2 Homometallic Alkoxides

Heterometallic Alkoxides 213
The X-ray structural study of LiNb⊲OEt⊳6 212,213 and [⊲Pr i OH⊳2BafNb⊲OPr i ⊳6g2] 214
reveals familiar patterns. However, LaNb2⊲OPr i ⊳13 reveals 214 the bonding of the central
La atom by a bidentate and another tridentate fNb⊲OPr i ⊳6g group in addition to coordination
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 nonaalkoxodizirconates
of alkali metals such as KZr2⊲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, NaSn2⊲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 SnCl4 with NaOEt.
It was Mehrotra 41 who drew attention to the uniqueness of [MfZr2⊲OPr i ⊳9g](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 [M2Zr3⊲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 crystallizable
derivative of composition MfTi⊲OPr i ⊳5g. Interestingly, a similar reaction
of MOBu t and Zr⊲OBu t ⊳4 also resulted in a crystallizable volatile dimeric product
[fKZr⊲OBu t ⊳5g2]. The basic information on the more dominant was thus set out in
nona-alkoxodimetallate fM2⊲OR⊳9g , penta-alkoxometallate fM⊲OR⊳5g , and hexaalkoxometallate
fM⊲OR⊳6g 2 derivatives were thus laid in this early publication.
For the highly interesting and novel fZr2⊲OPr i ⊳9g 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⊳][Nb2⊲OMe⊳9]. The relationship
of the above structure of fZr2⊲OPr i ⊳9g anion to the edge-sharing structure 186 of
Zr2⊲OPr i ⊳8⊲Pr i OH⊳2 was depicted by Evans et al. 187 in 1997. The flexibility of the
above type of fM2⊲OR⊳9g (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