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METAL OXIDE ALKOXIDE POLYMERS: PART II. THE HYDROLYSIS OF TANTALUM PENTAETHOXIDE
Abstract
The hydrolysis of tantalum pentaethoxide has been studied in boiling benzene and in boiling ethanol. Soluble tantalum oxide ethoxides were obtained over a considerable range of hydrolysis. Infrared studies suggest that Ta—OH groups are absent in these products. Ebulliometric studies have shown that the number-average degree of polymerization varies with the degree of hydrolysis in a characteristic manner and this has been quantitatively explained in terms of fundamental structural models based on octahedrally 6-co-ordinated tantalum.
The average degree of polymerization depends on the average degree of hydrolysis as over a large range of for all metal oxide alkoxides and oxide trialkylsilyloxides that have been formed by hydrolysis and examined in solution. A number of general theoretical models are developed which permit quantitative interpretation of the parameters a and b. Mixtures of linear or branched chains in proportions governed wholly or partly by the degree of polymerization of the unhydrolyzed alkoxide can explain all the data on oxide alkoxides in solution in their corresponding alcohol, and the model leads to a plausible mechanism for disproportionation. Similar models with more complex structure can be involved in the metal oxide trialkylsilyloxides. Heterogeneity in the composition of the repeating units or in the number of metal–oxygen–metal bridges between successive repeating units can also be significant for both oxide alkoxides and oxide trialkylsilyloxides. Models involving cyclic chains are possible, but steric effects make them improbable in many cases. Mixtures of chains with equal average degrees of hydrolysis for each type of chain have been assumed in previous analyses, but are shown to be special cases of the more general model.
Tetrakis-(trimethylsilyloxy)-titanium Ti(OSiMe3)4 has been hydrolyzed under controlled conditions in dioxane. The initial products of hydrolysis undergo facile disproportionation, e.g. 3Ti2O(OSiMe3)6 → 4Ti(OSiMe3)4 + polymeric Ti2O3(OSiMe3)2. Molecular weight determinations were made on the titanium oxide trimethylsilyloxide polymers (polytrimethylsiloxanotitanoxanes) obtained by thermal disproportionation. Structures have been suggested for the polymers on the basis of the variation of number-average degree of polymerization with the degree of hydrolysis.
The polymers formed by the hydrolysis of stannic isopropoxide, ceric isopropoxide, and uranium pentaethoxide have been studied by means of an ebulliometric method. The metal oxide alkoxide polymers formed from the tin (IV), cerium (IV), and uranium (V) alkoxides conform to the requirements of a structural theory. In view of the fundamental nature of the theory a formal mathematical presentation is now given.
Ebulliometric studies on the hydrolysis of tantalum pentaalkoxides Ta(OR)5, where R = Me, Prn, Bun, and Bus, have furnished important data on the polymeric nature of tantalum oxide alkoxides. It is shown from the variation of number-average degrees of polymerization as a function of degree of hydrolysis that the polymers conform to certain structural models. The structural models involve 6-co-ordinated tantalum.
The hydrolysis of zirconium alkoxides with secondary or tertiary alkoxide groups Zr(OR)4, where R = Pri, Bus, Bul, and Amt, has been studied. The relatively low polymers formed by the metal oxide alkoxides have been interpreted on the basis of structural models involving octahedrally 6-co-ordinated zirconium.
The nature of the alkyl group has an important influence on the hydrolytic behaviour of titanium alkoxides Ti(OR) 4, where R = Pr n, Pr i, Bu n, Bu i, Bu s, Bu t, and CMe 2Et. Hydrolyses involving the addition of dilute aqueous alcohol to a boiling solution of the alkoxide in the appropriate alcohol in the ebulliometer revealed a remarkably low degree of polymerisation in the products, apparently owing to solvation; the results are generally consistent with a structural theory based on the 6-co-ordination of titanium.
The hydrolysis of titanium tetraethoxide in ethyl alcohol has been studied by an ebulliometric technique. The molecular complexities of the titanium oxide ethoxides produced by hydrolysis have been explained in terms of a structural theory based on the assumption that titanium exhibits the coordination number 6 in these compounds. The theory explains the unusual properties of titanium oxide ethoxides reported previously (J., 1955, 721).
The molecular weights of tantalum alkoxides Ta(OR) 5, where R = Me, Et, Pr n, or Bu n, have been determined ebullioscopically in toluene, acetonitrile, pyridine, and diisopropyl ether. These results are discussed in comparison with the values previously found in benzene and in the parent alcohols and it is suggested that the donor power of the solvent has the major influence in determining the molecular complexity of the alkoxide.
Tantalum penta-alkoxides, Ta(OR) 5 where R=Me, Et, Pr n, or Bu n, have been prepared by the action of an alcohol and ammonia on tantalum pentachloride. Ebullioscopic measurements showed that the alkoxides were dimers in benzene, but in the parent alcohols the complexity depended on the alcohol. These data together with the boiling points under reduced pressure are discussed from the theoretical standpoint.
Ebulliometric studies have been made on the hydrolysis of some zirconium alkoxides, Zr(OR)4 where R = Et, Prn, Bui, and Bun. The variation of the degree of polymerization of the zirconium oxide alkoxides so produced was determined as a function of the degree of hydrolysis. The results are interpreted in terms of structural models based on octahedrally 6-co-ordinated zirconium.
- D C Bradley
- D G Carter
D. C. BRADLEY and D. G. CARTER. Can. J. Chem. 39, 1434 (1961).
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- . J Can
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Can. J. Chem. Downloaded from www.nrcresearchpress.com by DIRECTORATE OF COLDWATER FISHERIES RES on 10/22/15
For personal use only.