Chemistry Reference
In-Depth Information
bridging modes in addition to
2
-or
3
-bridging alkoxide groups. The formation of
oxo-derivatives under stringently anhydrous conditions indicates the possibility of their
formation by some alternative pathway, such as elimination of ether or alkene.
Chelation of a central metal atom by ligands (represented in general by L) like
fAl
OR
4
g
, fNb
OR
6
g
, fTa
OR
6
g
,andfZr
2
OR
9
g
gives rise to a large number
of homoleptic bimetallic alkoxides, which can be represented by ML
x
(where
x
is the
principal valency and 2
x
is the coordination number of M); this coordination number
would be higher (3
x
or 4
x
) when the alkoxometallate ligand is tri- or tetradentate:
e.g.
(before six-coordinate) nickel in Nif
-OMe
3
Al
OMe
g
2
(Section 3.3.1).
In addition to the homoleptic bimetallic derivatives like [MfAl
OPr
i
4
g
x
]hetero-
leptic tri- and tetrametallic derivatives with the general formula,
[MfAl
OPr
i
4
g
a
fNb
OPr
i
6
g
b
fZr
2
OPr
i
9
g
c
X
x
a
b
c
] (where X is a ligand of the type
Cl, OR, acac, etc.) have also been synthesized in the laboratories of Mehrotra
7
,
14
,
15
,
17 - 25
since 1985. Many of these have been shown to volatilize unchanged under reduced pres-
sure. These “stable” derivatives have thus added a novel dimension to heterometallic
coordination systems.
23
The prominent effect of steric factors on the stability and struc-
tural features of some bimetallic species has been observed in a comparison of similar
pairs: (i) [NaZr
2
OPr
i
9
]and[fNaZr
OBu
t
5
g
2
];
41
(ii) [Nif
-OMe
3
Al
OMe
g
2
]and
[Nif
-OPr
i
-OBu
t
Al
OBu
t
2
g
2
].
42
Some other novel types of heterometallic
alkoxide/glycolate/aminoalkoxide derivatives have also been synthesized.
43
In view of their volatility and solubility in organic solvents, these heterometallic
alkoxides have become attractive precursors for ceramic materials by the MOCVD
26
and sol - gel
27
processes (Chapter 6). Interesting observations that the framework of
these heterometallic alkoxides, for example MgfAl
OEt
4
g
2
44
and BafZr
2
OPr
i
9
g
2
,
45
remains unaltered at least during the early stage(s) of hydrolysis, has further underlined
the stability of these systems. The above far-reaching observations and the variety of
routes now available for various heterometal alkoxide systems have led workers to
anticipate the synthesis of “single source” precursors,
22 - 25
with a composition corre-
sponding to the targeted ceramic material, although the possibilities of alteration(s) in
composition during the sol - gel treatment have to be monitored carefully.
In the following periodic table, the underlined elements are those of which mixed
metal - oxide ceramic materials have been prepared,
46
whilst the encircled ones indicate
the elements whose heterometallic alkoxide chemistry has been investigated:
H
He
Li
Be
B
C
N
O
F
Ne
Mg
Na
Al
Si
P
S
Cl
Ar
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
K
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Rb
Ln
∗
Cs
Ba
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
Fr Ra An
∗∗
∗
Ln
=
La Ce Pr Nd Pr Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
∗∗
An
=
Ac Th Pa U
