Liliane G. Hubert-Pfalzgraf
IRC, Université de Lyon1 - 69626 Villeurbanne Cedex (France)

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-Si^{35, 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(O^tBu)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 BaTi_0.5Zr_0.5O₃ 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.