New Precursors for Sol-Gel Processing
The creation of powerful new materials for innovative applications is one of the big scientific and technical challenges of our days. The role of preparative chemistry is to provide the compounds, which eventually will be shaped into new devices by materials scientists and engineers.
One of the modern methods for preparing novel inorganic and inorganic/organic materials is sol-gel processing. It is based on molecular precursors (building blocks), mostly metal or semi-metal alkoxides, from metal oxide-based materials are obtained via hydrolysis and polycondensation reactions. "Design" of sol-gel materials - and their material properties - is to some extend possible by variation of the chemical composition and arrangement of the molecular building blocks and by deliberate tailoring of their nano- and microstructure.
Materials composed of both organic and inorganic components (organic-inorganic hybrid materials) are particularly useful, because new materials properties can be obtained by the combination of organic and inorganic building blocks. To fully exploit the possibilities of this materials class, precursors are needed where functional or non-functional organic groups are bonded to hydrolyzable (semi-)metal alkoxide moieties.
Organically modified transition metal alkoxides
While the chemistry of organoalkoxysilane derivatives R′Si(OR)3, with a great variety of functional or non-functional organic groups R′, is well developed, the corresponding chemistry of metal alkoxide derivatives is less investigated. Contrary to alkoxysilanes, the organic groups R′ cannot be linked to metal atoms by metal-carbon bonds, because most of these bonds are not hydrolytically stable. Thus, the chemical link between the metal alkoxide moiety and the organic group, usually a bidentate organic group, plays an eminent role. β-Diketonate- and carboxylate-modified metal alkoxides are widely used. We are currently focusing our attention to the less investigated derivatives with nitrogen-containing ligands.
Primary amines strongly interact with metal alkoxides. The strength of the metal-nitrogen interaction is not only determined by the basicity of the amine but also by stabilization through hydrogen bonding. Reaction of metal alkoxides with di- and triamines may result in the formation of coordination one- or two-dimensional polymers (Figure 1).
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Figure 1. 2D structure of {[Ti2(OiPr)8]3[N(CH2CH2NH2)3]2}∞ with isolated Ti2(OiPr)8 molecules in the voids of the network.
Many titanium alkoxide derivatives are based on the following structural motif:
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This is known for compounds with β-diketonate or isoeugenolate ligands, i.e. X = Y = oxygen. We found related structures for several derivatives with X = N and Y = O, such as aminoalcoholates, oximates, or amino carboxylates (Figure 2).
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Figure 2. Molecular structures of Ti2(OEt)6(glycinate)2 (left), Ti2(OR)6(aminoethanolate)2 (center) and Ti2(OR)6(oximate)2 (right).
However, the structural chemistry of the precursors may be more complicated than anticipated. A very illustrative example is the comparison of the aminoethanolate derivatives Ti(OiPr)3(OCH2CH2NH2) and Zr(OBu)3(OCH2CH2NH2). While the former (Figure 2, center, and Figure 3, left) has a dimeric structure, the composition of the latter (Figure 3, right) is in fact Zr4(OBu)10(OCH2CH2NH2)6⋅2 Zr(OBu)4, where Zr(OBu)4 groups are coordinated to the central aminoethanolate-substituted Zr4 unit via OR bridges.
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Figure 3. Comparison of the molecular structures of Ti2(OEt)6(OCH2CH2NH2)2 (left) and Zr(OBu)3(OCH2CH2NH2) (right).
Still a challenging task is the preparation of organofunctional derivatives of metal alkoxides of the type (RO)nM-BL-X-A, where BL is the bidentate chelating or bridging ligand, X an inert spacer, and A a functional organic group. Examples from our group include (RO)3M(OOCCH2CH2PPh2) (M = Ti, Zr), (RO)3Ti(lysinate), CoPc[4-SO3Ti(OR)3]4 (CoPc = cobalt phthalocyanine) or (RO)3M[(OCMe)2CCH2CH2CH2Si(OEt)3 (M = Ti, Zr).
Hydrolysis of Organically Modified Precursors
Studying the hydrolysis reactions of metal alkoxides is an important, but very difficult to investigate step between the molecular chemistry of the precursors and the solid state chemistry of the obtained gels. The evolution of (amorphous) gel structures by stepwise hydrolysis and condensation of precursors and oligomeric intermediates is extremely complex and is influenced by many parameters. We investigate how the structure of gels develops depending on structural features of the precursors. To this end, we are gathering structural information by isolating crystalline primary hydrolysis products and thus obtain a "library" of core structures.
Carboxylate-substituted metal alkoxide derivatives exhibit a very special hydrolysis and condensation behavior, because molecular oxo/hydroxo clusters of the type MaOb(OH/OR)c(OOCR)d are already obtained when metal alkoxides are reacted with carboxylic acids. The clusters can be used as nanosized building blocks for the preparation of inorganic-organic hybrid materials (see section on Cluster-Reinforced Polymers).
Selected examples for primary hydrolysis products of titanium alkoxide aminoalcoholate and oximate derivatives are shown in Figure 4.
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Figure 4. Structures of the primary hydrolysis products Ti4O4(OiPr)12(OCHPhCHMeNHMe)4 (left) and Ti6O6(OBu)6(ON=C5H8)6 (right).
Selected review articles on precursor chemistry
- U. Schubert, N. Hüsing, A. Lorenz, Chem. Mater. 1995, 7, 2010-2027; "Hybrid Inorganic-Organic Materials by Sol-Gel Processing of Organofunctional Metal Alkoxides".
- U.Schubert, J.Mater.Chem. 2005, 15, 3701-3715; "Chemical Modification of Titanium Alkoxides for Sol-Gel Processing".
- U.Schubert, Acc.Chem.Res. 2007, 40, 730-737; "Organically Modified Transition Metal Alkoxides - Chemical Problems and Structural Issues on the Way to Materials Syntheses".
- R. Lichtenberger, U.Schubert, J.Mater.Chem. 2010, 40, 9287-9296; "Chemical Modification of Aluminium Alkoxides for Sol-Gel Processing".
This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
Project P17220 (2005-2008): Modification of Metal Alkoxides with Functional Organic Groups
Project P20750 (2008-2011): Organically Modified Metal Alkoxide Precursors for Sol-Gel Processing
Project P22536 (2011-2014): Pre-organized Metal Alkoxide Precursors