Supported Ta-polyhydrides

As expected, initial studies on alkane hydrogenolysis found that these tantalum hydride species display a different alkane product distribution than the Zr-H species. Moreover, this catalyst was able to hydrogenolyse ethane into methane, suggesting a novel elementary step for this group-V transition metal an a-alkyl transfer occurs in competition with the observed fi-alkyl transfer for ZrH /SiOj [31]. These observations led to the discovery of silica-supported tantalum hydride [TaH /Si02] as an efficient catalyst for alkane metathesis [32]. To conduct this 

The cross metathesis between propane and methane to transform methane into higher alkanes has also been reported [35]. Mechanistic studies performed on a mixture of propane and C-labeled methane (1/1250) in a batch reactor afforded isotopomers of ethane (unlabeled, mono-, and dilabeled), confirming the incorporation of the labeled methane ( 85%). Moreover, metathesis of the C-monolabeled ethane catalyzed by [TaH, /Si02] in a batch reactor was examined. The results revealed that the degenerative and productive alkane metathesis processes concomitantly occur the alkane product distribution afforded a 1/1 mixture of unlabeled and monolabeled methane [36]. Further 

These mechanistic studies involving a metal carbene hydride are supported by the work of Villemin et al. using a W catalyst. They found that the thermal decomposition of different mixtures of WClg -l-RMgBr produced traces of linear alkanes [41-43]. To generate these products, they postulated the formation of a W-alkylidene hydride from an in situ, W-alkyl intermediate. 

Another approach using supported, early transition metals involves the use of tantalum clusters on oxide surfaces. Gates et al. reported the formation of trinuclear tantalum by exposing partially dehydroxylated silica treated with 

Ta(CH2Ph)5 to ethane or hydrogen [48]. EXAFS spectroscopy confirmed the existence of a Ta-Ta species, demonstrating the formation of tantalum clusters on silica. This supported that Ta sample catalyzed the metathesis of ethane at high temperature (250 C) [49]. The initial conversion of ethane metathesis was 24% the molar ratio of methane to propane exceeded 1, and small quantities of butanes were also observed. This catalytic system was also found to convert propane into ethane and butanes [48, 50, 51]. The authors found that the observed catalytic activity was dependent on average cluster size. Increasing the average cluster diameter by 0.2-0.3 nm led to a decrease in activity [51]. 

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Figure 2.1 Basset s supported Ta polyhydride catalyst systems.

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. 

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). 

Interestingly, clearly distinct results were observed when the MCM-41-supported [=SiOTa(=C Bu)(CH2 Bu)2] and silica-supported 17 were treated with hydrogen at 150°C (Scheme 8). In the case of silica-supported tantalum monohydride is formed via the intermediacy of tantalum polyhydride which was proved by EXAFS [42], whereas in the case of MCM-41-supported tantalum complex, a mixture of tantalum monohydride (major) and tris-hydride (minor) was formed via tantalum polyhydride [40] (Scheme 9). Upon further heating from 150 to 500°C under hydrogen atmosphere, progressive decrease of the Ta-H peak was observed in IR spectra, and a new surface complex corresponding to [(=SiO)3Ta] was formed. This can be explained by the fact that at higher temperature, a hydride transfer from... 

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