Some representative metathesis catalysts listed in this review.
Olefin metathesis is an important reaction not only in petroleum chemistry but also in fine chemistry. Professors Grubbs, Schrock, and Chauvin obtained the Nobel Prize in 2005 for the development of this reaction (determination of the mechanism and synthesis of homogeneous catalysts). This reaction can be described as the redistribution of carbon chains of olefins via a breaking of their C═C double bonds. It is catalyzed by metal carbenes and the catalytic cycle passes through a metallacyclobutane. The purpose of this chapter is to give an overview of catalysts based on tungsten or molybdenum active for this reaction. Numerous tungsten and molybdenum organometallic complexes displaying a carbene functionality were synthesized. Some of them are highly active in olefin metathesis. Industrially, tungsten oxide on silica is used as a precursor of the propene production by olefin metathesis of but-2-ene and ethylene. However, the active sites are not well known but they can be modeled by grafting, via surface organometallic chemistry, perhydrocarbyl complexes of molybdenum or tungsten on oxide surfaces. After a review of the complexes used in homogeneous catalysis, a review of the industrial catalysts and their models will be given.
- olefin metathesis
- heterogeneous catalysis
Olefin metathesis can be described as the redistribution of the two fragments obtained by breaking the double bond of an olefin (Figure 1). This reaction is of great interest not only for industry (for example, for the production of propene from ethylene and butene) but also for organic chemistry, mainly for the formation of cycles .
Historically, the reaction was not recognized as a metathesis reaction. At the end of the 1950s, there were many studies in industrial laboratories on the catalytic effects of systems containing transition metal ions on unsaturated hydrocarbons. These works had been initiated partly by the results of Ziegler and Natta in the field of ethylene and propene polymerization and partly by those obtained by Phillips and Standard Oil in ethylene polymerization by heterogeneous systems. Many observations were made which could not be explained by the reactions known at this period. Finally, it was really Calderon and Ofstead, at Goodyear, who obtained the first conclusive results, which led to the formulation of metathesis as a general principle of reversible scission and recombination of carbon-carbon double bonds.
The olefin metathesis reaction can be divided into three different reactions (Figure 1): (i) the homo-metathesis and the cross-metathesis which involve the exchange of fragments of acyclic olefins; (ii) the ring opening metathesis polymerization (ROMP), which involves the opening of a cyclic olefin, and (iii) the ring closing metathesis (RCM), which corresponds to the formation of a cyclic olefin by reaction of a diene. Three other classes of olefin metathesis are (iv) ring-opening metathesis polymerization, (v) acyclic diene metathesis, and (vi) ethenolysis.
The mechanism of the olefin metathesis reaction remained unknown for several years and various intermediates were postulated. In 1968, Calderon proposed a cyclobutane coordinated to the metal as an intermediate species (Figure 2) . Pettit proposed the formation of a tetramethylene complex , while Grubbs postulated the formation of a metallacyclopentane .
Finally, Chauvin proposed the now admitted and experimentally proved mechanism of the olefin metathesis reaction and obtained the Nobel Prize in 2005 with Grubbs and Schrock for this discovery . This mechanism necessitates the presence of a metallocarbenic species which can coordinate an olefin, leading to the formation of a metallacyclobutane. Upon rearrangement this cycle will lead to the formation of a new olefin and restore the metal carbene species (Figure 3).
This mechanism implies that the reactions are equilibrated and the metallacyclobutane can lead to new products (productive metathesis) or to the starting olefins (degenerative metathesis). This mechanism has been confirmed by the synthesis of homogeneous complexes containing a nucleophilic carbenic function and the formation of a metallacyclobutane by their reaction with an olefin [6, 7]. These species display a good activity in the olefin metathesis reaction, in agreement with a mechanism involving them. In addition, numerous complexes with metallacyclobutane intermediates were isolated and gave additional proofs to the Chauvin’s mechanism [8, 9]. During last years, the development of highly active homogeneous and heterogeneous catalysts made the olefin metathesis reaction a powerful tool in numerous domains such as petrochemistry, polymers synthesis, fine chemistry, and synthesis of natural products.
