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TUTORIAL


Toward molecular design of oxide precursors for advanced materials

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

 

III. TOWARD “SINGLE-SOURCE” PRECURSORS

Homogeneity can be absence of insoluble species in a medium or homogeneity at a molecular level with formation of molecules in which different metals –generally two M and M’- are present together (“single-source” precursor). Multicomponent oxides are a priori accessible by using mixtures of precursors or "single-source" ones. For the latter, the ratio between the metals should match that of the desired material.4,5,46,47 Such MM’ species can overcome the insolubility of some reagents (alkoxides of late transition metals, copper for instance), or provide a better control of the microstructure of the material and decrease its temperature of crystallisation. The main strategies for access to mixed-metal species are Lewis acid-base reactions or substitution reactions.

III.1 Lewis acid base reactions: metal alkoxides and/or other metallic species

General features

Such reactions are based on the mixing of alkoxides and/or of other more accessible oxide sources such as acetates, b-diketonates or nitrates 4, 48 (eq 14a-b) of different metals. The stoichiometry of the compounds in solution is only known after isolation and characterization (usually by single crystal X-Ray diffraction) since such reactions are under thermodynamic control. This is a drawback when stoichiometry is important, that criteria is less important when only atomic scale mixing of the metals is necessary. With the exception of silicon alkoxides (which must be prehydrolysed for formation of M-O-Si linkages), most reactions between metal alkoxides can lead to heterometallic species.46, 47, 49 They are often more soluble than the starting materials. Reactions with mixing precursors with different ligands (Z = OAc, b-dik,….) are more complicated: if the heterometallic species is unstable, homometallic M(OR)n-xZx (or M’) species with different ligands are obtained (14b)

The difficulty of reactions between alkoxides is to analyze the system. NMR is one of the best tool provide one of the metals is NMR active and sensitive. Nuclei having spin I = 1/2 (giving high resolution spectra) especially 29Si have being largely used. 27Al is another useful nucleus but its NMR signals are broadened by quadrupolar effects. Infrared can bring information for systems with ligands having diagnostic absorption bands such as the nCO stretching frequencies of carboxylates or E-diketonates.

All alkoxide reactions

Mixed-metal species incorporating alkali metals can be formed easily. They can be side products in the synthesis of metal alkoxides from halides when excess of lithium alkoxide is used (eq 15b). LiNb(OR)6 (R = Me, Et, C2H4OMe,...), MgNb2(OEt)12(EtOH)2, BaNb2(OPri)12(PriOH)250 are examples of MM' species whose formula is that of adducts.

The nature of the OR ligand can modify the stoichiometry between the metals as illustrated by the Ba-Zr system (eq. 17-18)49

Stabilisation of mixed metal-species often requires oxo O2- ligands allowing to increase the coordination number of the metals. Their formula become more complex: mixing Ti and Pb isopropoxides does not lead to PbTi(OR)6 but to a tetranuclear species (eq 19).50

2/m [Pb(OiPr)2]m + 2 Ti(OiPr)4  Pb2Ti24-O)(OiPr)10 + ….  (19)

Oxoalkoxides can also form mixed-metal species. Ln-M (M = Ti, Zr) species are formed by mixing lanthanide oxoisopropoxides Ln5O(OPri)13 (Ln = Y, Nd, Sm...) and titanium or zirconium isopropoxides at RT, they can be seen as adducts between M(OR)4 and Ln4O(OR)10 via a central oxo ligand.12

However, mixing metal alkoxides does not always lead to heterometallic species, especially when polymeric and insoluble metal alkoxides are involved. Bismuth ethoxide or isopropoxide for instance are inert toward the Ti analogues. Microhydrolysis can induce association via an oxo ligand. Species such as BiTi2O(OiPr)9 or Bi4Ti3O4(OEt)16 were obtained, the stoichiometry of the latter matching that of Bi4Ti3O12.51 It is interesting to observe that dissolution and thus depolymerisation can be promoted by water. This is encountered in material science when commercial , non-anhydrous solvents, are used. Soluble transition metal alkoxides are often inert toward each other at RT, typical examples being Ti and Zr butoxides, Ti and Ce or Nb and Ce isopropoxides. Formation of Ti-Zr species can be promoted by carboxylic acids.52 Polyols such as pinacol are also effective in formation of heterometallic species and furthermore able to control their stoichiometry (see IV-2.2). For insoluble and inert metal alkoxides, colloidal suspensions for instance of zinc isopropoxide, generated in situ by ultrasonic activation were reactive toward tantalum isopropoxide (eq 19b).53

