Metal alkoxides applications

Tab. 10.8 summarizes the application of rhodium-catalyzed allylic etherification to a variety of racemic secondary allylic carbonates, using the copper(I) alkoxide derived from 2,4-dimethyl-3-pentanol vide intro). Although the allyhc etherification is tolerant of linear alkyl substituents (entries 1-4), branched derivatives proved more challenging in terms of selectivity and turnover, the y-position being the first point at which branching does not appear to interfere with the substitution (entry 5). The allylic etherification also proved feasible for hydroxymethyl, alkene, and aryl substituents, albeit with lower selectivity (entries 6-9). This transformation is remarkably tolerant, given that the classical alkylation of a hindered metal alkoxide with a secondary alkyl halide would undoubtedly lead to elimination. Hence, regioselective rhodium-catalyzed allylic etherification with a secondary copper(l) alkoxide provides an important method for the synthesis of allylic ethers. 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

Heterocycles treated in this section belong to several structure types. The metal atoms in compounds under discussion can exist not only in the tetravalent but also in the carbenoid divalent state and the heterocycles differ not only in the nature and number of metal atoms but also in the nature of ring bonds of the latter. Thus, besides conventional organometallic compounds having only M—C bonds there are heterocycles with M—M bonds and those with C—M—X or X—M—Y fragments (X and Y—common heteroatoms like O, S, N). The last type is the most numerous and important group of five-membered Ge-, Sn- and Pb-heterocycles in which the metals are involved in M—X and M— Y bonds similar to those in respective metal alkoxides, thiolates and amides. This feature not only affects the structural parameters of these compounds but determines their chemical properties, synthetic routes and applications. 

The preparation, composition, structure and leaching characteristics of a crystalline, ceramic radioactive waste form have been discussed, and where applicable, compared with vitrified waste forms. The inorganic ion exchange materials used such as sodium titanate were prepared from the corresponding metal alkoxide. The alkoxides were reacted in methanol with a base containing the desired exchangeable cation and the final powder form was produced by hydrolysis in an acetone-water mixture followed by vacuum drying the precipitate at ambient temperature. 

Metal alkoxides are of interest as precursors to oxide materials with ceramic or electronic applications.1 Barium incorporation in such a precursor is particularly interesting because of the known high-Tc superconductor, YBa2Cu307. The title compound has vast possibilities for reactions of the type... 

The numerous substitution reactions lay in the background of the application of metal alkoxides in inorganic and metal-organic synthesis (preparation of halides, hydrides, sulfides, carboxylates, alkyl derivatives, certain classes of coordination compounds, etc.). One advantage of their application is the formation of only alcohols as by-products ... 

Over the last three decades an intensive development of the synthetic approaches to various oxide materials has been based on the application of metal alkoxides, M(OR) . In principle, a number of ways leading to the formation of oxides from metal alkoxides are possible — for example,... 

Concerning the other areas of application for metal alkoxides, see Chapter 12. 

Synthesis of metal alkoxides has been studied for more than a century by different researchers, who have proposed a large variety of synthetic techniques aimed at preparation of different distinct derivatives. In the 1950s in the course of the systematic studies Bradley suggested a few general techniques [196]. In the present review we would like to differentiate the methods that had technological applications and those that are mostly used in laboratory practice. 

On the other hand, application of alkali metal amalgam permits the slowing down of the reaction of metals with alcohols, which is used in the industrial production of alkali metals alkoxides. Production of NaOR by Mathieson Alkali Works is based, for instance, on the reaction of sodium amalgam (formed as a result of the electrolysis of aqueous NaCl solution with the mercury cathode) with alcohol NaOR ROH is isolated from the solutions. Na residue in the amalgam is hydrolyzed, the obtained mixture is returned to the electrolyz-... 

Both reactions are used for the commercial production of alkaline and alkaline-earth alkoxides from very cheap raw materials. As far as the metal alkoxides thus formed are soluble in alcohols, both reactions are reversible. Thus, application of these methods is expedient in the case of alcohols with the boiling temperature higher than 100°C (water is distilled off). When low-boiling alcohols are used the reaction time increases greatly, water is eliminat-... 

Alcoholysis of metal oxides may also be used for the synthesis of multivalent metal alkoxides nevertheless, application of this method is restricted to covalent oxides with low values of lattice activation energies. Usually these are derivatives of M in the higher oxidation states, and their interaction with alcohols is complicated by oxidation-reduction processes — for example,... 

