Mixed metal alkoxides

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 ... 

Titanium—Vanadium Mixed Metal Alkoxides. Titanium—vanadium mixed metal alkoxides, VO(OTi(OR)2)2, are prepared by reaction of titanates, eg, TYZOR TBT, with vanadium acetate ia a high boiling hydrocarbon solvent. The by-product butyl acetate is distilled off to yield a product useful as a catalyst for polymeri2iag olefins, dienes, styrenics, vinyl chloride, acrylate esters, and epoxides (159,160). 

These reactions have been studied in detail for materials such as silica, and understanding of reaction mechanisms, as well as of the role of the precursor and catalyst (acid or base), has been well documented.63,64 Similar studies have been carried out in other material systems, most notably, lead zirconate titanate [Pb(Zr,Ti)03 PZT].52,65-68 For multicomponent (mixed-metal) systems such as those noted, prehydrolysis of less reactive alkoxides is sometimes employed to improve solution compositional uniformity. Other synthetic strategies to achieve molecular level mixing of reagents have also been employed. Here, synthesis of mixed-metal alkoxides has been a focus of investigators.40-42 A key point is to restrict the amount of water and to control how it is added to form solubalizable precursor species, rather than to induce precipitation.1,52,69,70... 

In some instances, this approach has proven successful, with comparatively low crystallization temperatures being observed. For example, Eichorst and Payne in the synthesis of LiNb03 noted crystallization temperatures of 400-500 °C for a mixed-metal alkoxide precursor.111 In other instances, these attempts have proven less successful. Numerous attempts have been made to synthesize Pb-Zr and Pb-Ti precursors, each with the 1 1 cation stoichiometry of the desired PbZr03 and PbTi03 compounds.83,84 Unfortunately, 1 1 stoichiometric ratio compounds have not always been obtained, with crystalline compounds of other stoichiometries precipitating from the solution, as illustrated in Fig. 2.11.83 This figure shows the crystal structure of PbTi2[p(4)—... 

Mixed liquor volatile suspended solids (MLVSS), in biological waste treatment, 25 829-830 Mixed-metal alkoxides, titanium-vanadium, 25 100... 

Alkoxides and aryloxides demonstrate similar chemistry to that of hydroxides in that it is possible to prepare mixed metal or double metal derivatives similar to hydroxo salts such as Na2[Sn(OH)6]. The formation of mixed metal alkoxides, e.g. Na2[Zr(OEt)6] and Ln[Al(OPri)4]3, is typically a result of the electron deficiency (Lewis acidity) of the metal centers in units of the type M(OR)x or M(OAr)JC (x < 5). This then leads either to oligomerization via alkoxide bridges or, in the presence of other alkoxides, to the formation of mixed metal compounds. 

Although these mixed metal alkoxide complexes are a means of obtaining atomic mixing of the various metals, the stoichiometric ratio of metals may or may not be that desired for the ceramic powder. A different stoichiometric ratio, n, for the same two metals is not likely to be precipitated out just because the initial solution contains a different stoichiometric ratio. Also for many electronic ceramic compositions it is desirable to have many other metals incorporated at the ppm level into the ceramic powder as sintering aids, grain growth inhibitors, and crystal phase stabilizers. Adding these otiier metals is very difficult... 

Sheng, H., Xu, R, Yao, Y, Zhang, Y, and Shen, Q. (2007) Novel mixed-metal alkoxide clusters of lanthanide and sodium synthesis and extremely active catalysts for the polymerization of e-caprolactone and trimethylene carbonate. Inorganic Chemistry, 46, 7722-7724. 

Mixed metal alkoxide systems are also of interest as a means of creating additional hybrid systems. However, recognition of the large differences in their hydrolysis and condensation rates is crucial. For example, if titanium isopropoxide is made to react under the same conditions as might be used for TEOS, hydrolysis and condensation rapidly occur and lead to particulate rather than network formation of Ti02- Cocondensation with TEOS under these conditions does not occur because of the fast precipitation of the titanium dioxide species. Indeed, of the general metal alkoxides, those based on silicon tend to be more easily controlled because of their slower hydrolysis... 

