Alkoxides solution-state species

Using a tetradentate ligand, Rees and Moreno have proposed the first example of a monomeric barium bis-alkoxide (26). The proposed solution structure (Fig. 2) was determined based on ciyoscopic and spectroscopic data. One notable characteristic of this monomeric barium alkoxide is that it is an ambient temperature liquid. Although the compound exists as a monomer in the solution state, oligomeric species prevail in the absence of solvent. This phenomenon has been ob-... 

Sol-gel processing describes a type of solid materials synthesis procedure, performed in a liquid and at low temperature (typically T< 100 °C). The development of sol-gel techniques has long been known for preparations of metal oxides and has been described many times [30-38, 40-46, 65]. The process is typically used to prepare metal oxides via the hydrolysis of reactive precursors, usually alkoxides in an alcoholic solution, resulting in the corresponding hydroxide. It is usually easy to maintain such hydroxide in a dispersed state in the solvent. Condensation of the hydroxide molecules with loss of water leads to the formation of a network When hydroxide species undergo polymerization by condensation of the hydroxy... 

In a very comprehensive study by Finn and co-workers the synthesis, properties and reactions of several titanium-substituted ylides are described. The complexes were prepared from titanium alkoxides and (Me2N)3P=CH2 (scheme 9). The structure of one of the complexes was determined crystallographically which showed it to exist as a chloride bridged dimer (53). The ylidic P-C distance and /pc coupling constant are charactersitic of a P-C single bond which, together with the six-coordinate nature of the Ti centres, verify the zwitterionic nature of the species in both the solid state and in solution. The titanium ylide... 

Most of the metal alkoxides of interest for electrooptical ceramics are solids (less often liquids) that can be purified by recrystallization, sublimation, or distillation. They are all moisture sensitive, and handling under an inert atmosphere and with anhydrous solvents is thus required. Their unequivocal characterization and formulation are best achieved by x-ray diffraction studies (on monocrystals). Studies on solutions (molecular weight data, nuclear magnetic resonance, NMR, with H, or metal nuclei) are a means either to establish whether the solid-state structure is retained or, in the absence of x-ray data, to establish the molecular structure and eventually stereolability [48]. Mass spectrometry provides information on the stability of the oligomers or the het-erometallic species in the vapor phase. The information gained by infrared spectroscopy is limited the technique is mostly useful for the identification of solvates M(OR) (ROH)x (vOH absorption 3400-3100 cm-l or of chemically modified (heteroleptic) alkoxides (probe for the vCO stretching of P-diketonate or carboxylate ligands, for instance). 

As will be discussed in more detail below, the reactivity of metal alkoxides towards water is influenced by n. It must be pointed out, however, that the degree of association found in the solid-state structures does not necessarily correspond to that in solution. In solution, equilibria between differently associated species may exist and, more importantly, polar solvents may interact with the metal atoms and thus reduce the degree of association. For example, M(OPr )4 (M = Ce, ... 

The sol-gel chemistry of metal salts is more complex than that of metal alkoxides because of the numerous molecular species that can be formed depending on the oxidation state of the metal, the pH of the reaction solution and the concentration of the reactants. Since the sol-gel polymerization of inorganic salts varies widely among the different metal ions, this section will only present a general summary of the topic. For a detailed description of the mechanism of condensation and gelation, the reader is referred to the following review on the sol-gel chemistry of transition metal oxides [7]. 

More recently, a copper-bifluoride complex has been described, starting once again from the alkoxide species. Subsequently, [Cu(NHC)2] [HF2] complexes were developed (Figure 8.2) [91]. Both complexes were highly air stable in the solid state and moderately stable in solution compared to their analog [Cu(F) (IPr)] [92]. Remarkably, catalytic activity was observed in several transformations such as the reduction of ketones, the 1,4-conjugated borylation and silylation, and the diastereoselective allylation. 

---