Supported W-polyhydrides

0 TaHySilica WHySilica TattyAlumina WHyAlumina TaHySilica- Alumina WHySilica- Alumina 

I TaH /Silica WH /Silica TaHx/Alumlna WH /Alumlna I WH /Silica-Alumina I TaHy/Silica-Alumina 

Recently, the metathesis reaction of -butane employing supported W-polyhydrides on silica-alumina was reported using a continuous-flow reactor at 150 °C under pressure (P=20bar) [61]. In these harsh conditions, -butane was converted into a mixture of Uquid, linear alkanes (Cj to C12) as the major products. Unfortunately, the catalyst was rapidly deactivated ( 40 h). Using similar reaction conditions, supported Ta-polyhydrides on siUca-alumina were also active, but less selective, for the heavier alkanes. Ultimately, it follows a general trend that supported, W-polyhydride catalysts on alumina oxide outperform supported, Ta-polyhydrides on silica. This has been attributed to better catalyst stability and the possible formation of novel, W-oxo polyhydrides. 

Extensive studies on the coordination sphere of the metal using Mo-bisdiphenylamido and dipyrrolyl complexes for olefin metathesis revealed how to improve the activity and selectivity of these catalysts [62-68]. This seminal work led to the synthesis of a new generation of highly active olefin metathesis catalysts [69, 70]. Using a similar strategy, various surface-metal alkylidene and alkylidyne complexes have also been tested in alkane metathesis to determine if they can be employed as catalyst precursors [11, 71]. This section describes the structure-activity relationship of different metal-alkyl complexes with oxide surfaces in alkane metathesis. 

As mentioned earUer the mechanism of alkane metathesis occurs via (i) a C-H bond activation, followed by (ii) metal alkylidene and olefin formation, and finally (iii) the olefin metathesis step and alkene hydrogenation. Thus, it was of interest to commence this transformation with an alkylidene precursor rather than metal 

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Using a procedure related to that for the preparation of tantalum catalysts, supported W-polyhydride species were prepared to evaluate their catalytic activity in alkane metathesis. Hydrogenolysis of the grafted, alkylidyne d°-complex W(=fBu)(CH2tBu)3 [52] led to W-polyhydride complexes (Scheme 2.8) [53]. 

Figure 2.7 The proposed structure of supported, W-polyhydrides on different surface oxides.

Al-H bond vibrations detected by infrared spectroscopy suggest the formation of a W-polyhydride species directly bound to y-alumina for a supported, silica-alumina oxide. Thus, the alumina surface could favor the generation of unusually stable, yet reactive, metal hydride species, such as a trishydride-oxo, W species (Figure 2.7) [15]. The strong adsorption of alkenes onto alumina could also be enhancing and/or greatly modifying the reaction rates, which would explain the efficiency of these supported catalysts [60]. 

Cross-Alkane Metathesis In a parallel study on cross-alkane metathesis using supported, Ta polyhydrides [34], Schrock and coworkers recently reported the dual-catalytic, homogenous, cross-alkane metathesis of n-octane, and ethylbenzene. The authors [136] employed various W-monoaryloxide pyrrolide complexes in combination with several iridium pincer complexes. For example, employing Ir-2(H2) and W-1 with a ratio of n-octane/ethylbenzene (1 1.33, in v/v) produced the best productivity toward alkylbenzenes, with good selectivity over linear alkanes (Scheme 2.20). 

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