Metal alkoxides reactions with silanols

Similarly, alcoholysis of Mo(NMc2)4 allows a ready entry into Mo alkoxide chemistry, although reactions with silanols and phenols typically produce adducts in which the liberated dimethyl-amine remains coordinated to the metal center (equation 12). 

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. 

Hamaide and co-workers grafted metal alkoxides (eg Al, Nd, Y, Sm, Zr) onto silica and alumina (57,94). Nevertheless, the alcohol that is released in the reaction medium can react with the silanol groups of silica and form water. Therefore, part of the active species can be lost by hydrolysis. In case of alumina, Al—0—R species can be additionnally formed and participate in the polymerization. 

A complicating factor affecting the reactions of metal alkoxides with silanols is the condensation tendency of silanols to yield hexa-alkyldisiloxanes and water (Eq. 2.200) this may be to some extent avoided by adding the silanol slowly to the reaction mixture. 

A number of compounds of the types RBiY2 or R2BiY, where Y is an anionic group other than halogen, have been prepared by the reaction of a dihalo- or halobismuthine with a lithium, sodium, potassium, ammonium, silver, or lead alkoxide (120,121), amide (122,123), a2ide (124,125), carboxylate (121,126), cyanide (125,127), dithiocarbamate (128,129), mercaptide (130,131), nitrate (108), phenoxide (120), selenocyanate (125), silanolate (132), thiocyanate (125,127), or xanthate (133). Dialkyl- and diaryUialobismuthines can also be readily converted to secondary bismuthides by treatment with an alkali metal (50,105,134) ... 

The preparation and structures of siloxanediol derivatives has been reviewed29,184 but some representative examples of reactions are given below. Two general methods have been used to make metal siloxides from silanols one involves the direct reaction of a silanol with the metal derivative, usually an amide, alkoxide, halide or alkyl (to give amine, alcohol, HX or alkane byproducts respectively), while the second involves the preparation of a simple alkali metal derivative which can then be used as a siloxide transfer reagent for further synthesis. Only the direct route will be considered here. Two of the most useful siloxide transfer species, (MOPh2Si)20 (M = Li or Na), are... 

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