Alkane tantalum hydrides

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

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

Alkane C-C Bond Activation by Tantalum Hydrides. Low Temperature Catalytic Hydrogenolysis of Alkanes... 

Obviously, the alkane hydrogenolysis reaction highlights a clear difference in behavior between supported-hydrides of group 4 metals and (=SiO)2Ta(H),g (3) tantalum hydride. This difference in behavior can further be illustrated as follows ... 

On the one hand, a comparison of initial rates for various alkane hydrogenolysis shows a marked difference between zirconium and tantalum hydrides (Table 3.2). Indeed, the reaction rates for [(=SiO)(4.,<)Zr(H)J, x = 1, 2) are weakly dependent on the nature of the alkanes. They are higher than those obtained... 

Tantalum hydride(s) also catalyzes the hydrogenolysis of cyclic alkanes (substituted or not) but the reachvity order decreases with the cycle size as cycloheptane > methylcyclohexane > cyclohexane > methylcyclopentane > cyclopentane for the latter no reaction is actually observed (Figure 3.8). Activity decreases with hme and becomes low after 20 h. 

A study of the stoichiometric cyclopentane reaction over Ta-H has revealed that tantalum hydride very easily achvates cyclopentane, forming the corresponding cyclopentyl derivative. However, the latter is very quickly transformed into a cyclo-pentadienyl compound, as shown by NMR and EXAFS studies. This cyclopenta-dienyl derivative presents no achvity in alkane hydrogenolysis ... 

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... 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

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 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. 

In 1997, Basset et al. introduced catalytic transformation of acyclic alkanes into their lower and higher homologues using sihca-supported tantalum hydrides [1] in the absence of hydrogen at low temperature (150°C). 

As aheady mentioned, it was observed that one mole of hydrogen is liberated when methane is reacted with the tantalum hydride with the formation of tantalum methyl. The reaction with methane above 150°C leads to the formation of the Ta-methyl, Ta-methylene, and Ta-methylidyne species plus H2 (M=Ta) [40-42, 54]. These observations are a proof that the first step of alkane metathesis is the formation of metal alkyl intermediate via cleavage of the C-H bond of the alkane likely by sigma bond metathesis. Further, detailed mechanistic [22, 55] and experimental kinetic studies revealed that the alkenes and hydrogen are the primary products [56]. Initially, it was believed that the active site was a bis-siloxy tanta-lum-monohydride, but progressively, evidence came in favor of an equilibrium between bis-siloxy tantalum-monohydride d and bis-siloxy-tantalum-tris-hydride d° [57], and the mechanism would fit much better with a bis-siloxy-tantalum-tris-hydride [58]. 

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... 

Having set out the properties of tantalum and zirconium hydride toward C-H bond activation of alkanes we now describe the catalytic hydrogenolysis of C-C bonds. It was previously shown in the laboratory that supported-hydrides of group 4 metals, and particularly of zirconium, catalyze the hydrogenolysis of alkanes [21] and even polyethylene [5] into an ultimate composition of methane and ethane. However, to our initial surprise, these zirconium hydrides did not cleave ethane. (=SiO)2Ta-H also catalyzes the hydrogenolysis of acyclic alkanes such as propane, butane, isobutane and neopentane. But, unlike the group 4 metals, it can also cleave ethane [10], Figure 3.7 illustrates this difference of behavior between (=SiO)2Ta(H) and [(=SiO)(4.j,)Zr(H) ], x= or 2). With Ta, propane is completely transformed into methane by successive reactions, while with Zr only equimolar amounts of methane and ethane are obtained. 

Mono(siloxy) metalhydrocarbyl species can be converted into bis- or tris(siloxy) metal hydrides by reaction with hydrogen, as shown for zirconium and tantalum. Such species are less susceptible to leaching and this route can be extended to titanium and hafnium surface species that are potential precatalysts for hydrogenolysis of C-C bonds, alkane metathesis and epoxidation reactions. 

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