Vanadium alkoxides

Ihere are also some reports on the use of metal alkoxides (vanadium [39, 40], titanium [15, 41, 42], zirconium [15, 43], hafnium [15], and niobium [44]) as pre-catalyst along with organometalllc alumimun com-poimds (chlorodiethyl [39], chlorodimethyl [40], triethyl [41], tri-isobutyl [42] and tri-octyl [42], methylaluminoxane [ 15,43,44]) in olefin (ethylene [15,39-44], propylene [15], 1-hexene [39]) polymerization. The role(s) of metal alkoxides was (were) not studied precisely in these reports and so will not discuss more in this section. 

Vanadium Alkoxides. Except for the soHd methoxide, the lower vanadium alkoxides are slightly colored, yeUow, or yeUow-brown Hquids. They are easily hydroly2ed and decompose on heating above 100°C they darken. They are made from V20 or VOQ. -Vanadium alkoxides are used mostly in olefin polymeri2ation as catalysts also as hardeners and for coatings. 

Titanium—Vanadium Mixed Metal Alkoxides. Titanium—vanadium mixed metal alkoxides, VO(OTi(OR)2)2, are prepared by reaction of titanates, eg, TYZOR TBT, with vanadium acetate ia a high boiling hydrocarbon solvent. The by-product butyl acetate is distilled off to yield a product useful as a catalyst for polymeri2iag olefins, dienes, styrenics, vinyl chloride, acrylate esters, and epoxides (159,160). 

Mixed liquor volatile suspended solids (MLVSS), in biological waste treatment, 25 829-830 Mixed-metal alkoxides, titanium-vanadium, 25 100... 

Niobium Products Co., 50 m /g). Many different synthesis methods have been used to prepare supported metal oxide catalysts. In the case of supported vanadium oxide catalysts, the catalysts were prepared by vapor phase grafting with VOCI3, nonaqueous impregnation (vanadium alkoxides), aqueous impregnation (vanadium oxalate), as well as spontaneous dispersion with crystalline V2O5 [4]. No drastic reduction of surface area of the catalysts was observed. 

To investigate the effect of the synthesis method on the structure-reactivity relationship of the supported metal oxide catalysts, a series of V205/Ti02 catalysts were synthesized by equilibrium adsorption, vanadium oxalate, vanadium alkoxides and vanadium oxychloride grafting [14]. The dehydrated Raman spectra of all these catalysts exhibit a sharp band at 1030 cm characteristic of the isolated surface vanadium oxide species described previously. Reactivity studies with... 

If we consider the d0 metal-N,JV-dialkylhydroxylamino complexes (79), (80) and (81) as valid models for the reactive but unstable alkyl peroxide species Mo02(OOR)2, VO(OOR)3 or V203(00R)4, and Ti(OOR)4 presumably involved in catalytic oxidations, the low activity of vanadium and titanium for the epoxidation of simple alkenes could be interpreted by the fact that these alkenes cannot displace the O.O-bonded alkyl peroxide groups in the coordinatively saturated Vv- and Tiiv-alkyl peroxide species, whereas allylic alcohols can displace the alkyl peroxide groups by forming bidentate allylic alkoxides as in equation (75).162... 

VO(OR)3 — the alkoxides of vanadyl (V) or otherwise the esters of vanad-ic acid — belong to the most easily available derivatives and can be easily purified by distillation under low vacuum. The synthesis of these derivatives — from ethoxide to t-amyloxide—via alcoholysis of V2Os was first described at the beginning ofthe twentieth century by Prandtl and Hess [1313,743], The same work contained the first description ofthe hydrolysis of VO(OR)3, which laid basis for the numerous studies of the processes of formation of sols and gels of vanadic acid. The studies of the low-valent derivatives of vanadium that are very sensitive to oxidation were started in the 1960s by the works of Bradley on V(OR)4 and V(OR)3 [226]. The preparation, structure, and magnetic properties of vanadium(II) alkoxides were investigated only quite recently. 

