Silica-supported tantalum hydrides

Alkane metathesis was first reported in 1997 [84]. Acyclic alkanes, with the exception of methane, in contact with a silica supported tantalum hydride ](=SiO)2TaH] were transformed into their lower and higher homologues (for instance, ethane was transformed into methane and propane). Later, the reverse reaction was also reported [85]. Taking into accountthe high electrophilic character ofa tantalum(III) species, two mechanistic hypotheses were then envisaged (i) successive oxidative addition/reductive elimination steps and (ii) o-bond metathesis. Further work has shown that aLkyhdene hydrides are critical intermediates, and that carbon-carbon... 

Chabanas M, Vidal V, Coperet C, ThivoUe-Cazat J, Basset J-M (2002) Low-temperature hydrogenolysis of alkanes catalyzed by a silica-supported tantalum hydride complex, and evidence for a mechanistic switch from group IV to group V metal surface hydride complexes. Angew Chem Int Ed 39 1962... 

Maury O, Lefort L, Vidal V, Thivolle-Cazat J, Basset J-M (1999) Metathesis of alkanes Evidence for degenerate metathesis of ethane over a silica-supported tantalum hydride prepared by surface organometallic chemistry. Angew Chem Int Ed 38 1952... 

The catalytic properties of this silica-supported tantalum hydrides are noteworthy. First, H/D exchange in D2/CH4 mixture is fast (0.2 mol/mol/s at 150 °C), which shows that these systems readily cleave and reform the C-H bonds of alkanes (Scheme 36(a)). Second, it also converts alkanes into its lower homologs and ultimately methane in the presence of H2 (hydrogeno lysis) at relatively low temperatures (150 °C). " The key step of carbon-carbon bond cleavage probably corresponds to an a-alkyl transfer on a Ta(m) intermediate followed by successive hydrogenolysis steps (Schemes 36(b) and 37). In the case of cycloalkanes, hydrogenolysis yields smaller cycloalkanes, but deactivation is very fast. This phenomenon has been associated with the rapid formation of cyclopentane and, thereby, with the formation of cyclopentadienyl derivatives videsupra Scheme 35), which are inactive for the hydrogenolysis of alkanes. 

Third, besides hydrogenolysis properties, the silica-supported tantalum hydride catalyzes the metathesis of alkanes, which transforms a given alkane into its higher and lower homolog (Scheme 36(c)). This reaction... 

Scheme 24 Cross metathesis of toluene and ethane with silica-supported tantalum hydride...

Cross metathesis between two different alkanes represent one of the most difficult challenges in organic chemistry [53]. In 2001, Basset et al. first demonstrated the possibilities of sigma bond metathesis between two different alkanes [55]. In 2004, this same group has reported the cross metathesis between ethane and toluene [81] and methane and propane [82]. Silica-supported tantalum hydride catalyst [(=SiO)2TaH] [(=SiO)2TaH3] was employed for cross-metathesis reaction between toluene and ethane at 250°C. Under static condition, it produced mainly ethyl benzene and xylenes as major product along with propane and methane (Scheme 24). 

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... 

Solans-Monfort X, Chow C, Goure E, et al. Successive heterolytic cleavages of H2 achieve N2 splitting on silica-supported tantalum hydrides a DFT proposed mechanism. Inorg Chem. 2012 51 7237-7249. 

Various tungsten-hydrido compounds prepared on silica [38], silica-alumina [39] or alumina [40] supports have been tested in propane metathesis under batch conditions to compare their properties with those of the silica or alumina-supported tantalum hydride(s) 3 [41]. 

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... 

Synthesis and characterization of several catalyst precursors of [Ta] and [W] on different supports like silica [44], alumina [45], or silica-alumina [70] have been discussed in the previous section. These catalyst precursors were tested mainly in propane metathesis under similar condition in batch reactor at 150°C for 120 h. Catalytic performances revealed that the [Si02-Al203.5oo-W-H] and [AI2O3.500-W-H] have similar and better activity (TON) than the corresponding silica-supported tantalum-based catalyst precursors (Table 2). It was also observed that the product selectivity for tungsten hydrides is narrower than for tantalum hydrides [71]. 

Table 4 Hydrogenolysis of alkanes catalyzed by tantalum hydride supported on silica...

In contrast, tantalum hydride supported on silica gives mainly neopentane (31%), which indicates that the mechanism of carbon-carbon bond cleavage must involve the removal of one carbon at a time (in contrast to /3-alkyl transfer, which involves the removal of at least two carbons). 

Results in terms of TON indicate that silica-alumina or alumina-supported tungsten hydrides give very similar results but are twice as active as (=SiO)2TaH (3) and even more active than tantalum hydride on alumina or tungsten hydride on silica (Figure 3.12). 

Scheme 18.8 shows the results of Basset, which involve two types of metathesis catalyzed by a tantalum hydride supported on silica. In one case, an alkane forms methane and propane in the other case, methane and an alkane combines to form ethane and a higher alkane homolog. In practice, the reaction of ethane (Scheme 18.8, top) has been shown to form methane and propane, and the reverse reaction of propane (Scheme 18.8, bottom) with methane has been reported to form two equivalents of ethane. 

Scheme 28 Dinitrogen cleavage and partial hydrogenation mediated on a silica surface-supported tantalum(lll) and tantalumfV) hydride...

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