Chemistry Reference
In-Depth Information
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):
benzene
! Ti(OR)
4
C 4NH
4
Cl #
2
.
33
TiCl
4
C 4ROH C 4NH
3
(excess)
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):
benzene
! M(OR)
x
C
x
NH
4
Cl #
MCl
x
C
x
ROH C
x
NH
3
2
.
34
Owing to the highly hydrolysable nature of most of the alkoxide derivatives, strin-
gently 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 anhy-
drous 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 ammo-
nium 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
CF
3
2
g
3
138
and LnfOCH
CF
3
2
g
3
.
2NH
3
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 chlo-
rides (such as LaCl
3
)
140
tend to form a stable and insoluble ammoniate M
NH
3
y
Cl
n
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 ThCl
4
and alcohols, Bradley
et al
.
142
could