A review of the literature reveals that there are many claims regarding the formation of mixed-metal alkoxide species under different reaction conditions, but frequently supporting experimental evidence (spectroscopic or otherwise) is not included. Notable exceptions include the report of Kato et al."° on SrBi2Nb209 and SrBi2Ta209 precursors and the work of Pelligri et al. " on PZT precursors, both of which provide detailed information on precursor structural characteristics. 

Mixed-metal alkoxide complexes of thallium are also known. For example, Sn(/u-t-BuO)3Tl has both Sn(II) and T1(I) ions." The thallium site is unreactive as a donor for metal carbonyls. However, as indicated earlier, the indium(I) site of the indium analogue shows Lewis-base character. The Sn(IV)/Tl(I) mixed alkoxide [Sn(EtO)eTl2] exists as a one-dimensional polymer. This adduct reacts quantitatively with SnCl2 to form the homoleptic, mixed-valent [Sn2(OEt)6]n. Thallium-titanium double alkoxides have been synthesized using thallium alkoxide as one of the starting materials. ... 

The mixed metal alkoxide Bi(/y-OPrI)2(Ai3-0)( ri2(/J-OI>iJ)2(OPrl)s) was obtained from the controlled hydrolysis of a reaction mixture of Ti(OPr )4 and BiO(OPr ). Both Ti centers are six-coordinate while the Bi center is four-coordinate with a stereochemically active lone pair of electrons.476... 

Tin(II) tert-butoxide furnishes an excellent acid-base system that reacts with another metal alkoxides to yield mixed metal alkoxides. 

Mixed-metal alkoxides are generally covalent [4]. As a result, they are soluble in organic nonpolar solvents (often less in alcohols), generally more than the parent alkoxides. Some can be sublimed without disproportionation, the volatility following the same variation as for the homometallic alkoxides (OtBu > OiPr > OEt > OMe). Mass spectrometry, for instance, established that heterometallic LiNb fragments were retained in the gas phase for LiNb(OEt)6. 

Data on the reactivity of mixed metal alkoxides are limited. Their reactivity remains dominated by the lability of the metal alkoxo bond, but a problem that needs to be addressed is that of the maintaining or not of the heterometallic unit, of the stoichiometry between the different metals, and finally of the ho-... 

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. 

Information on the hydrolysis of mixed-metal alkoxides is even scarcer than for the homometallic. Partial hydrolysis of [LiNb(OEt)6]< leads to the dimeric heterometallic alkoxide [LiNbO(OEt)4(EtOH)]2, in which both the stoichiometry between the two metals and their coordination numbers are retained [93]. On the other hand, the partial hydrolysis of solutions of methoxyethoxides of Ba and Ti (1 1 molar ratio) offers Ba4Tii30i8(0C2H40Me)24 (30% yield). Its structure corresponds to a tetrahedron of BaOs units surimposed on a Ti06(Ti03)i2 core this Tii3042 core is related to that of the aluminum salt [A1i304(0H)24(H20)]2+ [83]. The important modification of the stoichiometry between the two metals illustrates the complexity of the sol-gel chemistry of multimetallic systems and shows that important structural rearrangements can occur. 

All alkoxide routes appear to be limited to obtaining PNM [88a] and lithium or potassium niobate or tantalate [102a]. For the latter, the formulation of the mixed-metal alkoxide MM (OR)6, M = Nb, Ta, is in agreement with that of the material. Epitaxial thin films of LiNbOa on sapphire could be obtained [103]. The use of a mixed-metal MgNb2(OR)i2 species (R = Et [105], C2H40Me [104]) has been shown to promote the formation of the perovskite phase for PNM. 

This chapter focuses only on the syntheses of oxide ceramic powders. The precursors of these powders are metal-organic compounds, mainly metal alkox-ides. The different varieties of ceramic powders synthesized by mixed metal alkoxide precursors are the focal points of this chapter. Numerous references on the fundamental principles of sol-gel processing are available in the literature and within other chapters of this book [10-12]. 

The oxide ceramic powders produced through mixed-metal alkoxides have many advantages over powders prepared conventionally some of the advantages are lower temperature processes, higher purity, more homogeneous distribution of constituents, finer particle size, and easier compositional alteration. Most metal alkoxides exposed to moisture and/or heat cause decomposition of the alkoxide and thus provide forming methods for fine ceramic powders (notable examples for decomposition are thermal decomposition and hydrolytic decomposition). 

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