2 Homometallic Alkoxides

Heterometallic Alkoxides 203
fAl⊲OPr i ⊳3g4 has since been confirmed by X-ray crystallography. 190,191 It also provided
a clue for the suggestion in 1968 by Mehrotra and Agrawal 192 of a similar structure,
Lnf⊲ -OPr i ⊳2Al⊲OPr i ⊳2g3 for volatile tetraisopropoxoaluminates of lanthanons (and
other trivalent elements); this has also been confirmed crystallographically for
ErfAl⊲OPr i ⊳4g3 in 1996 by Wijk et al. 40
In addition to the chemistry of heterometallic derivatives of alkoxometallate(IV), 188
and tetraalkoxoaluminate 92 ligands, the properties of hexa alkoxoniobates and
-tantalates 87–89,113–116 of various metals have also been extensively studied. An account
of these different alkoxometallates is presented in Section 3.3, after presentation of their
general properties in Section 3.2.
3.2 General Properties
As mentioned earlier, the heterometal alkoxides tend to form compact units, which
are volatile and generally monomeric in organic solvents. In view of some inherent
difficuties 193 in the X-ray structural elucidation of metal alkoxide systems, most of the
earlier work (till the early 1980s) on identification and characterization of heterometal
alkoxides was based on chemical analyses, colligative properties, volatility (indicating
stability to heat and ease of purification), and physicochemical investigations like
UV-Vis, IR, NMR ( 1 H, 13 C, 27 Al), and mass spectroscopy coupled with magnetic
measurements (particularly of paramagnetic later 3d systems). An account of these has
already been published in review articles 3,4,97 and these are, therefore, only referred to
in specific cases. During the 1990s, NMR studies based on a few other metals ( 119 Sn,
110 Cd, 89 Y, etc.) 16,17,28,29,47,103 have also played an important role in throwing light on
the coordination geometries of such metals in heterometal alkoxide systems.
Mention may also be made of the extensive phase-rule type of studies based on the
solubility isotherms of M⊲OR⊳m–M 0 ⊲OR⊳n systems in the research school of Turova, 194
as illustrated by Gibbs–Roseboom triangular plots of NaOMe–Fe⊲OMe⊳3–MeOH,
Ba⊲OBu n ⊳2–Ti⊲OBu n ⊳4–Bu n OH, Bi⊲OEt⊳3–WO⊲OEt⊳4–EtOH, Ba⊲OMe⊳2–Ta⊲OMe⊳5–
MeOH, NaOMe–Al⊲OMe⊳3–MeOH, Ca⊲OEt⊳2–Ta⊲OEt⊳4–C6H6, and Al⊲OPr i ⊳3–
Hf⊲OPr⊳4–Pr i OH systems.
The chemical properties of heterometallic alkoxides are in general similar to their
homometal counterparts: (i) hydrolysis, (ii) alcoholysis, (iii) trans-esterification reactions,
(iv) reactivity with carboxylic acids 195 and enolic forms of chelating ligands such
as ˇ-diketones/ˇ-ketoesters. 196–198 The hydrolytic reactions are now widely employed
for the preparation of homogeneous mixed metal–oxide ceramic materials and the
rest have found wide applications for the synthesis of a variety of novel metalloorganic
derivatives (sometimes unique), which are not often available through any
other synthetic route. 197
Just as in the case of homometal alkoxides, the alcoholysis reactions have also
provided a convenient techique for throwing light on the structural features of
heterometallic alkoxides. Whereas alcoholysis reactions are quite facile with simple
primary alcohols, resulting in the replacement of all the alkoxide groups of the
heterometal alkoxides, the reactions with sterically demanding alcohols enables
the distinction between terminal (nonbridged) and bridged alkoxide groups, as the
intermediate product surrounded by sterically demanding alkoxide groups (obtained by
initial replacement of terminal and possibly some of the bridged alkoxide groups) does