III.2 Control of
the stoichiometry between the metals via metalloligands
For Lewis acid-base reactions, the
stoichiometry of the MM' species is determinated a posteriori. Control of
the stoichiometry between the metals can be achieved by substitution
reactions (eq 24) or by using the reactivity of coordinated ligands, the
most common being alcohols (eq 25).2 The driving force of such reactions is
elimination of insoluble or volatile by-products.46 In the latter case, the
strategy can be used during processing.

Heterometallic alkoxides based on alkali
metals are required for eq 24. The strategy of eq 25 or 26 is based on the
acidity of alcohol which is enhanced by coordination.57 If solvates are not
available with usual alcohols, reactions between metal alkoxides and polyols
such as triethanolamine or pinacol can lead to hydroxyl functionalities in
the coordination sphere and the sites for anchoring another metal (eq
27a,b). Despite the insolubility of those intermediates (due to hydrogen
bonding), soluble mixed-metal species such as LaNb3(tea)2(OPri)12 can be
formed .58

Triethanolamine allowed also the synthesis of
an heterometallic precursor to the MgAl2O4 spinel directly from Al(OH)3, MgO
or Mg(OH)2 in ethyleneglycol at RT (eq 28).59
MgO+ 2Al(OH)3
+ 3teaH3 |
 |
 |
(28) |
III.3. General structural features
Fig 5 collect some common frameworks of
heterometallic species identified by X-Ray studies on single crystals. These
frameworks are function of the stoichiometry MM’ between the metals. One can
see the basic building blocks observed for the homometallic species :
trinuclear open-shell (G) or triangular units either isolated or associated
for instance via oxo ligands as in the Zn2Ta4 species (structure H). Tetranuclear M2M’2 species have structures related to a rhombohedral core
(E, fig 2) or to a tetrahedral one (I) as for the Pb2Ti2 oxo species.
Acetate ligands clamp the different metals together since they are in
bridging positions ( structures J, K). The various metals tend to attain
usual coordination numbers namely six for early transition metals (Ti, Nb,
Ta) or magnesium. Higher coordination numbers are often required for barium
or bismuth whereas lead displays often coordination numbers lower than six.
These heterometallic species can be fluxional in solution but their
structures are usually retained in non-polar media (IR or NMR evidence).
III. 4. Reactivity of MM' species, processing
into materials
III.4.1 General trends
Studies of reactivity of MM’ species remain
scarce.4 Modification of mixed-metal alkoxides by substitution reactions can
a priori allow tailoring of their properties. The reactions proceed often
with modification of the MM’ stoichiometry if an insoluble species can be
formed. The reaction between Pb2Ti2O(OiPr)10 and controlled amounts of
acetic acid or acetylacetone (eq 29) is a rare example of modification with
retention of the stoichiometry (only small structural variations occur and
all metals remain assembled around a central oxo ligand, see structures I,
K, fig 5).50
Pb2Ti2(µ4-O)(OiPr)10 + 2 ZH |
 |
Pb2Ti2(µ4-O)(OiPr)8Z2 + 2
iPrOH |
(29) |
Z = AcO, acac
Hydrolysis-polycondensation is often
performed in the parent alcohol. This is not always the best choice since
alcohol can promote dissociation if stable solvates M(OR)n(ROH)x can be
formed. Precipitation of insoluble species by alcoholysis can also be a
drawback: Addition of 2-methoxyethanol to a Nb-Pb ethoxide modifies the
stoichiometry in solution by precipitation of lead methoxyethoxide. Some
knowledge about the chemistry of the system is desirable for selection of
appropriate solvents and/or additives.
What about reactions with other metallic
species?
The formation of heterometallic species
involving three different metals M, M’, M” in processing conditions (without
precipitation of halides, i.e. eq 24) requires a special set of ligands (eq
30, 31).49 Thus if three or more different metals are involved, the medium
is likely to be a mixture of heterometallic species based on two different
metals only.
Zr2(OiPr)8(PriOH)2 + 2
Ti(OPri)4
|
 |
2 TiZr(OiPr)8(PriOH)2 |
(30) |
TiZr(OiPr)8(PriOH)2+ 2/m
[Ba(OiPr)2]m
|
 |
Ba2TiZr(OiPr)12 + 2PriOH |
(31) |
Knowledge of the intermediates on the way to
the material remains scarce. Do hydrolysis-polycondensation reactions
proceed via heterometallic M-O-M' or via homometallic M-O-M and M'-O-M'
species? EXAFS or NMR studies are essentially limited to M-O-Si35, 60a or
M-O-Al systems.60b Investigations of the first steps of hydrolysis reactions
have shown retention of the stoichiometry between the metals for LiNb(OEt)6
or BaZr(OtBu)5(THF)2 species related to LiNbO3 or BaZrO3 respectively.2c,
49, 61
III.4.2. Condensation reactions
All alkoxides route
The value of a mixed-metal species with
respect to mixtures of precursors can be illustrated by the Bi-Ti system.
Hydrolysis of Bi4Ti3O4(OEt)16 in ethanol offers Bi4Ti3O12 which crystallizes
at 470°C. This temperature is ~ 200°C lower than those observed when using
mixtures of Ti(OR)4 and Bi(OAc)3 or bismuth nitrate in 2-methoxyethanol due
to the non-formation of Bi2Ti2O7 as intermediate. Bi4Ti3O12 is obtained as a
powder from the ethoxides, the rheology of the system can be tailored by
using 2-methoxyethoxides. Hydrolysis affords physical gels which are
converted into sols which can be spread on silica wafers by spin-coating.51
The BaTi0.5Zr0.5O3 phase was already well developed at 400 °C and devoid of
secondary phases by hydrolysis of the Ba2TiZr alkoxide. The use of mixtures
produces either pure oxides which crystallize over 1000 °C or contamination
with BaCO3.62
Semi-alkoxide routes: influence of ancillary
ligands
The existence of Pb-Ti species such as
Pb2Ti2O(OiPr)8Z2 (Z = OiPr, OAc, acac) allows to estimate the effect of
ligands Z. Hydrolysis in similar conditions gave amorphous powders with
different characteristics. The IR spectra of those resulting from the
acetate or acetylacetone derivatives show
nCO absorptions (1600 - 1400 cm-1)
indicating differential hydrolysis. These residual ligands are eliminated
below 400°C (TGA evidence) but allow controlled transformation into pure and
crystalline PbTiO3 which is obtained at 500°C (XRD). By contrast,
crystallization of the powders resulting from Pb2Ti2O(OiPr)10 gives first
Pb2Ti2O6, heating up at 700°C is required for pure PbTiO3.50 Acetone is a
solvent able to promote non-hydrolytic condensation of titanium alkoxides.
Its use as a solvent associated to isopropanol (1/4 in volume) as hydrolysis
medium decreases particle size (from 9800 to 235 nm) and the crystallization
temperature of lead titanate in the case of Pb2Ti2O(OAc)2(OiPr)8. It also
avoids pyrochlore in the transformation of Pb2Ti2O(OiPr)10. Acetone can thus
be an handle in sol-gel chemistry.
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