| II.3.3 Metal
alkoxides modified by unsaturated ligands
ORganically MOdified SILanes (ORMOSILS) of
general formula (RO)4-xSiZx allow tailoring of
porosity, of surface properties, of refractive index, of coatings thickness,
to improve mechanical properties etc.29 ORMOSILS are based on the
stability of the Si-C bonds (Z is a functional alkyl group) versus the
hydrolysable Si-OR ones. Numerous silicon derivatives with polymerizable
functionalities or chromophores for NLO applications are available. The
epoxide q-glycidyloxypropyltrimethoxysilane
CH2(O)CHCH2O(CH2)3Si(OMe)3
(GLYMO) or methacryloxypropylsilane OMcC3H6Si(OMe)3
(OMc = O2CMe=CH2 = methacrylate) (MEMO) are examples
of derivatives used for cross-linking by epoxy or methacryl polymerization.
Many reviews have emphasized the use of those derivatives and these aspects
will not be developed here. 29-30
Homo- or copolymerisation reactions involving
a polymerizable moiety Z are required for extended organic arrays and access
to hybrid materials. If covalent association between the networks is a goal,
the M-Z linkage should resist to processing and be stable thermodynamically
and kinetically. The approach used for silanes is, to some extent, valid for
tin but the reactivity of metal-carbon bonds makes it useless for most
metals. A better strategy is to link the polymerizable functionalities to
the metal via O-donor ligands forming M-Z bonds which are less susceptible
to hydrolysis than metal-alkoxide ones. Chelating or bridging-chelating
ligation is preferable. The hydrolytic stability of the metal
b-diketonate bond is generally higher than that of
the metal carboxylate one but functional b-diketones
are less readily available than functional carboxylic acids. Carboxylic
acids (acrylic or methacrylic),
b-diketones such as
vinylacetylacetone, represent reasonable choices for unsaturated O-donor
moieties. Scheme 1 collects some common unsaturated ligands. Accessibility
of the polymerisation sites, the nature of the unsaturated functionality (acrylate
ones are among the best for polymerisation) are also of importance for
reactivity. 30
 |
The structural units are often similar to
those obtained with related ligands without unsaturation. Polynuclear
oxocarboxylatooxoalkoxides such as Ti6O4(OEt)8(µ-O2CR)8,23
or Nb4(µ4-O)2(µ- O2CR)4(OiPr)8
have been obtained. with acetic31a as well as methacrylic acid31b.
Derivatives resulting from complete substitution were reported for
zirconium.32 The unsaturated moieties are accessible for
copolymerization reactions. Reagents such as 2-hydroxyethylmethacrylate (HEMA)
can undergo transesterification in the case of oxophilic metals (Ti, Nb..) (eq
6) with cleavage of the C-O bond and loss of the acrylate functionality from
the coordination sphere even at RT.33 Soluble metal
ethyleneglycolate derivatives are obtained. These observations illustrate
the difference between silicon and oxophilic metals.
| M(OR)n + OHC2H4OC(O)CMe=CH2
|
 |
[M(OR)n-2(OC2H4O)]m + RCO2CMe=CH2
|
(6) |
R = iPr, M = Ti, m = 5;
Nb, m = 4
II.3.4 Hydrolysis: How to control
hydrolysis
rates?
Growth of the M-O-M network proceeds via
several steps namely hydrolysis (eq 7) [giving unstable hydroxyalkoxides
M(OH)(OR)n-1], then polycondensation reactions via olation
(preferential elimination of water eq 8) or oxolation (preferential
elimination of alcohol eq 9).1d
By contrast to silicon alkoxides whose
hydrolysis requires catalysts for efficient gelation rates, hydrolysis of
most metal alkoxides is rapid and can lead to uncontrolled precipitation.
The electronegative alkoxide groups make the metal highly prone to
nucleophilic attack by water. The more electrophilic metal centres –as
compared to silicon- as well as a larger and thus more stereolabile
coordination sphere result in a higher hydrolytic susceptibility. The
following sequence of reactivity is usually found Si(OR)4 <<
Sn(OR)4 ~ Ti(OR)4 < Zr(OR)4 ~ Ce(OR)4.7
This order is dependent on the R group and a slightly different order of
hydrolytic susceptibility namely Al < Zr < Ti was reported for
n-butoxides.
