Metal alkoxides reactions with water

Hydrolysis of metal alkoxides is the basis for the sol-gel method of preparation of oxide materials therefore, reactions of metal alkoxides with water in various solvents, and primarily in alcohols, may be considered as their most important chemical properties. For many years the sol-gel method was mosdy associated with hydrolysis of Si(OR)4, discussed in numerous original papers and reviews [242, 1793,243]. Hydrolysis of M(OR) , in contrast to hydrolysis of Si(OR)4, is an extremely quick process therefore, the main concepts well developed for Si(OR)4 cannot be applied to hydrolysis of alcoholic derivatives of metals. Moreover, it proved impossible to apply classical kinetic approaches successfully used for the hydrolysis of Si(OR)4 to the study of the hydrolysis of metal alkoxides. A higher coordination number of metals in their alcoholic derivatives in comparison with Si(OR)4 leads to the high tendency to oligomerization of metal alkoxides in their solutions prior to hydrolysis step as well as to the continuation of this process of oligomerization and polymerization after first steps of substitution of alkoxide groups by hydroxides in the course of their reactions with water molecules. This results in extremely complicated oligomeric and polymeric structures of the metal alkoxides hydrolysis products. 

The initial species used in sol-gel processing are metal alkoxides (M(OR)y). Hydrolysis of metal alkoxides involves nucleophilic reactions with water as follows ... 

Alcohols display the properties of weak acids. The reactions of alcohols with active metals (such as Na and K) are slower than the equivalent reactions with water. During these reactions, basic salts of the alkoxide ion (RO —) are produced. 

The rate of reaction of alcohols with alkali metals decreases from primary alcohols to tertiary alcohols. Basic alkoxides react with water (hydrolyze) to produce their initial alcohols. 

The alkoxides of metals, in general, do not present any serions fire or explosion hazards, as metal alkyls do. Only a few are flammable compounds nndergoing violent to vigorons explosive reactions. The single most hazardons property attribnted to metal alkoxides is their exothermic reaction with water. An alcohol and a metal hydroxide are generated. The reaction is as follows ... 

This reaction is an oxidation-reduction reaction. Metallic sodium reacts with hydrogen atoms that are bonded to oxygen atoms to generate hydrogen gas, sodium cations, and the alkoxide anion. The reaction with water is vigorous and at times explosive. 

For most metal alkoxides, hydrolysis with excess water yields insoluble oxide or hydroxide precipitates that are useless for further polymerization reactions [Eqs. (5.21)-(5.23)]. However, if small amounts of water are added slowly to a sufficiently dilute solution, it is possible to form polymerizable molecular species from these alkoxides also. For example, when a dilute solution of boron alkoxide in alcohol is exposed to water, soluble transient molecular species such as B(0R)2(0H) and B(OR)(OH)2, representing various degrees of hydrolysis, form initially, e.g.. 

One of the ways to dispose of chemicals that are reactive with water is hydrolysis, that is, the reaction with water under controlled conditions. Inorganic chemicals that can be treated by hydrolysis include metals that react with water metal carbides, hydrides, amides, alkoxides, and halides and nonmetal oxyhalides and sulfides. An example of a waste chemical treated by hydrolysis is the reaction with water of sodium aluminum hydride (used as a reducing agent in organic chemical reactions) ... 

Pyrrohdinone forms alkaU metal salts by direct reaction with alkaU metals or their alkoxides or with their hydroxides under conditions in which the water of reaction is removed. The potassium salt prepared in situ serves as the catalyst for the vinylation of 2-pyrrohdinone in the commercial production of A/-vinylpyrrohdinone. The mercury salt has also been described, as have the N-bromo and N-chloro derivatives (61,62). 

Zirconium tetrachloride is instantly hydrolyzed in water to zirconium oxide dichloride octahydrate [13520-92-8]. Zirconium tetrachloride exchanges chlorine for 0x0 bonds in the reaction with hydroxylic ligands, forming alkoxides from alcohols (see Alkoxides, METAl). Zirconium tetrachloride combines with many Lewis bases such as dimethyl sulfoxide, phosphoms oxychloride and amines including ammonia, ethers, and ketones. The zirconium organometalLic compounds ate all derived from zirconium tetrachloride. 

