**4. Supported tungsten catalysts prepared by SOMC**

SOMC can be considered as a bridge between homogeneous and heterogeneous catalysis [55–57]. 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:


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, silica-alumina, etc.).

The structures of species **18** and **19** were confirmed by solid-state NMR (<sup>1</sup>

ual hydroxyl groups and the alkyl ligands of the supported species.

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

**Figure 15.** Species formed during the reaction of [W(≡C*t*Bu)(CH<sup>2</sup>

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 resid-

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

] with SiO2–200 and SiO2–700.

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

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

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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**).

H, <sup>13</sup>C, HETCOR,

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.
