider-

siderably increased (up to 50,000) upon addition of Al(octyl)<sup>3</sup>

or GaCl<sup>3</sup>

[27, 28].

ple, W(═O)(CHCMe<sup>2</sup>

Ph)─(Me<sup>2</sup>

able increase of both the activity and the selectivity.

**Figure 5.** Cross-metathesis of ethylene and methyl oleate.

**Figure 6.** Oxo alkyl complexes synthesized by Osborn et al.

to the cis syndiotactic polymer while complex W(═O)(CHCMe<sup>2</sup>

to the cis isotactic polymer [32]. Addition of a Lewis acid such as B(C6

As these complexes are not very sensitive to functional groups, their use was then extended to fine chemistry [21, 22], oleochemistry [23], and to the synthesis of functional polymers [24] while during many years olefin metathesis was confined to nonfunctional olefins. In these complexes, the metal is surrounded by the carbene moiety and by various electro-attractor and/or sterically encumbered ligands allowing a good stability in solution and good activity, selectivity, and stability. For example, complex **6**, which contains an imido group and a binaphthyl

**Figure 4.** Some W(VI) and Mo(VI) complexes bearing imido, aryloxy, and carbenic functions highly active in olefin metathesis.

ligand, is able to perform the cross-metathesis of methyl oleate with ethylene (**Figure 5**) with a good yield (Turnover number (TON) = 4750) [23]. The reaction products, dec-1-ene and methyl decen-9-oate, are important intermediates in the chemical industry and are used for the preparation of polyolefins, surfactants, and lubricating compounds. The TON can be considerably increased (up to 50,000) upon addition of Al(octyl)<sup>3</sup> [25].

and MoCl<sup>5</sup>

70 Alkenes

metathesis.

/AlEt<sup>3</sup>

metathesis, is [M(═CHCMe<sup>2</sup>

ating agent were described such as WCl6

1973 the first electrophilic metal alkylidene, [(CO)<sup>5</sup>

) were described by Natta et al. in 1964 [12]. Depending on the catalytic system

W═C(C6

, WCl6

H5 )2

/SnBu<sup>4</sup>

] where Rʹ and Ar are sterically encumbered

, or (ArO)<sup>4</sup>

], which is of the Fisher type

WCl<sup>2</sup>

/SnMe<sup>4</sup>

used, trans- or cis-polymers were obtained by polymerization of cyclopentene and tungsten leading to the trans compound. Later, various catalysts based on tungsten combined to an alkyl-

and are active for the metathesis of internal olefins such as cis-pent-2-ene [13–15]. Systems based on the combination of nitrosyl complexes of molybdenum or tungsten with alkylaluminum species were developed for the metathesis of terminal olefins (pent-1-ene, oct-1-ene) [16–19]. In parallel, many academic researchers tried to synthesize metal alkylidene species which should be active for olefin metathesis as proposed by Chauvin. Casey et al. isolated in

and is active in olefin metathesis at room temperature without any cocatalyst [20]. After 1980, Schrock has prepared all a family of molybdenum and tungsten complexes with nucleophilic carbene functions. The general formula of these complexes, which are highly active in olefin

As these complexes are not very sensitive to functional groups, their use was then extended to fine chemistry [21, 22], oleochemistry [23], and to the synthesis of functional polymers [24] while during many years olefin metathesis was confined to nonfunctional olefins. In these complexes, the metal is surrounded by the carbene moiety and by various electro-attractor and/or sterically encumbered ligands allowing a good stability in solution and good activity, selectivity, and stability. For example, complex **6**, which contains an imido group and a binaphthyl

**Figure 4.** Some W(VI) and Mo(VI) complexes bearing imido, aryloxy, and carbenic functions highly active in olefin

Ph)(═N─Ar)(ORʹ)<sup>2</sup>

groups. The structure of some of these complexes is depicted in **Figure 4**.

/EtOH/EtAlCl<sup>2</sup>

Osborn et al. have developed another family of complexes where the imido group is replaced by an oxo ligand. For that purpose, they prepared oxo alkyl complexes of molybdenum and tungsten. The idea was that these species were not stable, due to the small energy of the metal-carbon bond, and should lead to the formation of the carbenic complex upon *α*-H abstraction. Oxo complexes of molybdenum and tungsten with neopentyl and/or neosilyl ligands were prepared (**Figure 6**) [26] and are active in the metathesis of linear, cyclic, and functional olefins, but in presence of Lewis acids such as AlCl<sup>3</sup> , SnCl<sup>4</sup> , or GaCl<sup>3</sup> [27, 28].

Later, Schrock et al. have synthesized pentacoordinated complexes of tungsten(VI) with oxo and alkylidene groups stabilized by phosphine ligands (**Figure 7**) [29, 30]. These systems are active in metathesis of terminal and internal olefins in presence of a Lewis acid such as AlCl<sup>3</sup> [31].

