Tantalum metallacycles

Green et al. observed that the tantalum complex 101 is converted into the bicyclic complex 109 after prolonged exposure to sodium and trimethyl-phosphine.930 Two phosphine ligands of 101 are easily displaced by butadiene, affording the complex 110, while treatment of 109 with dihydrogen leads to the reduction of the metallic center. In both cases, the metallacycle remains intact.930... 

Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds i remarkable in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive metallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. 

Although there are few five-membered metallacycles reported for group 5, there are some examples of metallacyclopentane compounds for niobium and tantalum. The reaction of the labile vanadium naphthalene complex [CpV(Ci0H8)] with ethene provided the unusual binuclear complex [Cp2V2(p-C4H8)2] (34 [Eq. (13)], Fig. 9) in which the two vanadium centres are bridged by two butanediyl groups.38... 

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. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

This is to be compared with the products of codimerization of ethylene with 1-butene (R = Et vide supra). As for tantalum, the /3-substituted metallacycle preferentially forms over the a-derivative. 

Carbonyl Olefination Using Zirconium, Tantalum, Niobium, Molybdenum 1191 Tab. 4.18. Methylidenation of aldehydes with molybdenum metallacycles 57. 

The cycloolefin-tantalum complex (r] -C5Me5)Ta(cyclooctene)Cl 48 is capable of dimerization of various olefins into a mixture of the head-to-tail and head-to-head dimers 49 and 50 (Scheme 21) [27]. Some representative results are summarized in Table 10. It was shown that the dimerization process proceeds through series of reversible metallacycle formations such as tantalacy-clopentanes and tantalacyclobutanes, and jS-hydrogen eliminations. The dimerization ends in reductive elimination of the organyl moiety from the intermediate alkylhydridotantalum species. 

The tantalum hydride (18) reacts reversibly with ethylene and at 70 °C dimerises ethylene selectively to 1-butene. The isomerization to 2-bu nes occurs only after the ethylene is consumed. The reaction (Scheme 15) is slow. An alternative mechanism via metallacycles could not be ruled out. ... 

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