Group 13 elements alkoxides

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

The most common sol-gel process employs metal alkoxides of network forming elements (M(0R) where M is Si, B, Ti, Al, etc. and R is often an alkyl group) as monomeric precursors. In alcohol/water solutions the alkoxide groups are removed stepwise by hydrolysis reactions, generally employing acid or base catalysts, and are... 

Ziegler catalysis involves rapid polymerization of ethylene and a-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide (1,2). Catalysts of this type operate at low pressures (up to 30 atm), but often at 8-10 atm, and, in special cases, even under reduced pressure, and at temperatures up to 120°C, but often as low as 20-50°C. Approximately 2,200,000 tons of polyethylene and 2,900,000 tons of polypropylene are produced per year with the aid of such catalysts. The polyeth-... 

AUcoxy groups are rather hard jr-donor ligands. They stabilize the highest oxidation number of the metal. Therefore alkoxides of main group elements and d° transition metals are well known, while those corresponding to soft d late transition metals have been much less studied. The number and stability of metal alkoxides decrease from left to right across the periodic table. 

Sol-gel processing techniques have largely been applied to main group alkoxides, particularly Si and Al, but increasingly studies have dealt with early transition metal elements such as Ti and Zr. The preparation of metal alkoxides has been reviewed extensively.46 18 Self-condensation of alkoxides can occur, depending on the steric demands of the alkoxide group for example, Ti methoxide, ethoxide, and isopropoxide are tetrameric, trimeric, and dimeric, respectively,49 while the zirconium isopropoxide is tetrameric.50... 

There are a number of reactions known involving metal alkoxides and organometallic compounds of main group elements (Li, Mg, Al, Sn, or Zn). The fundamental reaction generally involves elimination of a main group metal alkoxide. The composition of the reaction products is considerably influenced by (i) the nature and chemistry of the reactants as well as their stoichiometry, (ii) the reaction conditions, and (iii) the presence of other reactive substrates in the reaction medium. Some typical reactions are illustrated by Eqs (2.301)-(2.319). 

Sol-gel chemistry offers a unique advantage in the creation of novel organic-inorganic hybrids. The sol-gel process begins with a solution of metal alkoxide precursors [M(0/f) ] and water, where M is a network-forming element, and R is typically an alkyl group. Hydrolysis and... 

The second approach (Equation(3)) has a number of advantages over the first one (Equation(2)). The alkyl complexes are more reactive than the related alkoxides, the latter being for group 4 elements generally associated into dimers or trimers 48 also, reaction (2) liberates an alcohol which may further react with the surface of silica, whereas the alkane ( Equation(3)) is inert. It was demonstrated by various spectroscopic techniques and elemental analysis that with a silica dehydroxylated at 500 °C under vacuum, the stoichiometry of reaction (3) corresponds to n = 1.45,46 Moreover, a better control of the surface reaction was achieved with the procedure represented in Equation(3). 

In such reactions the leaving group is a halide ion but many other groups such as sulphonates, sulphates or alkoxides have also been used. A bond is formed between R and some non-metallic or metallic element. 

This method for the preparation of metal alkoxides and phenoxides is of only limited usefulness given the relatively low pAa values of alcohols and even common phenols. Hence, the method is confined to the more electropositive elements where reaction can occur either directly with the alcohol or phenol or sometimes in the presence of a catalyst. For the Group I elements (Li, Na, K, etc.) the dissolution of the metal into the neat alcohol or phenol at close to reflux temperatures can lead to the pure alkoxides or phenoxides with the evolution of hydrogen (equation l).6,7... 

For alkoxides of the Group I and II elements and the lanthanides, typical adducts involve coordinating O-donors such as Et20 or THF. For example, the sparingly soluble, anhydrous aryloxide LiOAr (OAr = 2,6-di-r-butylphenoxide) will dissolve in ethers to give dimeric adducts of the type [(OAr )Li(L)]2 (L = Et20, THF, py), which have been structurally characterized.22 ... 

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