Usually, olefin metathesis catalysts contain elements from groups 6 to 8, typically molybdenum, tungsten, rhenium, and ruthenium. While catalysts based on ruthenium are widely used in organic synthesis at the laboratory scale, molybdenum and tungsten are used industrially at a larger scale. Systems based on rhenium were developed but their use remains marginal.
Due to the importance of catalysts based on group (VI) elements, this review will be limited to them, with the aim to have a better understanding of the nature of the active sites in the industrial systems.
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
As these complexes are not very sensitive to functional groups, their use was then extended to fine chemistry [21, 22], oleochemistry , and to the synthesis of functional polymers  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
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
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 AlCl3 .
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 phenoxide complexes. They are less active and less selective. This was attributed to a higher electronic density around the metal, due to a stronger
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)2B(ArF)4 (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 .
3. Solids containing group VI (Mo, W) metal ions used in heterogeneous catalysis
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, developed by Phillips (Figure 9), was the first commercial application using WO3 supported on silica for olefin metathesis . 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 acetone), new processes were developed for the production of propene.
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 on MoO3/Al2O3 or WO3/SiO2, the production ability being ca. 1.2 Mtons/year . Another 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 obtention of synthetic musks .
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 groups. The most often used catalyst is WO3/SiO2, due to the following reasons: (i) it is resistant to 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 , without decomposition of the active sites, in contrast to other systems such as MoO3/Al2O3 or Re2O7/Al2O3; and (iii) its preparation is easy, by impregnation of a high surface area silica by an aqueous solution of ammonium metatungstate [(NH4)6H2W12O40 • xH2O].
A lot of studies were made on the WO3/SiO2 system, before and after activation by propene, by using various spectroscopic methods such as Raman, UV-visible, EPR, XANES, and EXAFS. The first studies were made by Raman and led to the conclusion that the active site was an isolated surface complex of tungsten but of unknown structure . The first postulated surface species was a pentacoordinated tungsten complex but no experimental justification was given .
In 1991, Basrur et al. have proposed that the active species of the WO3/SiO2 catalyst was a bis-oxo bis-siloxy tungsten complex [(≡SiO)2W(═O)2] and that the activation by propene led to a reduction of tungsten and formation of acetone or to a transformation of the W═O double bond into a metal-carbene bond with liberation of acetaldehyde . A characterization by EXAFS at room temperature of WO3/SiO2 has shown that polytungstic species are present on the surface of silica . By using a combination of Raman and UV-visible spectroscopies
Some authors have studied propene metathesis on WO3/SiO2 and have shown that the reaction rate is linearly dependent on the propene partial pressure . It has also been reported that the amount of surface tungsten and the treatment of the catalyst by an inert gas (nitrogen, argon, helium)  or by hydrogen  have a significant effect on the catalytic activity.
Recently, Wachs et al. have studied the effect of the WO3 amount on silica on the catalytic activity in propene metathesis at 300°C. The results are depicted in Figure 12. The catalytic activity increases with the amount of WO3 until a value of ca. 8 wt.% and then remains quite constant. These results were explained as follows: At low coverage (WO3 < 8 wt.%) the catalytic activity is proportional to the amount of isolated mono-oxo and di-oxo species (which are all assumed to be active in olefin metathesis). At high coverage, the reaction rate is not dependent on the tungsten loading, due to the formation of WO3 crystallites which are inactive . There is also an effect of the WO3 loading on the amount to ethylene and butenes. At low coverage, the ethylene/butene ratio is equal to ca. 1 as expected while at high coverage it decreases strongly, due to the formation of C4─C6 alkanes. This was interpreted as due to the presence of Brönsted acid sites on the WO3 crystallites (or nanoparticles), which led to oligomerization and cracking .
Recently operando methods (UV-Vis, Raman, XANES/EXAFS) were used in order to characterize the catalyst during its pretreatment and in presence of propene, the aim being to establish a structure-activity relationship. Wachs et al. studied by Raman the effect of the pretreatment in air on a WO3/SiO2 catalyst as a function of the oxide loading . For low coverages, the Raman spectrum shows new bands at 1016 and 958 cm−1, which were attributed to the symmetric vibrations of di-oxo and mono-oxo species, respectively. The di-oxo species displays also an asymmetric vibration band at 968 cm−1. The absence of the W─O─W band at 200–300 cm−1 confirms the absence, at low coverage, of WO3 aggregates. These results are in agreement with the UV-visible results. When the tungsten amount is higher than 0.6%, the Raman peak at 990 cm−1 increases with the amount of tungsten. At high loadings, three new bands appear at 270, 720, and 805 cm−1, characteristic of tungsten oxide nanoparticles. The main conclusion of this study is that tungsten is well dispersed on the silica surface for WO3 loadings below 8 wt.%.
