Metal alkoxides insoluble

Many metal alkoxides are soluble ia the corresponding alcohols, but magnesium alkoxides are practically insoluble. Only the distillable alkoxides, like those of alumiaum, titanium, and zirconium, are soluble ia weaMy polar solvents. The double alkoxides are soluble ia alcohol K[Li(OC2Hy)2],... 

The reaction is driven to completion by distilling the lower boiling alcohol. Metal methoxides are frequentiy insoluble and caimot be employed as starting materials in this reaction by the same token, they can be convenientiy prepared from solutions of higher alkoxides by precipitation with methanol. Alcoholysis also gives mixed metal alkoxides ... 

The alkoxide group R is chosen so that the heavier alkali metal alkoxide MOR is soluble in ether or hydrocarbon solvents and so the by-product LiOR may easily be separated from the desired heavier alkali metal phosphide/arsenide (which is often insoluble in weakly or noncoordinating solvents such as ethers and hydrocarbons). 

To prepare solids using the sol-gel method, a sol of reactants is first prepared in a suitable liquid. Sol preparation can be either simply the dispersal of an insoluble solid or addition of a precursor which reacts with the solvent to form a colloidal product. A typical example of the first is the dispersal of oxides or hydroxides in water with the pH adjusted so that the solid particles remain in suspension rather than precipitate out. A typical example of the second method is the addition of metal alkoxides to water the alkoxides are hydrolysed giving the oxide as a colloidal product. 

Alcohols, (a) Monohydric The common impurities in alcohols are aldehydes or ketones, and water. [Ethanol in Chapter 3 is typical.] Aldehydes and ketones can be removed by adding a small amount of sodium metal and refluxing for 2 hours, followed by distillation. Water can be removed in a similar way but it is preferable to use magnesium metal instead of sodium because it forms a more insoluble hydroxide, thereby shifting the equilibrium more completely from metal alkoxide to metal hydroxide. The magnesium should be activated with iodine (or a small amount of methyl iodide), and the water content should be low, otherwise the magnesium will be deactivated. 

The hydrolysis of early transition metal alkoxides is an extremely facile reaction that makes the manipulation of these materials non-routine. The typical insolubility and high molecular complexity... 

It is noteworthy that solubility of the formed metal alkoxide is in many cases even more important for the reaction than the values of the metal standard electrode potentials or the mobility of protons of the alcohol. The following examples illustrate this statement. Despite higher acidity of MeOH in comparison with EtOH(pK= 15.5 and 16, respectively), the insoluble Ca(OMe)2 is formed very slowly in comparison with the soluble derivatives Ca(OEt)2 or Mg(OMe)2 (both latter compounds crystallize from solutions as solvates) [1646]. Aluminum readily reacts with PrOH with the formation of the highly soluble Al(OPrf)3 even in the absence of the catalyst (pK ROH = 18, E°AP7Al,ld = -1.66 V). On the other hand, polymeric Al(OMe)3 and Al(OEt)3 are formed only on prolonged refluxing of the metal with alcohols in xylene (140°C) in the presence of HgCl2 and I2 [1301]. 

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 ... 

To synthesize insoluble polymeric metal alkoxides (primarily methoxides) LiOR is usually used, such as... 

The interaction of the metal alkoxides with the salts of carboxylic acids or with p-diketonates of other metals is especially attractive for the synthesis of bimetallic molecular precursors in the cases, when the preparation of the alkoxide of the other metal is somehow hindered or it is insoluble or irreactive under the conditions applied. This method has been widely used for the sol-gel preparation of HTSC materials (because of low solubility and reactivity of Cu(OR)2) and lead-containing ferroelectrics (in the view of difficult synthesis and low stability of Pb(OR)2). It should be mentioned that the reaction between a metal alkoxide and a functional derivative does far not always lead to the formation of a mixed-ligand bimetallic complex ... 

A further development in the 1980s/1990s was the introduction of some newer catalysts. Narrow range or peaked ethoxylates can be made using acid activated metal alkoxides, metal phosphates or activated metal oxides as catalyst. These catalysts are insoluble and therefore heterogeneous in nature and the major process difference is that catalyst slurry is added to the reactor after which the conditions are exactly as with normal alkaline catalysts. The reactions are slightly quicker and need less catalyst but it must be filtered out. Most producers [ 12-20 ] have patents on these systems, the advantages of which are seen in the finished products as ... 

