Heterometallic alkoxides properties

A description of the various physicochemical properties (e.g., volatility, dipole moments, refractivity, density, viscosity, magnetic susceptibilities, IR, ESR spectra, mass spectra, and NMR spectra) for elucidation of the structural features of homo- and heterometallic alkoxides was already presented in a number of publications [e.g., (3, 4, 6, 19, 34)]. 

This strategy was adopted to alleviate tedious techniques of mixing multicomponent precursors with different properties. In view of their technological demands, tin(II) heterometallic alkoxides are the most studied class of compounds and their properties can be modulated by chelating ligands like /3-diketonates, carboxylates, and functionalized alkoxides for specific purposes. These compounds also facilitate the formation of oxoalkoxides, either by hydrolysis or by some side reactions. 

This chapter deals with the synthesis and general properties of homo- and heterometallic alkoxides that are or might be used in chemical routes to electrooptical ceramic materials. Some considerations concerning structure, reactivity, and tailoring of their properties are also given. Emphasis is given to the most recent results. 

The molecular precursors are extremely sensitive towards atmospheric moisture, on slight exposure to atmospheric moisture immediately changes into hydroxide due to electronegativity of the central metal ions and the type of substituted alkoxy group used. Heterometallic alkoxide shows different physical and chemical properties due to the presence of different types of metal ions to act as Lewis acid along with the type of alkoxy group used which differs from simple alkoxide. The nanomaterials prepared from monomeric simple alkoxide differ in properties prepared from bimetallic alkoxides as shown in Figure 2.4 [51-64]. 

The chemical properties of heterometallic alkoxides are in general similar to their homometal counterparts (i) hydrolysis, (ii) alcoholysis, (iii) trans-esterification reactions, (iv) reactivity with carboxylic acids and enolic forms of chelating ligands such as /3-diketones/j6-ketoesters. The hydrolytic reactions are now widely employed for the preparation of homogeneous mixed metal-oxide ceramic materials and the rest have found wide applications for the synthesis of a variety of novel metallo-organic derivatives (sometimes unique), which are not often available through any other synthetic route. ... 

In addition to using homometallic alkoxides as reagents in sol-gel chemistry, efforts are also directed to the design of heterometallic alkoxides that provide a high homogeneity and reactivity to the synthesized solution [14,15], This permits obtaining complex oxide thin films with better properties. 

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. 

The general properties of the significantly covalent behavior of metal alkoxides (both homo- and heterometallic), in spite of the polar character of the M5+—O5- bond, were already dealt with in some detail (6). The effect of steric and inductive factors on the extent of polarization of the M6+—O5- bond as well as the consequent degree of association and volatility can be exemplified by the boiling points (under 1-mm pressure) and the observed degrees of their association (given in parentheses) by the three isomeric butoxides of zirconium Zr(0-n-Bu)4 ( 250°C 3.5) Zr(0-sec-Bu)4 ( 150°C 2.0 Zr(0-/-Bu)4 ( 50°C 1.0). However, the similarities in the molecular association of the neopentyloxides of Ti, Zr, and A1 to the secondary rather than primaiy amy-loxides have been adduced to indicate the higher predominance of steric rather than inductive factors in the above directions. Similarly, the insolubility and... 

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