Tantalum hydrides

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). 

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-... 

Tantalum halides, 24 334—335 Tantalum hydride, 13 627 Tantalum junctions, 23 870-871 Tantalum metal, 24 314... 

All these results lead to a more precise representation of the heterogeneous environment of the metal in various sites of surface tantalum hydride, taking into account mono- and tris-hydrides as well as the varying presence of siloxy bridges close to metallic centers (Scheme 2.19). 

In situ IR monitoring of the reaction of the tantalum hydride with regular NH3 and and after H/D exchange, has yielded the determination of all the NH, ... 

Mechanistically, the unusual reactivity of the starting tantalum hydrides [(sSiO)2Ta H] and [(=SiO)2Ta (H)3] towards ammonia to yield the amido imido complex [(=SiO)2Ta(=NH)(NH2)] can be fully rationalized in terms of classical molecular organometallic elementary steps. Scheme 2.21 offers an example of a such sequence of elementary steps with the noteworthy close analogy with methane activation by the same hydrides described above. 

In summary, the tantalum hydride system adds to the few previously reported well-defined organometallic complexes capable of cleaving N-H bonds of ammonia to yield either an amido or an imido complex, and achieves unprecedented dinitrogen N=N triple bond cleavage with dihydrogen on isolated tantalum atoms to yield reduction of both N atoms. 

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

In the case of ethane, this mechanism cannot occur since the resulting metal-ethyl intermediate does not display any alkyl group in the P-position. Consequently, with tantalum hydride(s), 3, which cleave ethane, another process must take place, involving only one carbon atom at a time. Among various reasonable possibilities, we assume a carbene deinsertion from a tantalum-ethyl species because the reverse step is known in organometallic chemistry (Scheme 3.4) [22]. Note that this reverse step has been postulated as the key step in Fischer-Tropsch synthesis [23]. 

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... 

Table 3.3 Comparison of selectivities for the C5 products obtained with the zirconium and tantalum hydrides during 2,2-dimethylbutane hydrogenolysis at low conversion (<5%) .

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

Therefore, the major differences between zirconium and tantalum hydrides are... 

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 ... 

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]. 

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). 

Reaction between methane and propane requires the right conditions. Firstly, it has a positive free energy of AG° = -1- 2kcalmoT at 150 °C for amethane/propane ratio of 1 but this can be overcome by increasing this ratio, which for a value of 1250 allows 98% propane conversion at 250°C. Secondly, it has to be separated from other reactions catalyzed by tantalum hydride, such as propane hydrogenoly-sis, leading to 1 equiv. of methane and 1 equiv. of ethane, or propane metathesis, leading to 0.5 equiv. of ethane ... 

However, over the past decade, advances in, and in particular the availability of sophisticated instrumentation, and in the understanding of the instrumental techniques and the hosts and guests to which they are applied, mean that this need no longer be the case. A recent example in which a gamut of carefully chosen techniques, including such basic but essential measurements as elemental analyses, has led to the same precise characterization of surface species as has been the mainstay of molecular compounds is the study of the synthesis, characterization and reactivity of tantalum hydrides on silica, and their involvement in the dissociation of dinitrogen [203]. 

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... 

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