It is worth noting here that in contradiction to the data published by Bradley [227] on the formation of VC12(0R)2 alkoxide chlorides on alcoholysis of VC14 his reaction was proved to yield VOCl2 nROH as the major products [607, 1634]. To avoid this reaction, the vanadium chloride should be introduced as an ether solution into the reaction with NaOR for the preparation of vanadium (IV) alkoxides. 

The thermal decomposition of vanadium alkxoides [24, 983, 1296] and their behavior in the gas phase (with the aid of mass-spectrometry) [11, 1613] were also studied in detail to optimize the CVD-techniques, applying the vanadium alkoxides as precursors. 

The alkoxides of vanadium have been tested as substituents for the titanium component in the Ziegler catalysts for about 30 years ago. The most efficient were found to be VO(OBu )3 and also V(OBu )3, and V(OBu )4 (see, for example, [434, 1266]). 

One of the most important applications of vanadium alkoxides is connected with their catalytical action on the reactions of oxidation of hydrocarbons and, in particular, alkanes, cycloalkanes, and so on. [645, 172], Except for VO(OR)3 it was proposed that the complexes already containing the peroxide groups in their composition — such as VO(OBu )2(OOBu ) [1516], VO(OR)(OOH)2, V02(00R) [172], and VIY(OPri)3(OOBu ) [25] — be used. 

Recently interest has been shown in studies of the biochemical processes, which include the action of enzymes with vanadium-containing centers (like those of serine or tyrosine) that catalyze or inhibiting the redox and phosphorylation reactions, and the alkoxides were investigated as the structural models for the latter [1319, 424], A bimetallic phenoxide of vanadium (II, HI) was found to be an efficient reducive agent for molecular nitrogen in the methanol solutions [1034],... 

The phenoxides of Nb(II) and alkoxides of Ta(III), as well as the corresponding derivatives of vanadium should be stabilized by the sterically hindered ligands. The alkoxide chlorides of niobium (Eft) and (TV) are diamagnet-... 

The asymmetric oxidation of sulphides to chiral sulphoxides with t-butyl hydroperoxide is catalysed very effectively by a titanium complex, produced in situ from a titanium alkoxide and a chiral binaphthol, with enantioselectivities up to 96%342. The Sharpless oxidation of aryl cinnamyl selenides 217 gave a chiral 1-phenyl-2-propen-l-ol (218) via an asymmetric [2,3] sigmatropic shift (Scheme 4)343. For other titanium-catalysed epoxidations, see Section V.D.l on vanadium catalysis. 

Crans, D.C., R.A. Felty, O.P. Anderson, and M.M. Miller. 1993. Structure and solution properties of a dimeric tetrahedral vanadium(V) chloride alkoxide complex. Inorg. Chem. 32 247-248. 

Nunes, G.G., G.R. Friedermann, M.H. Herbst, R.B. Barthem, N.V. Vugman, J.E. Barclay, DJ. Evans, P.B. Hitchcock, G.J. Leigh, E.L. Sa, and others. 2003. The first hetero-binuclear alkoxide of iron and vanadium Structural and spectroscopic features. Inorg. Chem. Commun. 6 1278-1281. 

In reporting a Ziegler-Natta catalyst, the kind of transition metal compound should not be omitted. Group 4-8 transition metal compounds, such as halides, oxyhalides, alkoxides, acetylacetonates, etc., have been used as catalyst precursors with activators such as alkyl derivatives or hydrides of group 1-4 metals. Titanium chlorides and triethylaluminium are most commonly applied for the preparation of heterogeneous catalysts in an aliphatic hydrocarbon medium. Also, vanadium oxychloride or acetylacetonate and dialkyaluminium chloride are often used for the preparation of homogeneous catalysts in an aliphatic hydrocarbon or an aromatic hydrocarbon medium. 

Comparison of the XANES spectra of the vanadium layer on Ti02 prepared by two different routes (by alkoxide precipitation and via VOCl3) shows close analogy. This indicates that the simple precipitation technique without chemical reaction between the vanadium reagent and the surface leads to a disordered monolayer on Ti02. Similar data have also been reported for the V205/y-Al203 system (147). 

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