Metathesis reactions between Group 2 metal alkoxides and lithium amides in hexane yield the metal amide and lithium alkoxide according to Eq. (7).60 The lithium alkoxide is insoluble in hexane and is easily separated from the hexane-soluble amide. 

Answer 4.15 (i) The chemicals are readily available. The chloride is fairly insoluble in organic solvents and it is possible that chlorine is retained in the product, (ii) There is no need to make the alkali metal alkoxide, as the alcohol is the starting material. The metal may need careful cleaning and there may be the need for heating and a catalyst. One product is a gas. (iii) The lanthanide silylamide has to be prepared first. Because of the bulky nature of the ligand, it may be inert to substimtion, but there are no problems with chloride retention and the reaction should be clean. 

Once the alkali alkoxide solutions have been added and the dispersed precipitate has been homogenized, the remaining alkoxides are precipitated by addition of methanol to form insoluble methoxides. Care must be taken to avoid segregation during precipitation due to differing solubilities and reactivities between the various metal alkoxides. When the desired precipitate is heated, the A phase is again the first crystalline phase to form at temperatures below 600°C. The lambda-to-beta transition occurs at 700°C. No p" phase formation occurs below 1200°C. 

The general properties of the significantly covalent behavior of metal alkoxides (both homo- and heterometallic), in spite of the polar character of the M5+—O5- bond, were already dealt with in some detail (6). The effect of steric and inductive factors on the extent of polarization of the M6+—O5- bond as well as the consequent degree of association and volatility can be exemplified by the boiling points (under 1-mm pressure) and the observed degrees of their association (given in parentheses) by the three isomeric butoxides of zirconium Zr(0-n-Bu)4 ( 250°C 3.5) Zr(0-sec-Bu)4 ( 150°C 2.0 Zr(0-/-Bu)4 ( 50°C 1.0). However, the similarities in the molecular association of the neopentyloxides of Ti, Zr, and A1 to the secondary rather than primaiy amy-loxides have been adduced to indicate the higher predominance of steric rather than inductive factors in the above directions. Similarly, the insolubility and... 

More recently (1994), discrete early transition metal (Ti, V, Nb, orTa) al-koxide complexes containing homochiral trialkanolamine ligands (392) were prepared and their usefulness as highly enantioselective catalysts was demonstrated. It is noteworthy that earlier work on the reactions of tetradentate triethanolamine with transition metal alkoxides was reported to yield insoluble products (6). 

Oligomerization becomes more and more important as the N-Z difference increases, that is, as the oxidation state Mz+ decreases. Divalent metals give insoluble polymeric alkoxides [Af(OR)2]n (Af2+ = Fe, Co, Ni, Cu,. ..). This was a real drawback for the sol-gel synthesis of high-Tc superconducting ceramics, such as YBa2Cu307-e. Bulky ligands, such as 2-(2-ethoxy-ethoxy)ethoxide had then to be used to prevent oligomerization and obtain soluble copper oxide molecular precursors [11]. 

Common modifiers used in the sol-gel process are functional alcohols, acetic acid, and acetylacetone. Alcoholysis reactions are the most documented [LiNb(OEt)6] for instance, was converted to LiNb(0C2H40Me>6 in the presence of 2-methoxyethanol [91]. Assembling ligands, such as 2-methoxyethanol or acetic acid, seem more favorable than -diketones for the stabilization of mixed-metal alkoxides. However, the poorly controlled addition of an additive may sometimes modify the mixed-metal species by extracting, at least partially, one metal as an insoluble product or as a complex. 

Most metal chlorides undergo only partial metathetical halide/alkoxide exchange upon reaction with alcohols or no reaction at all even at elevated temperatures. The metal alkoxide chlorides thus obtained, MClx(OR), have not been used in sol-gel processing (see, however. Section 7.10.3.3.2). In order to achieve the preparation of homoleptic metal alkoxides from metal chlorides basic conditions are essential in order to trap the liberated HCl. This can be achieved by reaction of metal chlorides with alcohols in the presence of a base such as ammonia or, less often, trialkylamines or pyridine (Equation (11a)). The base also increases the equilibrium concentration of alkoxide ions, which are a more powerful nucleophile for reaction with the metal chloride than the parent alcohol. For this reason the use of alkali alkoxides (M OR), mostly lithium, sodium, or potassium alkoxides, proves to be more successful (Equation (11b)). The use of LiOR has advantages for the preparation of insoluble metal methoxides because LiCl is soluble in methanol and is thus easily separated from insoluble metal alkoxides. 

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