On the other hand, to accelerate alcoholysis of NaH and KH in solutions of benzene or THF [22.1] - [2.22] - ctyptands are used (which bind alkaline metal into a rather stable chelate complex) [1004], Quite stable, volatile per-fluorotert-butoxides were first obtained in reactions of LiH or NaH with (CF3)3COH they distill at atmospheric pressure at 218 and 232°C, respectively [467] (the application of metals would presumably lead in this case to condensation of Wurtz type). Li and Na hydrides are used as cheaper than metal raw materials for production of the corresponding metal alkoxides. In particular it has been suggested that the equipment used in production of MH could be cleaned from its residue by the mixture ofEtOH and the aromatic hydrocarbon (40 to 60% by volume). After hydrogen evolution is completed the solvent is eliminated under vacuum at < 90°C the residue is MOEt with the content of the main product > 98% [342],... 

Alkoxylation of MHal is performed by KOR, NaOR, or LiOR or by ammonia in solution in alcohol (formally ammonium alkoxide). Reaction with NaOR is accompanied by formation of NaCl, which is almost completely insoluble in organic solvents and is, therefore, used in the synthesis of soluble metal alkoxides. One of the first examples of application of such reactions has not lost its importance as a method of synthesis to this day ... 

Application of halogenides under the same conditions results inMHal/OR), which does not decompose in excess of M OR or under refluxing. The patent [833] describes the general method of preparation of the stable metal alkoxide solutions (which are used in technology) it comprises a heterogeneous reaction of Zn, Cd, Y, Ln, In, Pb, Sn, Zr, Sb, Bi and Mn carboxylates in alcohol solutions with NH3 or amines with subsequent purification by ionic exchange. 

Application of metal alkoxides in preparation of oxide materials assumes their high purity, which satisfies the requirements formodem electronic materials. Nevertheless, there are only few works that consider the question of purification of metal alkoxides and that give the characteristics of their purity [522]. Future studies of the syntheses of metal alkoxides should pay special attention to the problems of their purification. 

Some examples of application of metal alkoxides for synthesis of ferroe-lectrics such as BaTi03-based materials, complex niobates and tantalates, PZT andPLZT-materials, and high-temperature superconductors are discussed below. 10.1. Barium titanate and BaTiOj-based solid solutions... 

Barium titanate and BaTi03-based materials are most commonly used for ceramic capacitors with high dielectric permittivity. BaTi03 powder of extremely high quality (in respect of its purity, stoichiometry, particles morphology) is required for most of the modem applications. This characteristic may be considerably improved by the application of alkoxide precursors. Thus, it is of no surprise that synthesis of BaTi03 and BaTi03-based materials from metal alkoxides attracted considerable attention for several decades. The first works on... 

Application of metal alkoxides for the preparation of niobates and tanta-lates in the form of both powders and films has been first performed by Dand-liker [475]. In our study of hydrolysis of LiOEt andNb(OEt)5 solutions in anhydrous EtOH, it was demonstrated that although the freshly prepared powder is amorphous for X-ray, it already contains microcrystallites of LiNb03 [1778]. In addition to conventional techniques, the properties ofthe powder in the course of thermal treatment were studied by the second harmonic generation ofthe laser beam, which is a sensitive technique for detection ofnoncen-trosymmetric phase the first signal was registered at 350°C. 

The same metal alkoxide precursor solutions in EtOH and methoxyetha-nol, which were used for preparation ofpowders were also suggested for application of LiNb03 and NaNb03 films on Pt and quartz substrates [524, 1778]. The choice of solution concentration and thickness ofthe layer, which undergoes calcination, is very important for the preparation of films without cracks or bubbles. Oriented LiNbxTa,.x03 (0 < x < 1) films were obtained on the... 

It may thus be concluded that application of metal alkoxides for preparation of PZT-based thin films in many aspects may be nowadays considered to be a routine technique widely used in many laboratories. The research work is presently mostly focused on electrical and microstructural aspects of these films rather than on chemical routes of their preparation. Irrespective of the technique for the precursor preparation, when 10% excess of Pb is introduced (to prevent loss of lead due to the PbO volatility) the ferroelectric perovskite PZT films are obtained after annealing at about 700°C, demonstrating nonlinear properties of the level acceptable for most of the desired applications. Their typical microstructure is presented in Fig. 10.3. 

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