Bimetallic alkoxide

Dienols with 2 equiv. of 1 gave the corresponding bimetallic alkoxide organozir-conocene complexes however, protolysis allowed recovery of the alcohol functionality (Scheme 8-29) [107]. Alcohols can also be easily converted to ethers. Alkyl, aryl, silyl [85,112,183, 210] and THP [17, 153, 211, 219] ethers are stable under hydrozirconation conditions side products were observed only with the trimethylsUyl group [220, 221]. 

Chae, H. K. Payne, D. A. Xu, Z. Ma, L. 1994. Molecular structure of a new lead titanium bimetallic alkoxide complex [PbTi2( t4-0)(00CCH3)(0CH2CH3)7]2 Evolution of structure on heat treatment and the formation of thin-layer dielectrics. Chem. Mat. 6 1589-1592. 

The insertion of various isocyanates into chromium(lll) alkoxide M—O bonds has been reported.737 The complexes are prepared by refluxing the isocyanates with a suspension of the alkoxide in benzene. No structural data were given for the products. Unusual bimetallic alkoxides have recently been prepared738 by the reaction of Cr[Al(OPr )4]3 with alcohols and acetylacetone (166). A wide range of spectroscopic methods were used to study them. In general, the results were in accord with a monomeric formulation similar to (166) below Cr[Al(OMe)4]3 was grossly insoluble the small size of the methyl groups may permit extensive polymerization. 

The solvates of alkoxides and the bimetallic complexes are described in Chapter 8, and oxocomplexes in Chapter 5. The products and mechanisms of hydrolysis for mono- and bimetallic alkoxides are considered in Chapters 9 and 10. 

For several decades after this, the metal alkoxides remained objects of interest only for organic chemists, who applied them for reduction of carbonyl compounds and as catalysts for disproportionation of aldehydes, condensation and polymerization. Among these works, of special interest is the work of Meerwein and Bersin [1101], who were the first to prepare the bimetallic alkoxides that were then called Meerwein alkoxosalts in analogy... 

The appearance of the alkali alkoxide can lead to formation of bimetallic alkoxides or bimetallic alkoxide halides as impurities. For example, lithium oxoalkoxocomplexes were isolated as side products of the anodic dissolution of Mo and W when LiCl was used as conductive additive [908],... 

The electrochemical technique can be used also for direct synthesis of bimetallic alkoxides. For instance, the anodic dissolution of rhenium in the methanol-based electrolyte that already contained MoO(OMe)4, permitted to prepare with a good yield (60%) a bimetallic complex RevMov,02(OMe)7, with a single Re-Mo bond [904], Application of the same procedure permitted the preparation of complex alkoxide solutions with controlled composition for sol-gel processing of ferroelectric films [1777]. 

As has been mentioned above, of the many works devoted to the studies of the chemical properties of metal alkoxides, we here can consider only the reactions of partial substitution for the alkoxide groups (Chapter 7), decomposition with formation of oxocomplexes (Chapter 5), and hydrolysis with the formation of homo- and heterometallic hydroxides (Chapters 9 and 10). We discuss here the complex formation products of metal alkoxides — the adducts with neutral ligands, M(OR) mL, and the bimetallic alkoxides, precursors of complex oxides in sol-gel technology. 

Bimetallic alkoxides ( Meerwein salts ) have been known for more than 70 years, since the report made by Meerwein and Bersin [1101] on the formation of alkoxosalts (analogs of hydroxosalts MM m(OH)n) on titration of alcohol or benzene solutions of acidic alkoxides by those of basic ones to the equivalence points determined using the acid-base indicators ... 

The synthesis of bimetallic alkoxides can be achieved using the 2 routes first described by Meerwein and Bersin [1101] complex formation of2 alkoxides and the methathesis of a metal halide with an alkali metal alkoxide. 

To determine the composition of bimetallic alkoxides formed via complex formation in solution and in the solid phase and to estimate their stability, we have applied physicochemical analysis — the investigation of the solubility isotherms in the systems M(OR)m- M (OR)n - L (solvent alcohol, ether, hydrocarbon, etc.). This method, common for the studies ofthe interaction of inorganic salts in water solutions, turned out to be rather fruitful in the chemistry of alkoxides. It permitted the study of the interaction ofthe components... 

An important development of this method has recently been carried out by Mehrotra and Veith. The reaction with the excess of halide was found to provide bimetallic alkoxide halides. Their interaction with the alkoxometallate of the third metal provided access to trimetallic alkoxides ... 

In addition to the synthetic routes to the bimetallic alkoxides described above, in some cases the redox reactions have been applied, such as ... 

Therefore, in the course of the studies of bimetallic alkoxides it turned out that one of the most important features for their application in sol-gel films preparation lies in their ability to undergo partial decomposition via hydrolysis, oxidation, or elimination of ethers or esters with formation ofM-O-M bridges (see, e.g., Section 10.1), fragments of future complex oxide phases. Formation of bimetallic oxoalkoxides ensures homogeneity on the atomic scale of the complex oxide formed in the process of the transformation M(OR)n — M2Ob. 

Just as in the above described case of BaTi03 preparation, the reaction can be performed in two steps at first bimetallic alkoxide is hydrolyzed, and then Ba(OH)2 is adsorbed by the precipitate of hydrated oxides. Crystallization ofthe perovskite phase under these conditions occurs at 80 to KXPC [1779, 805],... 

Next are placed the classes of alkoxide halides, hydrides, and bimetallic alkoxides. The latter are considered in the chapters devoted to more electronegative metal (in those devoted to the other metal they are mentioned only in the footnotes after the table). 

