Tantalum amidate complexes

Di(carbene)gold(I) salts, oxidation, 2, 293—294 Dicarbido clusters, with decarutheniums, 6, 1036 Dicarbollide amides, with tantalum, 5, 184 Dicarbollide thorium complexes, synthesis and characterization, 4, 224—225 Dicarbollyl ligands, in nickel complexes, 8, 185 Dicarbonyl complexes arylation with lead triacetates diastereoselectivity, 9, 389 enantioselectivity, 9, 391 mechanisms, 9, 387 reaction examples, 9, 382 indium-mediated allylation, 9, 675 with iridium, 7, 287 reductive cyclization, 10, 529 in Ru and Os half-sandwiches, 6, 508 with Zr—Hf(II), 4, 700... 

These observations, along with kinetic isotope effect studies and Hammett correlation studies, support a concerted elimination by a-bond metathesis involving a four-membered transition state (Eq. 2) [23]. A large kinetic isotope effect is observed for the loss of methane from methyl amide complexes lb and lh (Eqs. 3 and 4), comparable to those observed by Buchwald and coworkers for formation of zirconium rj2-thioaldehyde complexes [25] and by Bercaw and coworkers for formation of tantalum rf- imine complexes [5a] through similar transition states. 

Scheme 3.58 Regioselective synthesis of trisubstituted a,(i-unsaturated esters and amides with tantalum-alkyne complexes.

The development of niobium and tantalum dithiocarbamate chemistry is relatively recent, the first well authenticated examples coming from the insertion of carbon disulfide into metal amide complexes to give [M(S2CNR2)4] (193, 666). Dithiocarbamates are now known to stabilize niobium and tantalum in the +3 to +5 oxidation states. 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

Closely related mixed amido/imido/guanidinato tantalum complexes of the type Ta(NR R )[(R R2N)C(NR )2]( = NR ) (R R = Me, Et R = Cy, Pr R = Pr", BuO were synthesized by the insertion of carbodiimides into to tantalum-amide bonds in imidotantalum triamide precursors, and the effects of ligand substitution on thermal properties were studied by TGA/DTA measurements. In addition, selected compounds were pyrolyzed at 600 °C and the decomposition products were studied by GC-MS and NMR spectroscopy. ... 

Besides the phosphoranes, the homoleptic amides of niobium(V) and tantalum(V) are the only pentamides known.1 These compounds, exemplified by M(NMe2)5 (M = Nb, Ta) are volatile, hydrocarbon-soluble solids.37 All physical and spectroscopic data are consistent with a strictly mononuclear existence for these complexes. 

Tantalum complexes alkyl alkoxy reactions, 358 amides synthesis, 165 imido... 

Mass transport may influence material growth in ionic liquids. 1-Butyl-l-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide, for example, is, at room temperature, about 60 times more viscous than water. At temperatures above 150°C its viscosity is similar to most molecular solvents at ambient conditions. Indeed, temperatures between 150 and 200 °C were best to deposit tantalum from TaFs in the presence of LiF. One has to keep in mind that the deposition of tantalum from TaFs or an anionic complex delivers one Ta atom and 5-7 fluorides. If the deposition is too fast F may not diffuse rapidly enough from the surface to the bulk of the solution and may be trapped in the deposit. This might explain why we only got crystalline tantalum layers at low current densities. 

Schrock discovered that the r-butylalkylidene complex of tantalum (and in lower yield, niobium) was a structural analog to the phosphorus ylide. The complex proved to be a reagent for r-butylalkene formation. Of the Wittig-type reactions tried, most notable was the ability of the complex to react with esters and amides to form the corresponding r-butylalkenes in good yields. 

Group 5 pyridonate complexes are also readily prepared by salt metathesis and protonolysis routes to access both mono- and bis(pyridonate) tantalum complexes (Figure 12). [41] Such complexes can be difficult to rigorously characterize in the solid state as they are much more soluble than their group 4 pyridonate, or their group 5 amidate, counterparts. 

Scheme 19 intramolecular hydroaminoalkylation of unactivated olefins with secondary amines catalyzed by mono- and bis(amidate) tantalum ami do complexes... 

The combined action of zinc and tantalum(V) chloride on alkynes generates low-valent tantalum-alkyne complexes, which react with carbonyl compounds to yield ( )-allylic alcohols stereoselectively. The tantalum-6-dodecyne complex, for instance, and j -phenyl-propionaldehyde afford the alcohol 370 in 96% yield. Mixtures of structurally isomeric a,/ -unsaturated ( )-amides 371 and 372 (R, = alkyl or Mc3Si) are obtained from the... 

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