Parameters1, 2
The hydrolysis ratio h (h = [H2O]/[M(OR)n]
allows to control the extent of hydrolysis. Precipitation remains often
difficult to avoid for early transition metals or lanthanides. Several
strategies have been developed in order to slow down hydrolysis rates. These
are:
|
STRATEGIES TO SLOW
DOWN HYDROLYSIS RATES |
| |
Changing the nature of the organic group R:
alkoxides with primary organic groups such as n-butoxides are less sensitive
to hydrolysis than secondary ones such as isopropoxides; |
| |
Increase of the metal coordination number
thus hindering attack of water and formation of the metal hydroxyl bond,
M-OH, necessary for the development of the network; |
| |
Decreasing the functionality of the
precursor by partial substitution of the OR ligands by anionic ligands such
as carboxylates or b-diketonates leading to M-Z bonds
less susceptible to hydrolysis (and to M(OR)n-xZx.
species) |
These different approaches in controlling
hydrolysis are often interdependent, for instance replacing R by a
functional group increases also the coordination number of the metal.
Differential hydrolysis is observed for M(OR)n-xZx
species (IR data show the retention after hydrolysis of the less
hydrolyzable M-Z bond ). Anisotropy of the network can be promoted as well
as porosity.27 Carboxylates are usually more labile than
b-diketonates but hydrogen-bonding can assist the
elimination of the latter and thus modify the behavior. The facility of
release of the ligands varies according to OPri > OC2H4OMe
> acac > OAc for yttrium derivatives.34 For Ti, Zr and Al normal
or secondary butoxides, the hydrolytic stability decreases according to
acetylacetonate > allylacetatoacetate > ethylacetatoacetate >
methacryloxyethylacetatoacetate (IR and 13C NMR evidence on monosubstituted
derivatives).35 Trialkylsiloxide groups R3SiO are also less susceptible to
hydrolysis than alkoxide ones such as butoxides, isopropoxides. Heteroleptic
metallosiloxanes undergo thus differential hydrolysis: one OSiMe3
group per Al atom as in [Al(OPri)2(OSiMe3)]m,
prevents aluminium hydroxide precipitation. 36
II.4 Non-hydrolytic condensation pathways.
Thermal condensations
Non-hydrolytic condensation reactions can be
alternatives for control of hydrolysis.37 Hydroxylation reactions
involving reactions between basic metal alkoxides (Ti, Zn) and organic
carboxylic derivatives, acetone for instance, can proceed at RT but their
mechanism can be complex.38 Building-up of the M-O-M network can
also be achieved by condensation reactions between species with different
ligands. Metal alkoxides and carboxylates (elimination of ester, eq 10),
metal halides MXn and alkoxides (formation of alkylhalide- eq 11)
or elimination of dialkylether (eq 12) as the source of the oxo ligand are
usual examples. Solubility problems of the reagents can be encountered for
non-silicon systems (requiring an appropriate medium) and extensive
condensation requires heating. This approach has allowed obtaining of
nanocrystalline anatase. Titanium alkoxide was added to titanium chloride in
the presence of trioctylphosphine (TOPO) in hot heptadecane. TiO2
precipitates but remains dispersed in dilute solutions, TOPO serves as a
passivating agent.39 Intramolecular elimination reactions
starting for instance from chloroalkoxides can also be exploited.
II.5 Metal alkoxides as precursors of
non-oxide materials
Fluorinated alkoxides M(ORf)n
(Rf = CH(CF3)2, C6F5,...)
have been prepared for many metals for MOCVD applications.9,40
They are soluble in organic solvents and less susceptible to hydrolysis than
classical alkoxides (they are hygroscopic due to formation of hydrogen bonds
with water). X-ray data have often shown the presence of M...F interactions
of lengths comparable to the M-O bonds. They can thus act as oxide or
fluoride precursors depending on conditions of hydrolysis and/or thermal
treatment. BaF2 was for instance obtained by hydrolysis of barium
fluoroisopropoxide in ethanol (notice that no alcohol interchange reaction
occurs).41 Sn(OR)3(ORf) species were used
for F-doped tin oxide.42 The reactivity of the M-OR bond allows
to to accede to phosphates43 (eq 13), sulfides or oxysulfides
materials44,45 as shown for aluminium, titanium, lead or
lanthanum.
| Ti(OiPr)4 + tBuPO(OH)2 |
|
[Ti(OiPr)2(tBuPO3)]4
|
(13)
|
|
