II.3. Reactivity
Metal alkoxides M(OR)n react
easily with the protons of a large variety of molecules. This allows easy
chemical modification and thus tuning of properties by organic hydroxy
compounds such as alcohols, silanols R3SiOH, glycols OH(CH2)nOH,
carboxylic and hydroxycarboxylic acids, hydroxyl surfactants etc. (eq. 1a).2
Thus the additives used in sol-gel processing are chemical modifiers and can
change the nature of the species. The "modifying" ligand ZH should have a pka
lower than that of the alcohol eliminated in the process (corresponding to
the alkoxide ligand). Complexation of metal alkoxides by neutral ligands L
is limited due to the poor stability of M(OR)nLx adducts. Lewis bases with
hard O or N-donor sites are required for coordination. One of the best
ligand L is the parent alcohol giving M(OR)n(ROH)x solvates (eq 1b). Such
solvates are those of the isopropoxides of tetravalent metals (Zr, Hf, Sn,
Ce), their stability is assisted by hydrogen bonding 2, 4
1/m
[M(OR)n]m + x ZH |
 |
1/m’ [M(OR)n-xZx]m' + x ROH |
(1a) |
Z = R’CO2, b-dik,...
Nb2(OR)10 + 2 ROH |
 |
2 Nb(OR)5(ROH) |
(1b) |
II.3.1 Alcoholysis
reactions: Modification by
alcohols and polyols
Alcohol interchange reactions (alcoholysis)
generally require heating for classical alcohols if complete substitution is
desired but they can occur easily at room temperature (RT) with functional
alcohols. Such reactions occur also when an alkoxide other than a silicon
one is dissolved in an alcohol different from the parent alcohol ie. Zr(OnBu)4
in 2-methoxyethanol. Species having M-O-Si bonds (M = Ti, Al, Zr..) namely
metallosiloxanes are formed by reaction between metal alkoxides and silanols
R3SiOH or by transesterification reactions using commercial
silylacetates.2 The latter allow to overcome the instability of
most silanols.
Functional alkoxide ligands such as O(CH2)nX
[X = OR’ (alkoxyalcohols), NR’2 (alkanolamines)] with
intramolecular O- or N- donor sites can be chelating or bridging
(assembling) (fig 3). Chelation requires usually formation of a cycle of
five atoms (by linking the alkoxide oxygen and the donor site X to the
metal), this needs a value of n = 2 (as for 2-methoxyethanol) .for the
spacer (CH2)n.. Depolymerisation, and thus solubilization of
insoluble metal alkoxides (Ni, Cu,..) can be achieved by functional
alcohols. This depends on their ability to act as a chelating ligand rather
than a bridging one. Aminoalcohols are often more efficient than
alkoxyalcohols in this respect. Starting from polymeric Cu(II) alkoxides
[Cu(OR)2]¥
(R = Me, iPr, tBu), alcohol exchange reactions afford
insoluble copper(II) 2-methoxyethoxide [Cu(OC2H4OMe)2]¥
whereas Cu(OC2H4NMe2)2
is a monomer, volatile and soluble. Solubility of copper(II) alkoxides with
alkoxyalcohols requires an alcohol with an additional O-donor site (HOCH2H4OC2H4OMe)
or with a longer carbon chain for the ether as achieved with nBuOC2H4OH.
Chelation can also be forced by steric
effects such as substitution in the
D-position,
a strategy used for volatility.13 Functional alcohols are also
able to provide a rheology suitable for gels, monoliths or coatings. From a
molecular chemistry point of view, this proceeds with increase of the
nuclearity but solubility is generally retained when soluble alkoxides are
modified. The reaction between Y5O(OPri)13
and 2-methoxyethanol gives a soluble decamer [Y(OC2H4OMe)3]10,
the largest oligomeric non oxoalkoxide reported so far.4
The quest for the replacement of
2-methoxyethanol in processing by a safer reagent, affording similar
rheological properties, has motivated investigations with diols14 such as
1,3-propane-, 1,4-butane-diols, ethylenglycol HOC2H4OH, etc. The reactions
between metal alkoxides and polyols are generally possible (pka lower than
those of isopropanol or ethanol). By contrast with the reactions with alkoxy-
or amino-alcohols, the solubility of the resulting species depends on the
extent of deprotonation of the polyol. Complete deprotonation affords
soluble species of higher nuclearity, partial deprotonation leaves residual
hydroxyl groups (evidenced in the IR by nOH »3400 - 3200 cm-1) which can be
involved in intermolecular hydrogen bonding, leading to poor solubility or
gels. Constrained polyols such as triethanolamine N(C2H4OH)3 (teaH3) allow
also decomplexation. The soluble Ba(teaH2)2,2EtOH or [Cu(teaH2)]4,3teaH3
species obtained by alcoholysis of insoluble methoxides illustrates these
features15 (notice that only partial deprotonation occuirs with
divalent metals). Triethanolamine being coordinated to the metals by all
donor sites, namely three oxygens and nitrogen is a tetradentate in both
cases. Such coordination behaviour and hydrogen bonds are a hurdle for its
elimination at low temperature. On the other hand, due this difficult
expelling, it can act as a template for mesopore formation of binary or
multimetallic oxides.16 Functional alcohols can also stabilize
metal alkoxides toward undesired precipitation during the polycondensation
process. Fig 3 collects the various coordination modes of 2-methoxyethoxide
and triethanolaminate ligands
2 Ti(OiPr)4 + 2 teaH3 |
 |
[Ti(µ-OiPr)(tea)]2 + 6 iPrOH |
(2) |
II.3.2 Modification by
b-diketones
and carboxylic acids
Modification by those reactants reduces the
number of M-OR bonds available for hydrolysis and thus hydrolytic
susceptibility. It is a means to control the sol-gel process and is achieved
often in situ by using b-diketones namely
acetylacetone (acacH) or carboxylic acids mostly acetic acid (AcOH) as
"modifiers".
