Metal silanolates reactivity

Base-Initiated Polymerization. Although the base-initiated mechanism for reaction 2 was once thought to proceed via free ionic intermediates 31), little evidence supported this mechanism. Conductivities of solutions of alkali metal silanolates in moderately polar solvents are essentially nil (32), and most workers now agree that reactive intermediates consist of ion pairs or charge-separated ion pairs 4-5). The key intermediate is believed to involve coordination of the countercation of the ion pair at the chain end, for example potassium, with the cyclosiloxane in a manner analogous to the crown ethers or cryptates (33), as shown by structure 1. 

The reactivity of the metal silanolate catalyst is dependent on the nature of the metal counter-ion, the larger metal ions giving rise to more active catalysts. For example, in the metal silanolate series the order of catalyst activity23 is Liquaternary ammonium and quaternary phosphonium silanolates having the same order of activity as caesium silanolate. Lithium and sodium silanolates are not very powerful catalysts for cyclosiloxane polymerization unless used in conjunction with an activating solvent such as tetrahydrofuran (THF) or dimethyl sulphoxide (DMSO). 

Before reaction, the silica is treated with acid (eg refluxed for a few hours with 0.1 mol dm-3 HC1). This treatment produces a high concentration of reactive silanol groups at the silica surface, and also removes metal contamination and fines from the pores of the material. After drying, the silica is then refluxed with the dimethylchlorosi-lane in a suitable solvent, washed free of unreacted silane and dried. This reaction produces what is called a monomeric bonded phase, as each molecule of the silylating agent can react with only one silanol group. 

This method of silanation, which uses organic solvent without the addition of water, is suitable for highly reactive silane derivatives, such as chlorosilanes, aminosilanes, and methoxysilanes. This procedure will not work for ethoxysilanes, as these compounds are not reactive enough without prior hydrolysis to create the silanol. This method is convenient to use for silica particle modification and for the functionalization of metallic nanoparticles having the requisite—OH groups present (see Chapter 14, Section 5). 

The fact that silsesquioxane molecules like 2-7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties. This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest. As mentioned above, incompletely condensed silsesquioxanes of the type R7Si70g(0H)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces. Metal... 

Zirconium and hafnium tetraalkoxides are highly reactive compounds. They react with water, alcohols, silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as j8-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML (OR)4 these reactions are discussed in the sections dealing with the chelating ligand. 

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