Sol-gel chemistry involves nucleophilic reactions The chemical reactivity of metal alkoxides toward hydrolysis, condensation and complexation mainly depends on the electronegativity of the metal atom, its coordination number and the steric hindrance of alkoxide groups Silicon alkoxides are poorly reactive Hydrolysis and condensation rates have to be enhanced by acid and base catalysis or nucleophilic activation Transition metal alkoxides are usually too reactive They have to be stabilized by complexation in order to avoid fast condensation The molecular design of alkoxide precursors opens the way to tailor-made materials

Sol-gel chemistry

The chemical modification of titanium alkoxides by alkoxy- and aminoalcohols, s-diketones, s-ketoesters, carboxylic and phosphonic acids, and related compounds, and the hydrolysis behavior of the organically modified precursors are reviewed Special focus is put on structural aspects and on the possibility to introduce functional organic groups Such precursors have a high potential for innovative materials syntheses because they permit control of the precursor reactivity in sol–gel processes and the preparation of titania-based inorganic–organic hybrid materials Coordination, solvation, aggregation and redistribution equilibria play an important role in the chemistry of the modified titanium alkoxides, and organic side reactions have to be taken into account

Chemical modification of titanium alkoxides for sol–gel processing

Titanium oxo-clusters, well-defined monodispersed nano-objects, are appropriate nano-building blocks for the preparation of organic–inorganic materials by a bottom up approach. This critical review proposes to present the different structures of titanium oxo-clusters referenced in the literature and the different strategies followed to build up hybrid materials with these versatile building units. In particular, this critical review cites and reports on the most important papers in the literature, concentrating on recent developments in the field of synthesis, characterization, and the use of titanium oxo-clusters for the construction of advanced hybrid materials (137 references).

Titanium oxo-clusters: precursors for a Lego-like construction of nanostructured hybrid materials.

This Minireview highlights advances in the Suzuki?Miyaura cross-coupling of secondary boron reagents for the creation of C[BOND]C bonds with control of stereochemistry. It also includes non-transition-metal coupling of secondary and tertiary boronic esters to electron-rich aromatics.

Stereospecific Couplings of Secondary and Tertiary Boronic Esters

This paper summarizes recent literature data and presents new experimental data on the mechanisms of chemical modification, hydrolysis and polycondensation of the alkoxides and demonstrates possibilities to approach new classes of materials, exploiting these mechanisms. Low reactivity of silicon alkoxides is improved by either basic catalysis exploiting an SN2 mechanism or acidic catalysis facilitating a proton-assisted SN1 mechanism as well as by modification with chelating ligands. Metal alkoxides are much stronger Lewis bases compared to silicon alkoxides and the acidity of water is strong enough to achieve their rapid hydrolysis via proton-assisted SN1 pathway even in the absence of additional catalysts. Introduction of the modifying chelating ligands is leading generally to increased charge distribution in the precursor molecules. Modifying chelating ligands are also appreciably smaller than the alkoxide ligands they replace. The modification with chelating ligands is thus facilitating the kinetics of hydrolysis and polycondensation. The size and shape of the primary particles formed in sol-gel treatment of metal alkoxides are defined not by kinetic factors in their hydrolysis and polycondensation but by the interactions on the phase boundary, which is in its turn directed by the ligand properties. The products of the fast hydrolysis and condensation sequence consist of micelles templated by self-assembly of ligands (mainly oxo-species). This concept provides explanations for commonly observed material properties and allows for the development of new strategies for the preparation of materials. We discuss the formation of inverted micelles, obtained by the appropriate choice of solvents, which allows for the formation of hollow spheres. The modifying β-diketonate ligands act as the surfactant and form an interface between the hollow sphere and the solvent. Retention of ligands inside the gel particles is possible only if ligands possessing both chelating and bridging properties are applied. Application of such ligands, for example, diethanolamine, permits to prepare new transition metal oxide based microporous membranes.

New insight in the role of modifying ligands in the sol-gel processing of metal alkoxide precursors: A possibility to approach new classes of materials

Characterization of Zr{sub 2}(O{sup i}Pr){sub 8}({sup i}PrOH){sub 2} by {sup 1}H and {sup 13}C NMR spectroscopy, ir spectroscopy, and single-crystal x-ray diffraction ({minus}100{degree}C) is reported. The unit cell contains four half-dimers in the asymmetric unit, all of which differ only in the rotational conformation about Zr-O and O-C bonds. In each dimer, the edge-shared bioctahedron has two {mu}-O{sup i}Pr groups. On opposite sides of this Zr{sub 2}({mu}-OR){sub 2} plane, each dimer forms two hydrogen bonds, one each between a coordinated alcohol and a terminal alkoxide. The NMR spectra at 25{degree}C are so simple as to be structurally uninformative, a result of rapid fluxionality which includes, as one component, proton migration among all O{sup i}Pr units. At {minus}80{degree}C in toluene, the NMR spectra are now too complex to be accounted for by a single edge-shared bioctahedral structure. The hafnium analogue is isomorphous with the zirconium compound. Although Ce{sub 2}(O{sup i}Pr){sub 8}({sup i}PrOH){sub 2} is not isomorphous, it exhibits an analogous hydrogen-bonded structure in which the O{hor ellipsis}O distance is as short as it is in the Zr analogue, in spite of a metal-metal separation which is longer by 0.28 {angstrom}. The generality of hydrogen bonding between M-OR and M-O(H)R groups whenmore » they are aligned parallel in a metal cluster is reviewed. 44 refs., 2 figs., 4 tabs.« less

S. Parraud

Papers

Alcohol adducts of alkoxides: intramolecular hydrogen bonding as a general structural feature