II.3.2.1 O-Capping ligands as surface
controlling agents (SCA)
b-diketones are prone
to a chelating behaviour.6b This leads to a decrease of the
nuclearity of the precursors. Small particles are generally obtained after
hydrolysis of M(OR)n-x(b-dik)x since these ligands are surface
capping reagents and polymerization lockers. Acetylacetone can for instance
stabilize nanosized colloids derived from the Sn(OtBu)4
- EtOH system which were used to elaborate transparent and conductive oxide
films.17 Reactions with acetylacetone have been considered as
simple. However, recent reports have indicated that acetylacetone can be
easily modified18, 19 or degrade oxo aggregates20 in
mild conditions, depending on the solvent. Changing the
b-diketonate
ligand can allow to adjust UV-V absorption bands of precursors for
photo-assisted techniques.
b-diketonate
ligands have been used for patterning of coatings by UV-curing.21
b-diketonates
and related ligands can stabilize polynuclear oxoaggregates generated by
hydrolysis as observed for instance for titanium.1d
II.3.2.2 Carboxylates as assembling ligands
The carboxylate ligands act mostly as
bridging-chelating ligands as shown by scheme
1. The difference in the IR
spectra of the frequencies of the carboxylate absorption bands (asymmetrical
and symmetrical streching modes) provides a tool for ascerting their
coordination type. For instance, a difference of
nasCO2
- nsCO2
< 200 cm-1 indicates a bridging behaviour.22 These
ligands favor extensive networks and gels. The reactivity of metal alkoxides
with carboxylic acids is more complex than that with
b-diketones
since competitive reactions can occur. Besides substitution (eq 3a),
generation of oxo ligands might occur either by non-hydrolytic condensation
and elimination of an ester from an unstable carboxylatoalkoxide (eq 3b) or
by hydrolysis subsequent to esterification (eq 4).1, 2 The issue
depends of the experimental conditions (stoichiometry acid/M(OR)n,
temperature, nature of the acid, solvent, duration). Temperature increases
the number of oxo ligands.23
M(OR)n + x R'CO2H
|
 |
M(OR)n-x(R'CO2)x
+ x ROH |
(3a) |
M(OR)n-x(R'CO2)x
|
 |
1/2 [(OR)n-x-1M-O-M(R'CO2)x-1] + RCO2R' |
(3b) |
ROH + R'CO2H |
 |
H2O + RCO2R' |
(4) |
Extensive studies have been done for titanium
alkoxides (dimeric, trimeric, tetranuclear and hexanuclear species have been
reported) and to a lesser extent for zirconium, tin and niobium. For
instance whereas Ti6O4(µ-OR)4(OR)4(µ-OAc)8
is obtained at RT,23 heating drives the reaction toward a more
oxo species Ti6O6(OEt)6(µ-O2CR)6
(eq. 5, Fig 4)., 24 The metals are generally 6-coordinate and
these clusters can be seen as various types of association of MO6
octahedra. Oxocarboxylatoalkoxides might also be obtained by reacting metal
alkoxides and metal carboxylates. Such reactions are examples of
non-hydrolytic condensations and were used for instance for access to new
tin derivatives (II.4, eq 10).25 The combination of oxo,
carboxylate and only few alkoxide ligands affords species which are quite
robust toward hydrolysis and can be used as Organically Modified Transition
Metal Clusters (OMTOCs)26. The poor lability of carboxylate
ligands can allow to get porous materials, porosity being tailored by the
size of the R group.27 Carboxylates are also largely used in MOD
processes.28
 |
Fig. 4: Titanium
Oxocarboxylatoalkoxide clusters (all metals are hexacoordinated)
|
|
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