Chemistry Reference
In-Depth Information
attack. The combined effect of these factors makes metal alkoxides in general a
highly reactive class of metallo-organic compounds. Metal alkoxides are, for example,
extremely susceptible to hydrolysis even by atmospheric moisture, and require careful
handling under stringently anhydrous conditions.
Metal alkoxides react readily with a wide variety of hydroxylic reagents such as
water,
3
,
21
,
617 - 619
alcohol,
3 - 8
,
21
,
617
alkanolamines,
4
,
21
,
620 - 627
,
632
carboxylic acids,
628
ˇ
-
diketones and
ˇ
-ketoesters,
629 - 631
Schiff bases,
632
,
633
oximes, hydroxylamines,
634
,
635
and glycols,
2
,
4
,
632
to afford homo- and heteroleptic derivatives involving metals in
novel coordination states and geometries. The reactions of metal alkoxides with mono-
functional reagents (LOH) can be illustrated by Eq. (2.182):
M
OR
x
C
y
LOH ! M
OR
x
y
OL
y
C
y
ROH "
2
.
182
These reactions can be carried out quantitatively in the case of metal isopropoxides
(or ethoxides) by the azeotropic fractionation of the liberated alcohol with a suitable
solvent (
e.g.
benzene), yielding mixed alkoxides or alkoxide-ligand (carboxylate,
ˇ
-
diketonate, aminoalkoxide) derivatives. Many of these mixed ligand derivatives can
be recrystallized and appear to be stable to heat (volatilizing without decomposition
under reduced pressure).
A large number of such derivatives synthesized between 1960 and 1980 in the
research group of Mehrotra
3
,
4
,
7
have been assigned interesting structures mainly on
the basis of NMR studies and colligative properties and await X-ray crystallographic
elucidation.
In addition to the general procedure illustrated by Eq. (2.182), another route has been
employed for the synthesis of metal alkoxides (and particularly analogous siloxides) by
the reaction of metal alkoxides with esters (and silyl esters) (Section 4.3). Further, the
reactions of metal alkoxides with acyl halides (or hydrogen halides) and unsaturated
substrates such as CO, CO
2
,CS
2
, organic-isocyanates and isothiocyanates, chloral, etc.
have been utilized for the preparation of halide-alkoxides (Section 4.11) and interesting
insertion products (Section 4.12), respectively.
Metal alkoxides are also sometimes reactive towards other substrates having reactive
protons of -NH
2
or -SH groups. In these cases, however, the reactions are controlled
mainly by thermodynamic factors and are governed by the comparative stability and
strength of M-O, M-N, and M-S bonds. For example, the -NH
2
groups of alkanol-
amines and even substituted analogues are reactive with tin(
IV
) alkoxides
4
,
632
but not
with analogues of titanium(
IV
).
4
,
632
It is worth mentioning that a number of new classes of metal-organic compounds
have become conveniently accessible only by the use of metal alkoxides as synthons.
For example, the synthesis of pure aluminium tricarboxylates, Al
O
2
CR
3
(even the
existence of which was seriously doubted in 1949 by Gray and Alexander),
636
after
numerous reported failures over the previous two decades was successfully achieved
in 1953 by the preparation of Al
O
2
CR
3
(R D C
15
H
31
,C
17
H
35
) through the reactions
of aluminium alkoxides
637
with carboxylic acids:
facile
! 4
RCO
2
2
Al
-OPr
i
2
Al
O
2
CR
2
C 8Pr
i
OH
2
.
183
[Al
OPr
i
3
]
4
C 16RCOOH