The catalyst containing 4 wt.% was also studied by operando Raman spectroscopy during the metathesis reaction of propene (1% in argon) at 300°C. The bands characteristics of the mono-oxo and di-oxo species (which are the sole species on the solid) decrease simultaneously with time and disappear after 100 minutes . This proves that the two species were activated by propene and led to the formation of carbene species with elimination of oxygen from the coordination sphere of tungsten (Figure 13). After reoxidation by an O2/Ar mixture, the initial bands of the tungsten oxide species are restored with their intensity and no formation of nanoparticle is detected by Raman. For catalysts with high loadings (8 wt.% WO3), the activation under propene leads to a strong decrease of the bands characteristic of the nanoparticles with formation of oxygenates such as formaldehyde or acetaldehyde but no acetone is formed. In addition, a study by ESR and UV-visible spectroscopy has shown that tungsten is mainly in the +VI oxidation state.
However, Bell et al.  have recently shown by an
EXAFS spectra of a pretreated 5.4 wt.% WO3/SiO2 catalyst show the presence of mono-oxo and di-oxo tungsten species with contributions in the Fourier transform at 0.12 nm (W═O) and 0.16 nm (W─O) (the true distances take into account a phase correction and are slightly larger by 0.04 nm than those deduced from the Fourier transform). After treatment at 600°C under inert gas (helium), Bell et al. observed a decrease of the peak at 0.16 nm . This decrease was attributed to the transformation of the mono-oxo species into the di-oxo one (Figure 14). This increase of the di-oxo concentration could explain the higher activity of this system compared to that obtained after activation under air.
Stair et al. reported recently that a pretreatment at high temperature of the WO3/SiO2 catalyst by a gas containing propene increased by two to three orders of magnitude its activity at low temperature . Surprisingly, these catalysts can be regenerated by a treatment under nitrogen at high temperature.
Even if some tentative structure-activity relationships were made for the MO3/SiO2 (M = Mo, W) catalysts, the structure of the true active species is not really known up to now. The main problem is due to the low amount of active sites. Spectroscopic methods such as in-operando Raman, UV-Vis, XANES, or EXAFS show all surface species, not only those which are active in olefin metathesis. As a consequence, it is very difficult to understand the activation mechanism of the catalyst (and also its deactivation).
The preparation of systems containing a higher amount of active sites could lead to more active (and easily regenerated) systems and could allow a better characterization and mechanistic study of the initiation and deactivation steps and their rationalization in terms of classical organometallic chemistry. Surface organometallic chemistry (SOMC) is a choice method for the preparation of silica supported complexes. Numerous tungsten complexes with a variety of ligands (alkyl, carbene, carbyne, oxo, imido, aryloxy, etc.) were immobilized on oxide supports in order to obtain single site species which can be applied for the valorization of hydrocarbons via various reactions (alkane or alkene metathesis, methane coupling, etc.) [55, 56]. These materials can be characterized by the same spectroscopic methods than the conventional catalysts (solid-state NMR, EXAFS, DRIFT, ESR, UV-Vis, etc.).
4. Supported tungsten catalysts prepared by SOMC
SOMC can be considered as a bridge between homogeneous and heterogeneous catalysis [55, 56, 57]. Its aim is to graft organometallic complexes on oxide surfaces (silica, alumina, titania, zirconia, etc.) or on metal surfaces. In the case of oxides, the complex can be linked to the support by one or more bonds with surface oxygen atoms. When the support has been previously functionalized, the bonding can be made via other atoms such as P, N, Si, etc. As it is the case in homogeneous catalysis, these surface organometallic species can be defined by their ligands around the metal. Two types of ligands can be considered, those which will be involved in the catalytic cycle and those which are only spectators (such as oxo, alkoxo, amido, or imido groups). The modification of both types of ligands can have a drastic effect on the activity and selectivity of a given catalytic reaction, allowing to establish structure-activity relationships. For example, pretreatment of the support at different temperatures will lead to the synthesis of surface complexes with one, two, or three bonds with the surface. This new approach has many advantages:
The catalyst can be easily separated from the reaction products and recycled.
