2 Homometallic Alkoxides

22 Alkoxo and Aryloxo Derivatives of Metals
2.5.1 The Ammonia Method (E-1)
The addition of a base, typically ammonia, to mixtures 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 134 in 1939 for the preparation of titanium tetra-alkoxides (Eq. 2.33):
TiCl4 C 4ROH C 4NH3
(excess)
benzene
! Ti(OR) 4 C 4NH4Cl # ⊲2.33⊳
Zirconium tetra-alkoxides were prepared for the first time in 1950 by the ammonia
method, 135 as earlier attempts 136 to use the alkali alkoxide method did not give a
pure product, owing to the tendency of zirconium to form stable heterobimetallic
alkoxides 137 (Chapter 3) with alkali metals.
The ammonia method has, therefore, been successfully employed 3,4,21,26 for the
synthesis of a large number of alkoxides of main-group and transition metals according
to the following general reaction (Eq. 2.34):
MClx C xROH C xNH3
benzene
! M(OR) x C xNH4Cl # ⊲2.34⊳
Owing to the highly hydrolysable nature of most of the alkoxide derivatives, stringently
anhydrous conditions are essential for successful preparation of the alkoxides.
Apart from careful drying of all the reagents as well as solvents, gaseous ammonia
should be carefully dried by passage through a series of towers packed with anhydrous
calcium oxide, followed preferably by bubbling through a solution of aluminium
isopropoxide in benzene.
Benzene has been reported to be a good solvent for the preparation of metal alkoxides
by the ammonia method, as its presence tends to reduce the solubility of ammonium
chloride, which has a fair solubility in ammoniacal alcohols. In addition, the ammonium
chloride precipitated tends to be more crystalline under these conditions, making
filtration easier and quicker. Although most of the earlier laboratory preparations have
been carried out in benzene, the recently emphasized carcinogenic properties of this
solvent suggests that the use of an alternative solvent should be explored.
2.5.1.1 Group 3 and f-block metals
To date no unfluorinated alkoxides of scandium, yttrium, and lanthanides 18,21 in the
common C3 oxidation state appear to have been prepared by the ammonia method.
By contrast, yttrium and lanthanides (Ln) fluoroalkoxide derivatives of the types
LnfOCH⊲CF3⊳2g3 138 and LnfOCH⊲CF3⊳2g3.2NH3 139 have been isolated by this route.
It might appear that the ammonia method is applicable to the synthesis of a large
number of metal alkoxides, but there are certain limitations. For example, metal chlorides
(such as LaCl3) 140 tend to form a stable and insoluble ammoniate M⊲NH3⊳yCln
instead of the corresponding homoleptic alkoxide derivative. Difficulties may also arise
if the metal forms an alkoxide which has a base strength comparable with or greater than
that of ammonia. Thorium provides a good example of this type where the ammonia
method has not been found to be entirely satisfactory. 141 For example, during the
preparation of thorium tetra-alkoxides from ThCl4 and alcohols, Bradley et al. 142 could