Tantalum alkoxide

A combination of DAT and a metal alkoxide other than titanium alkoxide serves as a poor catalyst for the epoxidation of allylic alcohols. However, the combination of DAT and silica-supported tantalum alkoxides (2a) and (2b) prepared from Ta(=CHCMe3)(CH2Cme3)3 and silica(5oo) shows high enantioselectivity in the epoxidation of E-allylic alcohols, though chemical yields are not very great (Scheme 4).3... 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

Studies ofniobium and tantalum alkoxides were started by Bradley in the 1950s as apart of the studies of a broad range of derivatives ofrare metals for the separation of the elements from the mineral raw materials. 

The bimetallic complexes are not formed on direct interaction of either molybdenum or tantalum alkoxides. The introduction of steichiometric amounts of water into the solution of corresponding methoxides in methanol with subsequent evacuation and recrystallization from toluene leads to the formation with high yield of the bimetallic polyoxocomplex [Mo02Taj02(OMe)8]2 [847]. 

While the molecular tantalum catalyst Ta(OCH2CH3)5 exhibited very poor activity for epoxidation under Sharpless conditions, the surface-supported analogue [a mixture of 70% =SiOTa(OCH2CH3)4 and 30% (=SiO)2Ta(OCH2CH3)3] was shown to have activity comparable with that of the molecular Ti catalyst. Furthermore, excellent enantiomeric ee values (up to 94%, compared with 96 % for Ti[OCH(CH3)2]4 under the same conditions) were obtained. An inversion of the the major enantiomer obtained was observed for both the molecular and supported tantalum catalysts, i. e., the association of tetraisopropyltitanium and (+)-diisopropyltartrate produces (/f)-epoxide whereas the Ti catalyst with (+)-diisopropyltartrate produces the (S)-epoxide. The putative active species, =SiOTa(OCH2CH3)2[(+)-(DET)] (Structure 18) has also been synthesized and tested (eq. (3) [23 a]) Further improvements of catalyst activity have been obtained by modification of the support and refinement of the synthesis of the supported tantalum alkoxide precursor. 

Tantalum oxide on silica, made by treatment of silica with tantalum alkoxides, is a catalyst for the Beckman rearrangement of cyclohexanone to caprolactam (6.9).49 The catalyst lasted at least 10 h in operation. This process eliminates the by-product ammonium sulfate from the present commercial rearrangement in sulfuric acid. Caprolactam is polymerized to produce nylon-6. Alkylations can also be performed with iron oxide catalysts (6.10).50Other alcohols, such as n-propyl and /t-butyl alcohols, also worked. Highly acidic catalysts can also be obtained by supporting tungsten oxide on iron, tin, and titanium oxides.51... 

Hubert-Pfalzgraf, L. G and Riess, J. G., Isolation of a mixed niobium-tantalum alkoxide, Inorg. Chem., 15, 1196 (1976). 

To obtain a fine, transparent film, HCl should be added during the hydrolysis of tantalum alkoxide. An amount between 0.5 and 4.0 mol% is best. 

The reduction of a tantalum alkoxide with sterically demanding ligands [(RO)2TaCl3] (R = 2,6-PriC6H3) by sodium amalgam in the presence of 2-butyne gives a dewar-benzene complex which can be seen as a model for the last intermediate in the cyclotrimerization of alkynes to arenes see equation (49). It... 

K. Thome, E. Liimatta, and J. D. Mackenzie, Synthesis of SiC/TaC ceramics from tantalum alkoxide modified polycarbosilane, J. Mater. Res. 1991, 6, 2199-2207. 

Mesoporous oxides from elements other than sihca have been reported as early as 1994. Cieslaetal. [169] found that metals such as Sb, Fe, Zn, Pb,W, and Mo also form mesoporous oxides. However, many of the mesophases obtained were lamellar and were not porous after template removal (calcination). Antonelli and Ying reported the transformation of titanium, niobium, and tantalum alkoxides into stable mesophases [170]. Subsequently, mesoporous oxides based on zirconium, hafnium, and manganese have been synthesized (for a recent review on these materials see [171]). Bagshaw and Pinnavaia [172] prepared mesoporous alumina with worm-like pores and a specific surface area of more than 400 m g . Mesoporous alumina with surface areas above 700 m g have been reported by Vaudry et al. [173]. 

The pentavalent niobium and tantalum alkoxides are reasonably volatile and can be distilled unchanged in the pressure range 0.05-10mm, e.g. pentamethoxides of niobium" and tantalum" have been distilled at 153°C/0.1 or 200°C/5.5 and 130°C/0.2 or 189°C/10.0 mm pressures, respectively. The data in Table 2.12 indicate that the boiling points are dependent on chain length furthermore, the methoxides and ethoxides of tantalum are more volatile than the niobium analogues whereas for higher n-alkoxides, the reverse is... 

The volatilities of both the niobium and tantalum alkoxides decrease with increasing chain length. The mixed alkoxides of tantalum have been found to be thermally stable Ta(OMe)(OPr )4, Ta(OEt)(OPr )4, and Ta(OMe)(OBu )4 distil unchanged at 81°C/0.07, 130°C/0.1, and 96°C/0.05mm, respectively. 

Barraclough et studied the infrared specda of a number of metal (aluminium, titanium, zirconium, hafnium, niobium, and tantalum) alkoxides and tentatively assigned the v(C-0)M and v(M-O) bands in these derivatives. It has been observed... 

On the basis of variable temperature NMR studies HoUoway reasserted that in dimeric niobium and tantalum alkoxides, the terminal-terminal alkoxide exchange occnrs at a faster rate than terminal-bridging exchange. ... 

Donat, M., Seisenbaeva, G A., and Kessler, V.G. (2008) Synthesis of highly sterically hindered niobium and tantalum alkoxides and their microhydrolysis in strongly basic medium. /. Sol-Gel Sci TechnoL, 48, 61-65. 

Figure 27.10 Ferroelectric strontium bismuth the larger grain size of the surface microstruc-tantalate (Sro BBi2.2Ta209) thin films prepared tures of the films prepared from the tantalum from the sols of Figure 27.9 (a) tantalum pen- pentaglycolate sols and the larger values of taethoxide sols and (b) tantalum pentaglyco- the remanent polarization (5Pr) obtained in late sols. SEM images of the top surfaces of these films compared with the films prepared the films are shown, as well as the ferroelectric from the tantalum alkoxide-based sol [47]. hysteresis loops measured in these films. Note...

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