Chemistry Reference
In-Depth Information
More than two decades after the preparation of a large number of the so-called
“alkoxo salts” by Meerwein and Bersin
1
in 1929, UfAl
OPr
i
4
g
4
was synthesized in
1952 by Albers
et al
.
30
and evidence for the formation of MfU
OEt
6
g
n
(M D Na, Ca,
Al) was obtained by Jones
et al
.
31
in 1956. Similarly, formation of a number of anionic
methoxide species was indicated in the potentiometric titrations of chlorides of metals
(B, Al, Ti, Nb, Ta) with lithium methoxide in methanol by Gut
32
in 1964. Ludman
and Waddington
33
studied the conductometric titrations of a wide variety of Lewis
acids with basic metal methoxides and reported the formation of “alkoxo salts” of the
type KB(OMe)
4
and K
3
Fe(OMe)
6
. Schloder and Protzer
34
also synthesized a number of
bimetallic alkoxides of aluminium with the formulae, MAl(OMe)
4
and M
0
fAl(OMe)
4
g
2
where M and M
0
are alkali and alkaline earth metals respectively.
The synthesis of volatile (under reduced pressure, indicating stability to heat) and
soluble (in organic solvents) derivatives of alkali metals and zirconium, with rather
unexpected compositions,
e.g.
NaZr
2
(OEt)
9
by Bartley and Wardlaw
2
gave a new orien-
tation to the field. Attention was drawn to the apparently covalent characteristics of
such derivatives, in spite of the strongly electropositive alkali and alkaline earth metal
component, in review articles by Mehrotra and Kapoor,
35
,
36
in whose group a large
number of such derivatives of alkali and alkaline earth metals were synthesized, with
confirmation
3
of the covalent characteristics of the derivatives by conductivity measure-
ments. Such covalent heterometallic alkoxides have also been given the classification
“molecular” in a review by Caulton and Hubert-Pfalzgraf.
16
The coordination models of the following types suggested for tetraisopropoxoalu-
minate derivatives:
Pr
i
O
OPr
i
M
Al
Pr
i
O
OPr
i
n
(M D Be,Mg,Ca,Sr,Ba,Zn,Cd,Cr,Mn,Fe,Co,Ni,Cu,In,Th,etc.),aswellas
Pr
i
Pr
i
O
Pr
i
O
Pr
i
O
OH
Al
K
OPr
i
OH
Pr
i
have been confirmed in a number of recent publications by the X-ray struc-
tural elucidation of molecular species such as: [
HOPr
i
2
Mg]f
-OPr
i
2
Al
OPr
i
2
g
2
,
37
[
Pr
i
OH
2
K
-OPr
i
2
Al
OPr
i
2
]
n
,
38
[fPrf
-OPr
i
2
Al
OPr
i
2
g
2
Pr
i
OH
-Cl
g
2
],
39
and
[Erf
-OPr
i
2
Al
OPr
i
2
g
3
].
40
It is interesting to note that in the penultimate example, two [fPrfAl
OPr
i
4
g
2
Pr
i
OH
-Cl
g
2
] groups are held together by chloride bridges. With the rapidly increasing appli-
cations of heterometal alkoxides for mixed metal - oxide ceramic materials, evidence
is accumulating for the formation of a large number of oxo-heterometal alkoxides
(Chapter 5), which may well be intermediate stages in the hydrolytic sol - gel process.
X-ray structural studies on a number of such oxo-derivatives have shown that in these,
alkoxo moieties of two or three elements are linked together
via
oxo-ligands of various
