Chemistry Reference
In-Depth Information
The X-ray structural study of LiNb
OEt
6
212
,
213
and [
Pr
i
OH
2
BafNb
OPr
i
6
g
2
]
214
reveals familiar patterns. However, LaNb
2
OPr
i
13
reveals
214
the bonding of the central
La atom by a bidentate and another tridentate fNb
OPr
i
6
g group in addition to coor-
dination with a free isopropanol molecule. This manner of novel double (bi- as well
as tri-) types of bonding indicates the need for further detailed investigations in such
systems.
3.4
Heterometallic Alkoxides Involving Alkoxometallate(
IV
) Ligands
3.4.1
Introduction
The synthesis of volatile and soluble (in organic solvents) covalent
2
,
3
,
35
,
36
,
183 - 185
nona-
alkoxodizirconates of alkali metals such as KZr
2
OR
9
(R D Et, Pr
i
), by Bartley
and Wardlaw
2
in 1958 initiated an entirely new dimension in the chemistry of
heterometallic alkoxides. Historically, a nona-alkoxostannate derivative, NaSn
2
OEt
9
had been described by Bradley
et al
.,
216
in 1957 as an intermediate during their efforts
to synthesize tin tetra-alkoxides by the reactions of SnCl
4
with NaOEt.
It was Mehrotra
41
who drew attention to the uniqueness of [MfZr
2
OPr
i
9
g](MD Li,
Na, K) derivatives based on their nonconducting behaviour in isopropanol. These
workers also synthesized similar derivatives of titanium and tin(
IV
) by reacting them
with alkali isopropoxides in 2:1 molar ratio; both of these could be recrystallized
without any change in composition. However, whereas the zirconium derivatives
could be distilled unchanged under reduced pressure, their titanium analogues tended
to disproportionate on heating yielding volatile Ti
OPr
i
4
; conductometric titrations
(Fig. 3.3) also did not give clear inflexions at 2:1 ratio of the reactants except in the
case of zirconium.
However, a derivative with the composition [M
2
Zr
3
OPr
i
14
] was obtained when
the molar ratio of MOPr
i
and Zr
OPr
i
4
.
Pr
i
OH was 1:1 or
>
1:1; this product could be
recrystallized without change in its composition and could be volatilized under reduced
pressure.
The 1:1 molar reaction of alkali and titanium isopropoxide also yielded a crys-
tallizable derivative of composition MfTi
OPr
i
5
g. Interestingly, a similar reaction
of MOBu
t
and Zr
OBu
t
4
also resulted in a crystallizable volatile dimeric product
[fKZr
OBu
t
5
g
2
]. The basic information on the more dominant was thus set out in
nona-alkoxodimetallate fM
2
OR
9
g
, penta-alkoxometallate fM
OR
5
g
, and hexa-
alkoxometallate fM
OR
6
g
2
derivatives were thus laid in this early publication.
For the highly interesting and novel fZr
2
OPr
i
9
g
ligand, a plausible structure
(Fig. 3.4) involving two face-sharing octahedra was suggested in 1971,
3
which was
capable of encapsulating the alkali ion. Some evidence for this was furnished by
alcoholysis with ramified alcohols and
1
H NMR spectroscopy
76
in 1972.
This type of structure in a simple anionic form is depicted
61
in a few niobium(
IV
)
solvent-separated ion-pair derivatives like [Na
MeOH
][Nb
2
OMe
9
]. The relationship
of the above structure of fZr
2
OPr
i
9
g
anion to the edge-sharing structure
186
of
Zr
2
OPr
i
8
Pr
i
OH
2
was depicted by Evans
et al
.
187
in 1997. The flexibility of the
above type of fM
2
OR
9
g
(M D Zr, Hf, Th, Ce(
IV
), U(
IV
), Sn(
IV
), Ti(
IV
), Nb(
IV
),
etc.) ligands in binding the central heterometal atoms in the tetra-, tri-, or bi-dentate
