Metal alkoxides physical properties

PHYSICAL AND SPECTROSCOPIC PROPERTIES OF METAL ALKOXIDES AND ARYLOXIDES, M(OR),... 

Of the vast array of physical techniques available to the modern chemist, many have been applied to the study of metal alkoxides and aryloxides. We have decided to summarize the physical and spectroscopic properties of alkoxides and aryloxides according to stoichiometry and include techniques only where they highlight a particularly interesting characteristic of these compounds. The mass of more routine data that has been accumulated cannot be covered in detail here. 

In order to highlight the major aspects of the physical and spectroscopic properties of metal alkoxides and phenoxides, representative examples of homoleptic compounds of stoichiometries M(OR)v or M(OAr)x will be considered. 

The properties of Ge(OR)4 allow them to be considered more likely to be the esters of an inorganic acid than metal alkoxides these are colorless volatile liquids, containing monomeric tetrahedral molecules. The solid crystalline form is known only for R = Bu, OC6Hnc, and also 2,6-substituted phenoxides. All the members of the Ge(OR)4 homologous series are characterized by thoroughly determined physical characteristics — density, refraction index, surface tension, viscosity (and calculated parachor values), dipole moments in different solvents [222, 857, 1537] (Table 12.9). The results of the investigation of vapor pressure, density, viscosity polytherms, and so on. permitted rectification for the preparation of samples of high purity for sol-gel and MOCVD applications [682, 884]. 

The metal oxides prepared by conventional baking or by the CVD method are, in general, chemically stable, crystalline materials, and show excellent mechanical, electrical, optical, and physical properties. Flexible porous gel films obtained by the surface sol-gel process are totally different. In this chapter, we described a new preparative method for ultrathin metal oxide films by stepwise adsorption of various metal alkoxides. We named this method the surface sol-gel process. Structural characterization of the gel films thus obtained, the electrical property, and formation of nano-composites with organic compounds, were also explained. The soft porous gel contains many active hydroxyl groups at the surface and interior of the film. This facilitates adsorption of organic compounds, and consequent preparation of ultrathin metal oxide/polymer nano-composite films and organization of functional small molecules. In the nano-composites, proper selection of polymer components leads to the design of new materials with unique electrical, optical, and chemi-... 

In stark contrast to other metal alkoxides, the kinetics for the hydrolysis of Si(OR)4 compounds often require several days for completion. As a result, acid e.g., HCl, HE) or base (e.g., KOH, amines, NH3) catalysts are generally added to the mixture, which also greatly affects the physical properties of the final product. Under most conditions, condensation reactions begin while the hydrolytic processes are underway. However, altering the pH, [H20/M(0R) ] molar ratio, and catalyst may force the completion of hydrolysis prior to condensation. 

The syntheses, physical properties, and molecular structures of alkoxides and aryloxides have been discussed in CCC (1987).161 The alkoxides of scandium and yttrium were reviewed in CCC (1987).1 There have been more recent developments in this area and the impetus for this chemistry has been the developments in materials research. Metal alkoxides and /3-diketonates can be used as precursors for oxide and nonoxide thin films.162 The stable M—O bond and the volatility of the metal alkoxides are important features of this area of chemistry. This has lead to more research in this area particularly in synthesis, NMR, and X-ray crystallography. 

Table 40 shows the physical properties of Zr/silica as a function of the calcination temperature. Also shown for comparison are values characterizing plain silica, titanated silica, and silica treated with two other metal ions. These samples were made by first drying the silica base, then treating it with a metal alkoxide or chloride, followed by calcination in dry air at the specified temperature. It is evident from both the surface areas and the pore volumes of these samples that silica-zirconia made in this way is quite stable, and more stable than silica-titania. 

Sol-gel chemistry offers flexible methods for the preparation of porous metal oxides such as the transition aluminas used as catalyst supports. The physical properties of sol-gel materials depend on the nature of the reactants, the rate of mixing and the conditions of drying. High surface area aluminas have been prepared from various alkoxide and salt solutions and their textures have been examined extensively. The interest in the use of these chemically prepared materials is largely for catalyst and absorbent applications [1]. The first user of alkoxide precursor was Teichner who prepared pure aluminas by the water vapour action on aluminimn methoxide [2] and reported materials with surface area of 200 m /g. Harris and Sing [3] reported gels prepared from aluminium isopropoxide with water and... 

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