Tantalum, hydride formation

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-... 

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. 

The emphasis in the treatment that follows is on compounds that can be isolated, and the transient tantalum hydride species postulated in, for example, the work of Schrock et al. on alkylidenes 184, 194) are only briefly mentioned. The subject of metal hydride formation during hydrogen elimination reactions is worth a review in its own right, and indeed several exist. One of the most recent deals primarily with osmium hydrides and alkyls but is relevant to tantalum also 195). It can be consulted for other references, and the subject is not treated further here. 

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. 

Because of their tendency to hydride formation, the embitterment of Ti, zirconium (Zr), and tantalum (Ta) by hydrogen is particularly pronounced. The proneness to gas absorption restricts the use of these metals. They are not only attacked by nascent hydrogen they also become brittle at... 

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]. 

Tantalum has a high solubility for hydrogen, forming two internal hydrides, but the exact mechanism of their formation is not precisely known. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

In some cases, hydrogenation of the alkylidenes and alkylidynes reduces the metal-carbon multiple bonds to single bonds. The alkyhdene hgand in (29) is converted to an alkyl gronp when exposed to H2, leading to the formation of an interesting tantalum hthium bridging hydride complex. 

Reduction of Ta(silox)3Cl2 with Na/Hg leads to a three-coordinate alkoxide complex Ta(silox)3. The coordinatively unsaturated tantalum complex is capable of cleaving H2 and O2 bonds resulting in the hydride and 0x0 complexes as illustrated in Scheme 7.14. Carbon monoxide is also split upon carbonylation of Ta(silox)3 generating the 0x0 and p-dicarbide complexes. This reaction models the C—O bond cleavage and C—C bond formation believed to occur in the Fischer-Tropsch reaction, and the ketenylidene complex Ta(silox)3(=C=C=0) was postulated as the key intermediate. On the other hand, when Ta(silox)3 was treated with pyridine and benzene, remarkable T -coordinated complexes were formed. 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

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