Tantalum dimer

In Discrete Niobium and Tantalum Dimeric and Trimeric Complexes... 

Inner-Transition Metal to Transition and Inner-Transition Metal Bond 67 9.2.8. Formation of Heavy Transition Metal Group V Metal-Metal Bonds 9.2.8.3. In Discrete Niobium and Tantalum Dimeric and Trimeric Complexes... 

I am not going to discuss early transition metal oxide or alkoxide chemistry here. Instead I would like to use the few minutes allotted me to describe out work with low valent dimers of tantalum and tungsten. Professor McCarley will no doubt see his influence on our work and forgive this digression from the topic of his excellent presentation. 

Now I would like to leave tungsten chemistry and describe a few of our results in low valent tantalum chemistry. Our entry into this area began with the synthesis (reaction 2) and structural characterization (Figure 3) of the tantalum(III) dimer,... 

In 1982, Cotton et al prepared the first phosphinocarbene tantalum complex 99 from the reaction of Ta2Cl6(SMe2)3 with bis(diphenylphosphi-no)acetylene. The most remarkable feature of this reaction is the formal dimerization of the alkyne moiety.91... 

Several studies have been concerned with the chemistry of the + ni oxidation state of these elements, and the characterization of the first tantalum(iii) compounds has been claimed. The diamagnetic dimer [TaCl3(MeCN)2]2 has been prepared and used to obtain [TaClafphen)], [TaCljfbipy)], and tris-(dibenzoylmethanato)tantalum(ni). NbFa has been characterized as the product of the reaction of Nb and NbF (1 1) at 750 °C under pressure. Electrolytic reduction of niobium(v) in ethanol,formamide, and dimethylformamide can afford preparative concentrations of niobium(iii) and the new compound niobium(iii) trilactate has been obtained from ethanol. 

Niobium(m) and Tantalum(in) Complexes.—The first tantalum(iii) compounds have been reported. TaCl and MeCN react at 100 °C to form the diamagnetic dimer [TaCl3(MeCN)2]2- The compound exhibits five i.r. bands in the 23(X) cm region, suggesting more than one environment for the MeCN groups the structure (44), analogous to that of [WCl3(py)2]2, has been proposed for the... 

Niobium and tantalum compounds form adducts with virtually all types of neutral ant anionic donors. The coordination chemistry of the higher halides is widely developed, and thei activity as Friedel-Crafts catalysts is another manifestation of their Lewis acidity. The stron acceptor capacity of the high valent metal compounds tends to favor the formation of dimers and sometimes of higher condensation products, which competes with coordination with othe donor molecules. Numerous simple anionic or heteropolyanionic species, but little cationi chemistry, and no simple metal salts, are known. 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

See also Scheme 17 for the template synthesis of tri(te/t-butyl)-Cp from a dimeric tantalum alkylidyne complex and 3,3-dimethyl-1-butyne. Schemes 8 and 9 show the syntheses following method 2. The 1,3,5-substi-tuted tri(/m-butyl)cyclopentadiene is obtained as the single isomer from both routes. 

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