Tantalum complex

C2.7.6.4 PROPANE METATHESIS CATALYSED BY TANTALUM COMPLEXES ANCHORED TO SILICA... 

Increasing the temperature to 260-300°C leads to the decomposition of the ammonium-tantalum complex fluoride ... 

Kuterev, Stoyanov, Bagreev and Zolotov [461] reported on the investigation, using IR spectroscopy, of the composition of niobium and tantalum complexes extracted by amines from fluoride solutions. It was shown that in solutions that contain hydrofluoric acid in concentrations ranging from 1M to 12M, both niobium and tantalum were extracted in the form of NbF6 and TaF6 ... 

L.A. Kamenskaya Phys.-Chem. investigations of interactions between niobium and tantalum complex fluorides and oxyfluorides and potassium and sodium fluorides and chlorides in molten conditions. - Abstracts of Dissertation, Kiev, 1976 (in Russian). 

Investigations into the use of niobium and tantalum complexes as Lewis acids [99]... 

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. ... 

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

The chemistry of tantalum is different. The tantalum pentaalkyl complex does not exists, because it transforms easily into a carbene complex by a-elimination. This complex reacts also with silica, leading to a supported tantalum complex.262,263 Their reaction proceeds first by the addition of the silanol OH group across the tantalum-carbon double bond followed by elimination of an alkane (Scheme 7.19). 

Several group 5 complexes have also been examined as MMA polymerization initiators. A series of 1,4-diaza-1,3-diene tantalum complexes have been investigated and a mixture of (206)/ Me3Al was found to polymerize MMA in a living manner at — 30 °C (rr = 78%).500 501 Cp2TaMe3 also polymerizes MMA when activated with two equivalents AlMe3.235 However, initiator efficiencies are low and molecular weight distributions are broad. 

Two other isoelectronic tantalum complexes, 156 [254] and 157 [255], have recently been reported to show a similar catalytic activity for the polymerization of ethylene (2 kg PE/h[Ta]mol and 12 kg PE/h[Ta]mol, respectively) when they were activated with MAO. 

Although monodentate isocyanide-tantalum complexes are available by alternative routes,204 the majority of studies are concerned with the resultant 2-imines. Conversely, similar reactions with nitriles give monodentate imine moieties, which can undergo reactions while remaining attached to the metal.195,203-209... 

An intermolecular cyclotrimerization of an acetylenic nitrile was reported to proceed via the same alkyne-tantalum complex. The resultant pyridine derivative was obtained in 73% yield (Scheme 56).210... 

Acetylenes 191 (R1 = C5H11, Me3Si or MeS R2 = C5H11 or C10H21) react with tantalum ) chloride and zinc to form tantalum complexes. Addition of the imine... 

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... 

TaCl593 reacted with metallic sodium in neat trimethylphosphine to give the phosphinocarbene tantalum complexes 100 and 101, respectively. These reactions are the first examples of double activation of coordinated trimethylphosphine via oxidative cleavage of a substituent methyl C-H bond. A similar process was also observed in the reduction of tantalum pentabrom-ide with magnesium turnings in the presence of dimethylphenylphosphine.94... 

In the case of the tantalum complexes 100, reversible hydrogen migration may occur at room temperature in the presence of carbon monoxide, or at 70°C with dihydrogen or dimethylphosphino ethane to afford complexes 106, 107, and 108, respectively.92,95 In contrast, the phosphametallacycle remains intact when 100 is treated with halogenated reagents such as CH3X (X = Cl, Br, I).92,95... 

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... 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

Probably the first isolated tungsten alkylidene complex active in metathesis and completely characterised is the one shown in Figure 16.10 reported by Wengrovius and Schrock the analysis included an X-ray structure determination by Churchill and co-workers [18], The alkylidene was transferred from a tantalum complex to yield the hexacoordinate tungsten complex containing two PEt3 ligands. One of these can be removed by the addition of half an equivalent of palladium chloride. The total turnover number of these catalysts with Lewis acids added was 50 in 24 hours. 

Stoichiometric Activation of Cyclic Alkanes by Tantalum Complexes... 

To follow the mode of grafting of an organometallic species onto Si02, the tris (neopentyl)neopentylidene tantalum complex has been used to show the reactivity of its groups with silanol groups of silica (dehydroxylated at 300 to 700 °C)... 

Scheme 7.1 Mode of grafting a tantalum complex onto silica.

A decrease in metal-carbon distances when going down a group has also been pointed out. However, if this observation can roughly be applied to Zr and Hf (owing to lanthanide contraction), it does not fit the data obtained for Ti (Table 11.1). In contrast, for tetracoordinated complexes of the third transition row, replacement of just one neopentyl ligand is reported only when silica was pretreated at 700°C [18, 19, 23, 24]. For the tantalum complex [Ta(=CHCMe3)Np3, its... 

Low-valent niobium and tantalum react with imines to give the corresponding imine-niobium or -tantalum complexes. Some of these are isolated, and structures of the complexes are confirmed by X-ray analyses. These imine complexes react with aldehydes to give /3-hydroxyamines in good to excellent yields (Equation... 

In a similar manner, the reaction of 6.28 with CuCl in CH2CI2-CH3CN produces the spherical complex [ Cp Ta(CO)2(t " -P4) 6 CuCl g] (6.31). The tantalum complex 6.31 is comprised of 32 core atoms forming four-membered P4 rings and six-membered P4CU2 rings in an extended cubic arrangement with... 

There are a number of ways that nitriles may be converted to alkylimido ligands. Low valent niobium and tantalum complexes have been shown to reductively couple nitriles to give bridging... 

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