Metal alkoxides binary

In view of the solubility of metal alkoxides (binary or homometallic as well... 

Most coordination catalysts have been reported to be formed in binary or ternary component systems consisting of an alkylmetal compound and a protic compound. Catalysts formed in such systems contain associated multinuclear species with a metal (Mt)-heteroatom (X) active bond ( >Mt X Mt—X > or — Mt—X—Mt—X— Mt = Al, Zn, Cd and X = 0, S, N most frequently) or non-associated mononuclear species with an Mt X active bond (Mt = Al, Zn and X = C1, O, S most frequently). Metal alkyls, such as triethylaluminium, diethylzinc and diethylcadmium, without pretreatment with protic compounds, have also been reported as coordination polymerisation catalysts. In such a case, the metal heteroatom bond active in the propagation step is formed by the reaction of the metal-carbon bond with the coordinating monomer. Some coordination catalysts, such as those with metal alkoxide or phenoxide moieties, can be prepared in other ways, without using metal alkyls. There are also catalysts consisting of a metal alkoxide or related compound and a Lewis acid [1]. 

Because of the extremely high susceptibility of metal alkoxides to even traces of moisture, stringent precautions for maintenance of anhydrous conditions (solvents, reagents, equipment, etc.) as well as careful protection from atmospheric moisture were generally regarded as essential conditions for the synthesis of simple binary or heterometallic alkoxides in a pure state. 

Besides binary compounds (MLa.) containing the uninegative ligand L = XRa-, a number of ternary (MLa-L y) and quaternary (ML L L" ) complexes were also isolated by use of supplementary ligands U and L", and there is scope for further exploitation of the field. In this account, we restrict the discussion to metal alkoxides and metal dialkylamides since the metal alkyls are the subject of papers by Lappert et al, (6, 7). 

The sol-gel process is particularly useful for nonvolatile metal alkoxides which cannot be used in the MOCVD process, and for forming heterometal oxides, where control of stoichiometry is important. Since 1980, numerous excellent reviews have been published on the sol-gel chemistry of metal alkoxides to which readers are referred for detailed bibliography. Numerous publications have appeared detailing the preparation of binary metal oxides and heterometal oxides using the sol-gel technique and a few representative examples only are given here. 

Elimination of Ester. The condensation reaction between metal or silicon acetates and metal alkoxides (Eq. 26-3) has been used for the preparation of various polymetallic oxo-clusters (Caulton, 1990). It has also been successfully applied to the preparation of Z1O2 (from a mixture of Zr(OPr")4 and Zr(OAc)4) and homogeneous Ti02-Si02 and Zr02-Si02 binary oxides (Iwasaki, 1994 Jansen, 1995). Similarly the condensation between... 

The metal chloride precursors are relatively cheap and easy to access. However, they result in halide impurities in the metal oxide nanoparticles. In order to avoid such impurities, halide-free metal oxide precursors such as metal alkox-ides, acetates, or acetylacetonates can be employed. The solvothermal treatment of metal alkoxides with benzyl alcohol offers an alternative route for the synthesis of a htige selection of nanocrystalline binary and ternary metal oxides such as... 

Single-source precursor approaches are becoming more and more attractive in the sol-gel synthesis of mixed oxide materials. Well-mixed Si/Ti binary oxides have been synthesized by Miller et al. They chose to use a diethoxysiloxane-ethyltitanate copolymer as a network-forming precursor since the Ti—O—Si linkage is already present in the precursor. This synthetic route avoids the problem associated with unequal hydrolysis rates of metal alkoxide precursors commonly used to prepare mixed oxide catalysts [60]. A more detailed discussion can be found in Section 25.4. 

Binary metal fluorides Many binary metal fluorides have been prepared via sol-gel fluorolysis [4, 7]. The applicability of the synthesis process is primarily limited by the ready availability of soluble metal alkoxides. However, the synthesis of metal fluorides, of which the metal ions are very weak Lewis acids, typically does not result in sol formation but finely dispersed solid fluorides, the XRD of which reflect the aimed-for compounds. 

It should be noted that the sol-gel process is particularly attractive for the synthesis of multicomponent particles with binary or ternary compositions using double alkoxides (two metals in one molecule) or mixed alkoxides (with mixed metaloxane bonds between two metals). Atomic homogeneity is not easily achieved by coprecipitating colloidal hydroxides from a mixture of salt solutions, since it is difficult to constmct double metaloxane bonds from metal salt. Hybrid... 

The thermolysis is strongly affected by structural features e.g. Westin and Nygren investigated the thermolysis of antimony-containing binary alkoxides with transition metals. The compounds approximate the composition MSb(OEt)5, M = Mn, Fe, Co and Ni. These materials are suitable for sol-gel processes. The gels were heated in the temperature range 50-950 °C and quenched at various temperatures to be examined spectroscopically (I.R. and X-ray powder diffraction). The thermolysis was carried out in both N2 and air. The final products are shown in the equations below ... 

The reactions of some simple as well as chelating alcohols (e.g., 2-alkoxy-alkanols) with metals, resulting in the formation of oxo-alkoxides (in place of simple binary alkoxides) will be dealt with in Sections IV and V. 

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. 

Oxo-alkoxides are partially condensed species and represent a new class of sol-gel precursors. These species are oligomeric and can be either homometallic or heterometallic. In many cases the oxo bridges produced in these compounds are multiply bridging, in order to achieve the most favored coordination number of six for metal atoms in alkoxide species. Isolation of compounds with similar multiply bridging oxygen atoms has been observed by the reaction of metals with the appropriate phenol or alcohol, with, in some cases, reaction with solvent molecules producing compounds quite different from the simple binary alkoxides expected.72 Besides isolation of intermediates, it is also possible to monitor the reactions by NMR and mass spectrometry 73... 

NKOiPr)s, VO(OiPr)3, Sb(OiPr)3, Mo(OiPr)s and Si(OEt)4 were the precursors adopted in the hydrolytic preparations. The syntheses were conducted under inert atmosphere by adding mixtures of water and 2-propanol to stirred mbctures of the alkoxides in 2-propanol. Each synthesis led to a stable sol, which turned into gel within few days. The metal precursors were mixed in order to obtain the following atomic ratios T.l for the binary systems Nb-V and Sb-V 1 1 1, 1 2 1 and 1 1 5, for the ternary systems Nb-V-Mo, Nb-V-Sb and Nb-V-Si, respectively. All the obtained xerogels were activated in air at 550°C for 5h. 

The hydridic metal hydrides react with alcohols to yield hydride-alkoxides or binary alkoxides depending on the amounts of alcohols used. 

Figure 2.13 Structural variety in binary and mixed alkoxide-glycolate derivatives of metals.

---