Chemistry Reference
In-Depth Information
2.1.1.2 Preparation of bimetallic alkoxides of some other metals (A-2)
In addition to the formation of alkali alkoxometallates by the reactions of alkali alkox-
ides (strong bases) with alkoxides of a variety of metals and metalloids (Lewis acids),
formation of heterometal alkoxides has been shown to occur even between alkoxides
of such similar metals as aluminium and gallium
69
as well as niobium and tantalum.
70
However, the formation constant of the latter derivative has been found to be statistical,
which precludes the isolation of this bimetallic alkoxide in view of the equilibrium:
MeO
4
Nb
-OMe
2
Ta
OMe
4
Nb
OMe
5
C Ta
OMe
5
3
.
4
An early conjecture of Dislich
11
in this direction is noteworthy. The extraordinary
homogeneity of the final ceramic product obtained by the sol - gel hydrolysis of a
mixture of alkoxides of various metals in alcoholic solution, led him to suggest that
in addition to the advantage of much more efficient mixing at the molecular level in
the initial precursor solution, some sort of complexation reactions must be occurring
amongst the alkoxides of various metals in the initial stages:
m
Si(OR)
4
+
n
B(OR)
3
+
p
Al(OR)
3
+
q
(NaOR)
Complexation in alcoholic medium
(Si
m
B
n
Al
p
Na
q
)(OR)
4
m
+3
n
+3
p
+
q
(3.5)
Hydrolysis by added water
−
(Si
m
B
n
Al
p
Na
q
)(OH)
4
m
+3
n
+3
p
+
q
(3.6)
ROH
Sol changing to gel by partial
dehydration
reactions
or dealcoholation
followed by
sintering
(Si
m
B
n
Al
p
Na
q
)(O)
(4
m
+3
n
+3
p
+
q
)/2
Ultrahomogeneous
oxide ceramic or glassy material
Although the equilibria involved in this type of complexation may be rather labile
and require further detailed investigations, yet examples of the isolation of a few
well-defined heterometallic alkoxides from mixtures of component alkoxides may be
mentioned here. For example, bimetallic isopropoxides of lanthanides with aluminium
can be conveniently volatilized out of a reaction mixture of Ln
OPr
i
3
and Al
OPr
i
3
(Eq. 3.7):
1
n
fLn
OPr
i
3
g
n
C
3
4
fAl
OPr
i
3
g
4
! [LnfAl
OPr
i
4
g
3
]
3
.
7
In fact, with the use of excess Al
OPr
i
3
in the initial reaction mixture, the more
volatile Al
OPr
i
3
distils out first followed by LnfAl
OPr
i
4
g
3
in the temperature range
160 - 180
Ž
C/1 mm (the volatility increasing with lanthanide contraction). However, if
the much less volatile fLn
OPr
i
3
g
n
is in excess, the bimetallic isopropoxide can be
distilled out first, leaving the excess (
>
1/3 mole) of Ln
OPr
i
3
behind.
71
,
72
Formation
of a similar product, InfAl
OPr
i
4
g
3
has been reported
69
,
73
in the reaction of In
OPr
i
3
with Al
OPr
i
3
in 1:3 molar ratio.
The reaction of thorium isopropoxide with excess aluminium isopropoxide yields
74
ThfAl
OPr
i
4
g
4
.
a
product
of
the
composition,
However,
similar
reactions
of
