Chemistry Reference
In-Depth Information
Pr
i
Pr
i
Pr
i
Pr
i
H
O
OH
O
O
O
O
+ 6HOH
− 6Pr
i
OH
Ln
Al
Ln
Al
Ln
Al
Standing
O
O
O
O
OH
Pr
i
3
3
3
Pr
i
Pr
i
Pr
i
H
Transient white precipitate
Clear solution
−Pr
i
OH, −H
2
O
Heating
LnAlO
3
3
.
120
The mechanism suggested above is confirmed by
1
H NMR studies, indicating that
as expected the terminal isopropoxide groups are replaced initially as indicated by
the insolubility of the intermediate products in Eqs (3.118) and (3.120), but these
methoxide/hydroxide groups are preferentially transferred to the bridging positions
(bringing about higher thermodynamic stability); the external environment of the final
products thus becomes similar to the original reactant, Lnf
-OPr
i
2
Al
OPr
i
2
g
3
which
brings about the observed solubility.
An interesting observation was made by Jones
et al
.
44
that in the sol - gel process
for the preparation MgAl
2
O
4
by the hydrolysis of MgfAl
OEt
4
g
2
in the presence
of triethanolamine, (Eq. 3.121) the original framework of the precursor bimetallic
ethoxide remains almost unaltered (as revealed by
1
Hand
27
Al NMR studies), indi-
cating the stability of the coordinated structure:
Et
H
H
OEt
OEt
OH
O
O
O
Mg
Al
Mg
Al
Mg
Al
MgAl
2
O
4
O
O
O
OEt
OEt
OH
2
2
2
Et
H
H
3
.
121
A similar observation about the retention of the original framework in the initial steps
of hydrolysis of MgfNb
OC
2
H
4
OMe
6
g
2
,
199
[LiZr
OPr
i
5
]
5
and [BafZr
2
OPr
i
9
g
2
]
45
also points to the stability of the original structure of the bimetallic alkoxide
precursor. Detailed investigations on the mechanism of the formation of BaTiO
3
from barium - titanium-ethoxide systems by Turova
et al
.
200
tend to point to similar
conclusions.
In view of their importance in the sol- gel preparation of spinel type
materials, detailed stepwise hydrolytic reaction studies of [CafAl
OPr
i
4
g
2
]
201
,
202
and
[MgfAl
OPr
i
4
g
2
]
203
have been carried out with similar results. With the synthesis
of heteropolymetallic alkoxides as precursors, the importance of extending hydrolytic
studies to such systems is obvious.
20
,
24
As illustrated by a number of examples in Section 1, the X-ray structural elucidation
of a rapidly increasing number of heterometallic alkoxides (Chapter 5) has in general
confirmed their coordination models
165
,
204
with chelating ligands like fM
OR
n
C1
g
(M D Al, Ga, Nb, Ta) and fM
2
OR
2
n
C1
g
(M D Zr, Hf, Sn(
IV
)) of metals with valency
n
, as suggested since 1971.
Heterometallic alkoxides, therefore, constitute a novel class of heterometal coordi-
nation systems, stabilized by alkoxide bridges without the support of any auxiliary
ligands (like CO) or metal - metal bonds. The factors responsible for this extraordinary
stability of heterometallic alkoxides are not yet fully understood, but the formation of
