Tantalum, hydride compound

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]. 

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]. 

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. 

Treatment of tantalum hydride complexes with hexamethylsilacyclopropane at 65 °C in the presence of a Lewis base gave the corresponding silyl-tantalum compounds in excellent yield. It appears that this reaction takes place via direct insertion of dimethylsilylene into the tantalum-hydride bond, which would make it the first example of silylene trapping by a transition-metal substrate (Equation (15)) <87JA6210>. 

Selective dimerization of ethene to 1-butene was reported for the tantalum hydride 51 [28]. The reaction proceeds through hydrotantalation of ethene to give alkyl tantalum compound, followed by the insertion of another molecule of ethene and j0-hydrogen elimination. 

We then studied group 5 metals, especially tantalum-for which the laboratory already had great experience. Because of the studied reaction, alkyl or hydride-type compounds such as those developed in the laboratory could not be employed. Consequently, we became interested in alkoxo-type derivatives, either synthesized by reaction of the grafted complex with an alcohol or obtained by direct synthesis starting from an alkoxy-tantalum compound grafted on silica. In all cases, resulting complexes have been characterized by surface organometallic chemistry techniques, especially EXAFS and solid-state NMR (ID and 2D with C-labeled compounds). Indeed various compounds bonded by one, two or three surface bonds have been prepared and characterized. 

Niobium and tantalum chemistries are frequently very similar, and it is not surprising to find close parallels in their hydrides with respect to which compounds are known and to their properties. However, the two sets of compounds have frequently been discovered by independent workers with different objectives, and this is reflected in the current status of our knowledge of the hydrides. 

Metallacyclic (see Metallacycle) complexes of niobium and tantalum play an important role in understanding several catalytic and stoichiometric transformations of organic compounds. Some group 5 metallacycles are formed from the inter- or intramolecular hydride abstraction reactions. Most of the Nb and Ta metallacycles are prepared, however, from reductive coupling (see Reductive Coupling) of unsaturated organic substrates. To be included in this section, the metallacyclic ligand must have at least one M-C bond. 

---