The classic study of Al(OR)3 was carried out by Tishchenko [1585] at the end of nineteenth century. His dissertation, entitled On the Action of Amalgamated Aluminum on Alcohols (Aluminum Alkoxides, Their Properties and Reactions), became a great resonance for the chemistry of alkoxides (see also Chapter 1). The synthetic approaches, that he developed are still in use the purity of the samples obtained by Tishchenko, taking into account the data he reported, was definitely not worse than that of those described at present. In 1929 Meerwein and Bersin [1101], performing the acidimetric titration of Al(OR)3 solutions by solutions of alkali alkoxides, discovered the existence of bimetallic alkoxides (Meerwein salts), which play an important role in the modern chemistry of metal alkoxides. 

The obtaining of tin(IV) alkoxides was first reported in a well-known publication by Meerwein and Bersin [1101] devoted to bimetallic alkoxides. At the end of the 1950s Bradley [222] and Make [1049] practically simultaneously devoted the synthetic approaches to and described the properties of nearly all major representatives of the Sn(OR)4homologous series. During the last 10 to 20 years interest in these compounds was renewed due to the prospect of their application in the synthesis of optically transparent and conducting films based on Sn02, and also of related ceramic materials. The alkoxides of Sn(IV) were considered in detail in a review by Hampden-Smith etal. [702],... 

The presence of halide ligands in the molecules of bimetallic alkoxide complexes hinders apparently their decomposition with formation of oxoalkox-oderivatives. Examples are CunClZr2(OPr )9 [1664], CdHalZr OPti), [1508, 1687], Veith etal. [1687,1691] used the the stepwise replacement of Cl andl atoms for preparation of tiimetallic zirconates, [MnCdZr2(OPr )12]2, M11 = Ca, Sr, Ba These octanuclear aggregates contain linear [Ba(p-OR)2Cd(p-OR)2Cd(p-OR)2Ba] chains in the center and [Zr2Ba] triangles at the ends of each molecule (see also Section 4.7.3). 

The compounds of these two elements belong apparently to the most studied and most fully characterized alkoxoderivatives of metals. The particular features of their synthesis and physicochemical properties are discussed in detail in a large number of review publications authored by Chisholm [370] who made tremendous contributions to the development of the chemistry of these substances. The synthetic routes to the bimetallic alkoxide derivatives of molybdenum and tungsten (VI) along with the discussion of their physicochemical properties and application prospects are also reviewed in [1638, 908],... 

Complex formation of molybdenum and tungsten alkoxides with the alkoxides of other metals formation of bimetallic alkoxides... 

The complex formation of tungsten alkoxides with the alkoxides of other metals than the alkali, bismuth, and rhenium (see Section 12.21) has not been investigated in detail, and therefore the major attention below will be paid to the description of the bimetallic alkoxides of molybdenum. 

The main route to rhenium alkoxides is the interaction of halids and oxy-halids with alkali alkoxides or alcohols in presence of amines (method 5). As the important starting reagents can serve also Re2(CO),0 and Re207 (method 3) The preparation ofrhenium (V) and (VI) oxoderivatives by the anodic oxidation of metal in alcohols has also been described (method 2) (see Table 12.22). The bimetallic alkoxides ofrhenium and heavy transition metals can most efficiently be obtained by interaction ofrhenium (VII) oxide with the alkoxides ofthese elements in refluxing toluene ... 

Antimony(V) ethoxide and isopropoxide have been found to be volatile compounds these are dimeric in cyclohexane or carbon tetrachloride solutions. By contrast, the low volatility of Sb(OMe)5 has been ascribed to its polymeric nature in solid state. The antimony atom in these pentaalkoxides prefers hexacoordination state which may be acquired either by molecular association or on coordination with suitable donors such as alcohol, ammonia etc.,246,248 249,251. Antimony pentaethoxide also reacts with sodium ethoxide to give the bimetallic alkoxide, NaSb(OEt)6249). 

Metal aUcoxide complexes with related alcohols are apparently edge-sharing bioctahedral dimers, on the evidence of the X-ray stmcture of the isopropyl complex. Some stable mixed alkoxides of the type Q[M3(OR)9] (Q = Li, Na, K, NH4, Ca/2) have been reported they distill in vacuo without decomposition. The X-ray stmctures of several bimetallic alkoxides obtained by Caulton,QZr2[0(/-Pr)]9,Q = Li[HO(i-Pr)], K(DME), Ba[0(i-Pr)], show a similar triangular stmcture with two 743 and three /r-OR bridges. The thallium salt of composition Tl2Zr((/R)6 obtained by the reaction shown in equation (18) has a distorted octahedral stmcture stabilized by six T1 F contacts. ... 

From 1971-1985, during which time the chemistry of bimetallic alkoxides was looked into in some detail (except X-ray crystal structure elucidation). 

Precaution has to be taken while using alkali metals as proton acceptors not to use them in excess, in order to avoid the formation of bimetallic alkoxides. For example,... 

Lewis acid-base reactions between component alkoxides have been used primarily for the synthesis of bimetallic alkoxides involving (1) alkali alkoxides and less basic alkoxides, and (2) between binary alkoxides of other metals. 

As indicated above, the nature of alkoxyl groups, alkali metals (6, 126), or molar ratios of the reactants were shown to influence the nature and composition of the bimetallic alkoxide formed. 

In the hydrolysis of Mg Al(0-/-Pr)4 2, carried out in the presence of etha-nolamine during the preparation of the polycrystalline MgAl204 (spinel) by the SG procedure, a careful 27 A1 NMR study (557) showed that the basic framework of the bimetallic alkoxide, Mg /x-0-/-Pr)2Al(0R)2 2, does not break down, at least in the initial stages of the hydrolysis. This result confirmed that the conversion took place at the molecular level in the process. Similar indications were obtained in the conversion of the bimetallic alkoxide Mg Nb(OC2H4OMe)6 2 to the perovskite phase of PNM (558). 

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