Mixed metal alkoxide systems

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... 

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... 

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

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. 

The innovative catalysts involve the use of mixed metal alkoxide complexes that incorporate the dual functions of base, giving rise to a chiral metal enolate, and Lewis acid, activating the aldehyde component. A large famOy of these systems has been examined by Shibasaki in a host of different transformations for complex molecule synthesis. The lanthanum binaphthoxide complexes LLB (213) are some of the earliest of these and were shown to give aldol adducts with high selectivities and yields. This process has been utilized elegantly in the kinetic resolution of aldehyde ( )-215 in the enantioselective syntheses of epothilones A and B (217/218, Scheme 4.24) [107]. 

Regarding preparation procedures, the grafting of metal alkoxides on surface hydroxy groups, the co-precipitation procedure and sol-gel synthesis can lead to systems where the mixed oxides can be either close to a classical supported impregnated oxide (for sub-monolayer coverages) or close to solid solutions or multi-layered supported oxides. So the frontier between supported oxides and mixed oxides cannot be well defined. 

In spite of considerable initial scepticism, the synthesis of an increasing number of well-characterized heterometallic alkoxides is now well established and attempts are being directed towards the synthesis of single-source precursors for preparation of mixed metal-oxide ceramic materials by the sol-gel or MOCVD processes. " " Before proceeding to a brief account of some novel types of heterometal glycolate and aminoalkoxide derivatives, it is appropriate to mention four review articles" summarizing the state of knowledge about the conventional heterometallic alkoxide systems at the end of the twentieth century. 

It is certainly beyond the scope of this book to discuss in any detail the solution chemistry pertinent to the large number of multicomponent silicate systems described in the literature. However, it is necessary to address at the minimum the synthetic strategies employed in multicomponent gel synthesis. In aqueous systems coprecipitation is so commonly used that it warrants no further discussion here. In alkoxide systems, there are two general approaches 1) hydrolysis of mixed-alkoxide or metal organic precursors and 2) sequential addition of alkoxides to partially hydrolyzed precursors. 

---