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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. 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]Ą (R = Me, iPr, tBu), alcohol exchange reactions afford insoluble copper(II) 2-methoxyethoxide [Cu(OC2H4OMe)2]Ą  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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