Niobium and tantalum alkoxides

Yamaguchi was the first to study hydrolysis ofW(OEt)6 [1766], Hydrolysis performed in situ at 20 and 80°C yielded amorphous blue precipitate or green-yellow crystals of W03 H20, respectively. The final crystallizationproduct formed on thermal treatment of both precipitates was monoclinic W03, while in the course of thermal treatment at 350°C, the cubic W03 was also registered. 

In the first study of hydrolysis of Fe(OEt)3 carried out by Thiessen et al. [1577], the precipitate of cubic crystals Fe203 4H20 was obtained after refluxing of diluted alcoholic solution of iron ethoxide for a few hours. Hydrolysis of iron ethoxide by excess of water results in amorphous precipitate, which on thermal treatment at first gives y-Fe203 and then a-Fe203 (in contrast to the precipitates of (X-FeOOH, which precipitate from aqueous solutions) [1776]. 

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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. 

The numerous alkoxide halides of niobium and tantalum containing the atoms of all the known halogenes from F to I and a broad range of radicals are considered in the Table 12.17. 

Preparation of heterometallic alkoxides between pairs of metals as similar to each other as aluminum and gallium (137) or even niobium and tantalum (138) were described. However, the formation constants of the latter derivative were found to be rather low, precluding its isolation in view of the dynamism of the equilibrium. 

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. ... 

Niobium and Tantalum Tidile 8 Double and Mixed Alkoxides... 

The alkali alkoxide method has been extended to the preparation of alkoxides of the hexanuclear niobium and tantalum cluster units, e.g., [M6Xi2](OMe)2.4MeOH (where M = Nb or Ta and X = Cl or Br), and M2[Ta2Cli2](OMe)6.6MeOH. 

The alkylation of alkoxides of niobium and tantalum rarely lead to total substitution. Typically the reaction is free from any complication and the products are pure up to the trisubstitution stage. 

In addition to the formation of alkali alkoxometallates by the reactions of alkali alkoxides (strong bases) with alkoxides of a variety of metals and metalloids (Lewis acids), formation of heterometal alkoxides has been shown to occur even between alkoxides of such similar metals as aluminium and gaUium as weU as niobium and tantalum. However, the formation constant of the latter derivative has been found to be statistical, which precludes the isolation of this bimetallic alkoxide in view of the equilibrium ... 

Bradley D.C., Holloway R.C. Nuclear magnetic resonance on niobium and tantalum penta-alkoxides. J. Chem. Soc. 1968 A 219-223... 

Alkaline-earth alkoxides are insoluble and involatile in contrast to the double alkoxides formed by dissolving the alkaline earth metal in a solution of niobium or tantalum isopropoxide in propan-2-ol. With the exception of the magnesium derivative which decomposes on heating, they can be sublimed in vacuo, thermal stability diminishing in the order Ba > Sr > Ca. The calcium and strontium derivatives are monomeric in refluxing benzene, the others are insoluble. Attempts to elucidate structures by n.m.r. spectroscopy were foiled by the rapid exchange of geminal dimethyl protons (see also p. 66). 

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

Reactions similar to those represented by Eq. (2.264) have been observed for primary and secondary alkoxides of beryllium, yttrium and lanthanides/" titanium and zirconium/" ° ° vanadium,niobium,tantalum, " uranium, iron, aluminium, gallium, silicon, germanium, as illustrated by the following equations in a few typical cases ... 

Following the procedure of Meerwein and Bersin and others, - - this method has been extended to the bimetallic alkoxides of alkali metals (Lewis bases) with those of less basic metals and metalloids, beryllium, zinc, - boron, aluminium, gallium,tin(n), " tin(iv), antimony(iii), bismuth, titanium," niobium(iv), zirconium, " thorium, niobium(v), tantalum(v) and copper. Equations (3.1)-(3.3) reflect a few typical reactions used in the synthesis of bimetallic alkoxides involving alkali metals (M) ... 

Johansson A., Roman M., Seisenbaeva GA., Kloo L., Szabo Z., Kessler V.G. The solution thermolysis approach to molybdenum(V) alkoxides synthesis, soUd state and solution structures of the bimetallic alkoxides of molybdenum(V) and niobium(V), tantalum(V) and tungsten(VI). J. Chem. Soc., Dalton Trans. 2000 387... 

Ammonia and alcohol may be used instead of sodium alkoxides to manufacture alkoxides of titanium and other metals such as tirconium, hafnium, germanium, niobium, tantalum, aluminum, and tin. 

Vinyl ethers have also been prepared by addition of alkoxides to acetylene,6 7 6 elimination from halo ethers and related precursors,6 8 and vinyl exchange reactions.6 Reaction of an electrophilic tungsten carbenoid with methylene phosphorane or diazomethane also produces vinyl ethers.9 Enol ethers have resulted from the reaction of some tantalum and niobium carbenoids with esters,10 and the reaction of phosphoranes with electrophilic esters.4... 

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