| 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 |
 |
Pb2Ti2(µ4-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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