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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)

 

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 Nb44-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)

 
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