The catalysts are single sites, as in homogeneous catalysis.
The metal complexes have a limited mobility on the surface, avoiding the bimolecular decomposition reactions which are often observed in homogeneous catalysis .
The catalysts can be characterized easily by use of spectroscopic methods, as all species are identical.
The good knowledge of the structure of the active site allows to propose a reasonable catalytic cycle and to determine how deactivation and regeneration will proceed.
A lot of organometallic complexes of groups 4–8 were grafted on a variety of surfaces such as amorphous inorganic oxides , zeolites , 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, silica-alumina, etc.).
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 support and (ii) grafting of an organometallic complex bearing oxo ligand.
4.1. Supported tungsten complexes with oxo and hydride ligands
The first carbynic complex of tungsten(VI), [W(≡C
The structures of species
The grafting reaction of [W(≡C
In contrast to complexes
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 WO3/SiO2 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 .
While an excess of ethylene is needed in the case of the industrial WO3/SiO2 catalyst to achieve a good selectivity to propene , 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
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 . The 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 WO3/SiO2 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)2W(═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) . Upon
The catalytic performances of
In the case of the imido complex, the deactivation mechanism has been determined: It is due to the decomposition of the 2-methyltungstocyclobutane by
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
Then, the new complex [WOCl(CH2SiMe3)3] was obtained in 70% yield from [W(≡CSiMe3)(CH2SiMe3)3] by hydrolysis at −78°C with 2 eq. of H2O in THF (resulting in the formation of the unstable intermediate [WO(OSiMe3)(CH2SiMe3)3] as reported by Xue) followed by reaction with 1 eq. of Me3SiCl/HCl. Thus, grafting of [WO(CH2SiMe3)3Cl] onto silica dehydroxylated at 200°C yields the well-defined bipodal species [(≡SiO)2WO(CH2SiMe3)2]
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, 90, 91].
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
In order to reveal the stability and robustness of the real model of industrial catalyst
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.
|[(CO)5W═C(C6H5)2]||Casey et al. , 31|
|[M(═CHCMe2Ph)(═N─Ar)(OR′)2] M = Mo, W||Malcolmson et al. , 30; Schrock and Czekelius , 107; Marinescu et al. , 108; Schrock , 109|
|M(═O)(CH2C(CH3)3)X M = Mo, W; X = Cl, Br, ONp, Np||Kress et al. , 111|
|W(═O)Cl2L(═CH─||Wengrovius and Schrock , 118; Wengrovius et al. , 117|
|W(═O)(═CHCMe2Ph)-(Me2Pyr)(OAr) (Me2Pyr = 2,5-dimethylpyrrolide, OAr = aryloxide)||Wengrovius and Schrock , 118; Wengrovius et al.  117|
|W(═O)(═CHCMe2Ph)(OR)2||Wengrovius and Schrock , 118; Wengrovius et al.  117|
|W(═O)(═CHCMe3)(OSi(OtBu)3)3||Mougel and Coperet , 122|
|[W(═O)(═CHCMe2Ph)(Mes)(OAr)(NCMe)2]+||Schowner et al. , 123|
|[(≡SiO)W(═C||Weiss and Lössel , 61|
|[(≡SiO)W(≡C||Le Roux, 2005 , 62|
|[(≡SiO)2W(≡C||Le Roux, 2005 , 62|
|[(AlsO)W(≡C||Le Roux, 2005 , 68|
|[(≡SiO)(W═O)(CH2||Mazoyer, 2010 , 94|
|[(≡SiO)(W═N(2,6-||Mazoyer, 2010 , 94|
|[(≡SiO)2WO(CH2SiMe3)2]||Grekov et al. , 162|
|[(≡SiO)W(═O)(═CH||Conley et al. , 156|
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, WO3/SiO2 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)2W(═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 WO3/SiO2 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.