2/m [Zn(OiPr)2]m + 4 Ta(OiPr')5 Zn2Ta4O4(OiPr)16 + 4 iPr2O (19b)

By contrast to the Li-Nb system, mixing barium and titanium alkoxides (R = Et, iPr) in a 1:1 stoichiometry, as required for BaTiO3, gives several MM' species but not the expected one [BaTi(OR)6]m, this stoichiometry being unable to satisfy the high coordination number required for an element as large as barium.2c This problem can be overcome with phenoxides.54 Chelating ligands such as E-diketonates can also increase the coordination number for barium. Reacting titanium ethoxide and barium tetramethylheptanedionate (1: 1 stoichiometry) offers Ba2Ti2(thd)4(OEt)8(EtOH)2 (thd = tBuCOCHCOtBu).48 Each metal bears a chelating diketonate, ethanol molecules are linked to barium. Strontium-titanium species of 1:1 stoichiometry (also not accessible by mixing of usual alkoxides) can be obtained by a similar route (eq 20)55 independently of the alkoxide, isopropoxide or ethoxide. The structures of the Sr2Ti2 and Ba2Ti2 species are derived from the rhombus E (fig 2) .

1/m [(Sr(b-dik)2]m + Ti(OR)4   Sr2Ti2(OR)8(b-dik)4   (20)

R= iPr, Et; b-dik = acac, thd.

Use of carboxylates as associated oxide precursors

Sol-gel techniques use often 2-ethylhexanoates as soluble carboxylates. Acetates are better in terms of ceramic yield but they are poorly soluble. They can however be dissolved in the presence of metal alkoxides by formation of heterometallic species.56 Dissolution of anhydrous acetates M(OAc)2 (M = Mg, Pb, Cd) in the presence of niobium alkoxides proceeds at RT in hydrocarbons (eq 21) giving trimetallic species Nb2M(µ-OAc)2(OR)10. Their formula corresponds to adducts (no esters are formed at RT). The choice of the solvent can be crucial: alcohols can act as ligands toward metal carboxylates precluding formation of heterometallic species. Barium acetate is inert even under refluxing or in the presence of acetic acid.

Reactions are more complicated when lead acetate is reacted with Ti or Zr alkoxides (R = Et, Pri). The formation of Pb-Ti and Pb-Zr oxo carboxylatoalkoxides, even at RT, indicates non-hydrolytic condensation with ethers as by-products (eq 22-23). The formulae of these oxo species is function of the metal (Ti and Zr alkoxides behave differently) and of the alkoxide group.50 The isopropoxide species Pb2Ti2O(OiPr)8(OAc)2 only matches the formulation of PbTiO3. The difference in the nasCO2 and nsCO2 stretching frequencies in the IR indicates that acetate ligands are bridging or chelating in all derivatives. The oxo ligand allows hexacoordination of the tetravalent metals Ti or Zr, (fig 5, structure K). The M-Pb heterometallic carboxylatoalkoxides are modified by heating whereas those with other divalent metals (Mg, Cd) are inert. Condensation proceeds with formation of ester (IR evidence) and lowers solubility. Heating can be required for dissolution of some metal acetates. This is the case for lanthanide acetates in the presence of zirconium isopropoxide.5

As a general feature, dissolution of anhydrous acetates in the presence of metal alkoxides proceeds with formation of adducts or of oxoacetatoalkoxides due to elimination of dialkylether. The selectivity might depend on the solvent as observed for the Pb-Ti system. Further condensation by heating can occur with elimination of ester. It should be noticed that the issue of thermal condensation applied to non homogeneous media (before spontaneous dissolution of the acetates) is generally different, leading to intractable, insoluble compounds.50

 
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