The exchange of lithium in a dililhium phthalocyanine is a useful tool to prepare metal (e.g., zinc) or metal-free phthalocyanines. For this purpose, the dilithium phthalocyanine is prepared by reaction of phthalonitrile and lithium alkoxide in an alcohol, e.g. pentan-l-ol. In most cases, the lithium phthalocyanine is not separated but directly converted into the respective phthalocyanine by treatment with metal salts or, in the case of metal-free phthalocyanine, with acid or water. 

The nucleophilic attack by alkoxides, amines, and water is of great interest to homogeneous catalysis. A dominant reaction in syn-gas systems is the conversion of carbonyls with water to metal hydrides and carbon dioxide ("Shift Reaction"), see Figure 2.27. 

Fig. 5.18 Schematic and TEM image of reaction scheme to prepare metal nanoparticles encapsulated within metal oxide coating on oxidized MWCNTs. Metal NPs are added to developing metal alkoxide sol followed by addition of oxidized MWCNTs and water for hydrolysis. Adapted with permission from [228], (2012) American Chemical Society.

The use of supported metal complexes in transesterification reactions of TGs is not new. An earlier patent claimed that supported metals in a hydroxylated solid could effectively catalyze transesterification. The catalyst preparation used an inert hydrocarbon solvent to attach transition metal alkoxide species to the support surface. The reaction, however, was carried out in the presence of water. The author claimed that water was essential in preparing materials with good catalytic activity. Among the metals employed, titanium catalysts showed the best activity. However, it was not clear from the preparation method if reproducibility could be easily achieved, an important requirement if such catalysts were to be commercially exploited. 

Nucleophilic metal alkoxide reacts rapidly with electrophilic water to generate the several hydrolysis products with complex intermediate species containing some residual alkoxy groups. The concentration ratio of the starting reagents and the nature of alkoxide and the solvent influence the morphology of hydrolysis products. Overall, the formation reaction for particles consists of hydrolysis of metal alkoxide and condensation of the hydrolysis species. The hydrolysis reaction of metal alkoxide... 

Various metal alkoxides are ideal starting materials for the preparation of metal (hydrous) oxides by the described aerosol techniques, because many of these compounds are in the liquid state at room temperature, easily vaporized, and exceedingly reactive with water vapor. Additional advantage is the purity of the resulting powders, because the only products of the chemical reactions are the metal (hydrous) oxide and alcohol. The particles are, therefore, free of impurities, such as various ions, normally present in solids prepared from different salts. 

Reaction of trialkoxyboranes with metal alcoholates, alcoholysis or hydride transfer reactions of tetrahydroborates with aldehydes or ketones all result in the formation of tetraalkoxobor-ates. Steric factors play an important role in these reactions. As a consequence, sec-alcohols react very slowly and tetra-r-alkoxoborates in general cannot be obtained by any of the reactions above. At elevated temperatures the tetraalkoxoborates revert to the trialkoxyborane and metal alkoxide.75 Thioalcoholysis of tetrahydroborates can also be effected but, in contrast to the situation in alcoholysis, the last hydrogen atom is more difficult to substitute, probably for steric reasons.119 Tetraalkoxoborates and tetramercaptoborates are readily hydrolyzed by water or moist air. 

Zirconium and hafnium tetraalkoxides are highly reactive compounds. They react with water, alcohols, silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as j8-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML (OR)4 these reactions are discussed in the sections dealing with the chelating ligand. 

Metal alkoxides constitute a useful class of starting materials for the synthesis of the metal / -diketonates. The ethoxides of Nbv, Tav and Uv react with diketones. Here, only partial substitution of the ethoxy groups occurs and materials of the type M(diketonate)3(OEt)2 are formed.194,195 Similar reactions with lanthanide alkoxides, however, provide pure, unsolvated lanthanide tris(diketonates). The virtue of such syntheses lies in their ability to yield anhydrous diketonate complexes. Removal of water from the hydrates without decomposition is sometimes difficult.196,197... 

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-... 

In this case isolation of metal alkoxide from water does not require any special measures liquid TIOEt is accumulated in the bottom of the flask in a 95% mixture with EtOH, while TlOH and water comprise the upper layer [1625]. Alkaline metals react in analogous way with alcohols that do not mix with water. For instance, reaction of KOH with Et2CHOH (pentanol-3) at 120°C also results in the formation of two layers the upper (alcoholic) layer contains 40 wt% of KOR and 2 wt% of KOH while the lower (aqueous) layer contains 54 wt% of KOH [1277]. 

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