Complex **9**, which contains two chloro ligands, has been used as a precursor for the preparation of numerous oxo carbenic complexes where chlorine was replaced by other ligands such as phenoxide, pyrolidyl, thiophenoxide, or siloxide. By this way, Schrock et al. have studied the effect of the modification of the coordination sphere of this complex on its activity and selectivity in metathesis of terminal olefins and ROMP of norbornene derivatives. For example, W(═O)(CHCMe<sup>2</sup> Ph)─(Me<sup>2</sup> Pyr)(OAr) (Me<sup>2</sup> Pyr = 2,5-dimethylpyrrolide, OAr = aryloxide) **13** is active in ROMP of 2,3-dicarbomethoxynorbornadiene (DCMNBD) and leads selectively to the cis syndiotactic polymer while complex W(═O)(CHCMe<sup>2</sup> Ph)(OR)<sup>2</sup> **10** leads selectively to the cis isotactic polymer [32]. Addition of a Lewis acid such as B(C6 F5 )3 leads to a considerable increase of both the activity and the selectivity.

Complexes with thiophenoxide ligands were also prepared and their activity in metathesis of oct-1-ene and polymerization of DCMNBD was compared to that of the corresponding

**Figure 6.** Oxo alkyl complexes synthesized by Osborn et al.

**3. Solids containing group VI (Mo, W) metal ions used in heterogeneous** 

Oxides of group VI (molybdenum and tungsten) and group VII (rhenium) are often used in industrial processes when they are supported on silica or alumina. The triolefin process,

on silica for olefin metathesis [36]. Initially, this process was developed in order to convert propene into ethylene and but-2-ene. Later, due to the increasing request of propene for the synthesis of numerous chemicals (polypropylene, acrylonitrile, propene oxide, cumene, and

Actually, the propene production by metathesis is mainly made by use of the OCT (Olefins Conversion Technology) process, developed by ABB Lumus Technology at Houston. This reaction is the reverse of the triolefin process, with quite the same catalyst [37, 38]. It produces ca. 6% of the world production (6.5 Mtons/year in 2014). The SHOP (Shell Higher Olefins Process) is one of the main industrial processes using olefin metathesis for the production of α-olefins, which are precursors for plasticizers and detergents [37, 39]. The catalyst is based

industrial process using olefin metathesis is the synthesis of neohexene from di-isobutene and ethylene (**Figure 10**). Neohexene is mainly used for perfumes where it is a starting material for

The main application of these heterogeneous systems is in petrochemistry and their use in other domains such as organic synthesis, oleochemistry, or polymerization remains very limited, mainly due to the drastic conditions which are required and to their intolerance of functional

poisoning by oxygenated and sulfided compounds due to the high reaction temperature (more than 350°C) [41, 42]; (ii) even if the coke formation is important at high temperature, the catalyst can be regenerated easily by calcination in air [42], without decomposition of the active sites, in

/SiO<sup>2</sup>

, the production ability being ca. 1.2 Mtons/year [37]. Another

, due to the following reasons: (i) it is resistant to

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

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supported

73

developed by Phillips (**Figure 9**), was the first commercial application using WO<sup>3</sup>

acetone), new processes were developed for the production of propene.

**catalysis**

on MoO<sup>3</sup>

/Al<sup>2</sup> O3

or WO<sup>3</sup>

the obtention of synthetic musks [40].

groups. The most often used catalyst is WO<sup>3</sup>

**Figure 9.** The Triolefin process developed by Phillips.

**Figure 10.** Synthesis of neohexene by olefin metathesis.

/SiO<sup>2</sup>

**Figure 7.** Oxo carbenic complexes synthesized by Schrock et al.

**Figure 8.** Synthesis of the cationic tungsten(VI) complex.

phenoxide complexes. They are less active and less selective. This was attributed to a higher electronic density around the metal, due to a stronger *σ*-donor effect of thiophenoxides compared to phenoxides [33]. The bis-siloxy oxo alkylidene tungsten complex **14** has been described recently. However, it displays a low initial activity (Turnover frequency (TOF) = 12 min−1) in metathesis of cis oct-4-ene at 80°C. This low activity has been explained by the low thermal stability of this complex in absence of phosphine ligands [34].

Buchmeiser et al. have increased the catalytic activity of these oxo complexes by increasing the electrophilicity of the metal by transforming them into cationic species. They have reported recently the synthesis and X-ray structure of the first stable cationic complex of tungsten by removing chlorine of the W─Cl bond by reaction with Ag(MeCN)<sup>2</sup> B(ArF)<sup>4</sup> (**Figure 8**). This complex is highly active (the TONs can reach 10,000) in metathesis of olefins functionalized by nitrile, sec-amine, or thioether groups [35].
