Metalloid alkoxides, synthesis

The strategy of metal alkoxides synthesis is entirely related to the electronegativity of the element concerned. Some electropositive metals, such as alkali metals, alkaline earth metals, and lanthanides, react directly with alcohols. But some less electropositive metals such as magnesium and aluminum require a catalyst (I or HgCy for successful reaction with alcohols. Use of electrochemical synthesis by anodic dissolution of some metals or metalloids (Sc, Y, Ti, Zr, Nb, Ta, Fe, Co, Ni, Cu, Pb, Si, Ge, etc.) in dry alcohol performs a promising procedure because it does not produce any by-products except hydrogen gas. Another applicable method for the synthesis of some alkoxides (B, Si, Ti, Zr, Hf, Nb, Ta, Fe, etc.) is the reaction of their chlorides with alcohols which require a base such as... 

The following pages present a brief summary of the general methods used for the synthesis of metal and metalloid alkoxides applicable to specific systems. Tables 2.1 and 2.2 in Section 2.1 (pp. 6-14) list some illustrative compounds along with their preparative routes and characterization techniques. 

For the synthesis of metalloid (B, Si) alkoxides, the method generally employed consists of the reaction of their covalent halides (usually chlorides) with an appropriate alcohol. However, the replacement of chloride by the alkoxo group(s) does not appear to proceed to completion, when the central element is comparatively more electropositive. In such cases (e.g. titanium, niobium, iron, lanthanides, thorium) excluding the strongly electropositive s-block metals, the replacement of halide could in general be pushed... 

The addition of a base, typically ammonia, to mixtnres of metal(loid) halides and alcohols allows the synthesis of homoleptic alkoxides for a wide range of metals and metalloids. Anhydrous ammonia appears to have been employed for the first time by Nelles " in 1939 for the preparation of titanium tetra-alkoxides (Eq. 2.33) ... 

The above technique has been quite successful for the synthesis of alkoxides of a number of s- and j)-block metals and metalloids such as sodium, boron, thallium, " silicon, " " " and arsenic. 

Although many routes for the synthesis of metal complexes of Schiff bases and P-ketoamines are available, " the facile reactivity of metal alkoxides has been utilized for the synthesis of homo- and heteroleptic Schiff base and -ketoamine derivatives of metals and metalloids with advantage. The most interesting lesnlts concern the Schiff bases and /6-ketoamines shown in Fig. 2.15, and other ligands obtained by snitably modifying them. 

Following the procedure of Meerwein and Bersin and others, - - this method has been extended to the bimetallic alkoxides of alkali metals (Lewis bases) with those of less basic metals and metalloids, beryllium, zinc, - boron, aluminium, gallium,tin(n), " tin(iv), antimony(iii), bismuth, titanium," niobium(iv), zirconium, " thorium, niobium(v), tantalum(v) and copper. Equations (3.1)-(3.3) reflect a few typical reactions used in the synthesis of bimetallic alkoxides involving alkali metals (M) ... 

Today, even after more than 20 years, almost all developed methods related to the synthesis of mesoporous materials by surfactant soft templates still use knowledge based on mesoporous silica materials. The synthesis of mesoporous oxides of TMs (i.e., Ti, Zr, and Mn), metalloids (i.e., Ge), posttransition metals (i.e., A1 and Ga), and lanthanides (i.e., Ce) has been adapted from the methods developed in mesoporous silica synthesis [44-49]. In other words, one can easily find a silica analog of any procedure for the synthesis of non-silicious mesoporous oxides. Flexible Si—O bonds made via well-known and easily manageable sol-gel chemistry, allow one to use various solvents or solvent mixtures (i.e., aqueous or alcoholic), pH (1-7), temperatures, and pressures to synthesize numerous mesoporous silica materials [50]. However, sol-gel chemistry of other elements especially TMs requires more controlled reaction conditions. The sol-gel chemistry (hydrolysis and condensation) of early (group I-IV) TMs can be controlled in alcoholic solutions with proper pH, temperature, and humidity adjustments [2,4,10,46,47,50]. Typical TM sources are either commercially available alkoxides (i.e., titanium isopropox-ide) or can be formed in situ by the reaction between anhydrous TM chloride salts and alcohols (i.e., WClg + EtOH W(OCH2CH3)6). 

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