Metalloid alkoxides

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. 

The traditional precursors for sol-gel reactions have been metal alkoxides. In most of the sol-gel coating formulations, metal alkoxides/organofunctional metal alkoxides are used either alone or in combination with a conventional polymer capable of participating in sol-gel reactions. Sol-gel reactions start with a solution of monomeric metal or metalloid alkoxide precursors M(OR) in an alcohol or other low-molecular-weight organic solvent. Here, M refers to a network-forming element such as Si, Ti, Zr, etc, and R is typically an organic moiety. " ... 

In this section the reactions of selenophenes and tellurophenes with metal alkoxides, alkyls, aryls and amides will be considered. These reactions result primarily in proton abstraction and, in the monocyclic and benzo[6] fused systems, protons are removed preferentially from positions a to the heteroatom. In conformity with the general format of the reactivity sections in this work, the reactivity of the metal derivatives formed is dealt with in Section 3.16.8.8 on metalloid substituents. 

With the exception of nitrogen, all of the Group Vb elements are expected to form pentacoordinate compounds in their 5+ oxidation state, and this is, indeed, the case with some of the halides, alkoxides, etc. It was not until the pioneering work of Georg Wittig and his collaborators, however, that the first examples of pentaorganyls of these borderline elements between metals and metalloids were reported (95-99, 102, 104). In this early investigation, a complete set of the pentaphenyls could be obtained and characterized (95-99, 102), but apart from the pentamethyl-antimony case, all attempts for the preparation of pentaalkyl derivatives failed (104). 

Similar to the conventional method for the preparation of organic esters by esterification reactions, the alkoxides (orthoesters) of mainly metalloids as well as organometallic moieties (e.g., RXB, R,Ge, RxSn, or PhHg) can be synthesized (6, 34) by the following types of reactions. These reactions can be pushed to the right by removal of water, which is formed, azeotropically with an organic solvent (e.g., benzene/toluene). 

A large number of compounds exist between the branches of organic and inorganic chemistry that involve metals and metalloids bonding with organic compounds, including organometallics, soaps, and alkoxides. 

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 precursors for synthesizing these colloids consist of a metal or metalloid element surrounded by various reactive hgands. Metal alkoxides are most popular because they react readily with water. The most widely used metal alkoxides are the alkoxysilanes, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides such as aluminates, titanates, and borates are also commonly used in the sol-gel process, often mixed with TEOS. 

Metal alkoxides [M(OR) ] (where M = metal or metalloid of valency x R = simple alkyl, substituted alkyl, or alkenyl group and n = degree of molecnlar association), may be deemed to be formed by the replacement of the hydroxyhc hydrogen of an alcohol (ROH) by a metal(loid) atom. 

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. 

In spite of the fact that a wide variety of structurally interesting mixed ligand-alkoxide derivatives of a large number of metals and metalloids are known, X-ray structural data are available only in a very few cases such as Ce2(OPr )6(M-OC2H4NMeC2H4NMe2)2, ° [Ti OCH(CF3)2 2(OEt)2(HOEt)]2,2 2... 

Extensive work on the reactions of alkoxides of different metals and metalloids such as alkaline earth metals, ° lanthanides,titanium,zircon-uranium, vanadium,niobium tantalum, iron, boron, aluminium, silicon, germanium, " tin, antimony, selenium, and tellurium with a wide variety of glycols has been carried out by Mehrotra et al. 

Reactions of iV,iV-diethylhydroxylamme with alkoxides of metals and metalloids as shown in Eq. (2.245) have also been cleaner and quite general. 

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. 

R.C. Mehrotra, Alkoxides and Alkylalkoxides of Metals and Metalloids, Presidential Address, 54th Indian Science Congress, Hyderabad, India (1967). 

R.C. Mehrotra, R.K. Mittal, and A.K. Rai, Alkoxides of Metals and Metalloids, Rajasthan University Studies, Science Section Chemistry (1964-1965). 

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) ... 

In addition to the formation of alkali alkoxometallates by the reactions of alkali alkoxides (strong bases) with alkoxides of a variety of metals and metalloids (Lewis acids), formation of heterometal alkoxides has been shown to occur even between alkoxides of such similar metals as aluminium and gaUium as weU as niobium and tantalum. However, the formation constant of the latter derivative has been found to be statistical, which precludes the isolation of this bimetallic alkoxide in view of the equilibrium ... 

The first step in the sol-gel process is the preparation of an alkoxide of the metal or metalloid that is going to be made into the ceramic. This can be illustrated with the yttrium(III) ion, which is used to prepare a yttrium-oxygen ceramic ... 

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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