Metal alkoxide complexes

During the last decade Shibasaki and co-workers focussed on the application of rare earth metal catalysts with special properties [2]. More recently, impressive studies by this group revealed the broad applicability of chiral heterobimetallic catalysts based on rare earth metal alkoxide complexes in asymmetric catalysis. Whereas initial... 

Although these mixed metal alkoxide complexes are a means of obtaining atomic mixing of the various metals, the stoichiometric ratio of metals may or may not be that desired for the ceramic powder. A different stoichiometric ratio, n, for the same two metals is not likely to be precipitated out just because the initial solution contains a different stoichiometric ratio. Also for many electronic ceramic compositions it is desirable to have many other metals incorporated at the ppm level into the ceramic powder as sintering aids, grain growth inhibitors, and crystal phase stabilizers. Adding these otiier metals is very difficult... 

Mixed-metal alkoxide complexes of thallium are also known. For example, Sn(/u-t-BuO)3Tl has both Sn(II) and T1(I) ions." The thallium site is unreactive as a donor for metal carbonyls. However, as indicated earlier, the indium(I) site of the indium analogue shows Lewis-base character. The Sn(IV)/Tl(I) mixed alkoxide [Sn(EtO)eTl2] exists as a one-dimensional polymer. This adduct reacts quantitatively with SnCl2 to form the homoleptic, mixed-valent [Sn2(OEt)6]n. Thallium-titanium double alkoxides have been synthesized using thallium alkoxide as one of the starting materials. ... 

As mentioned earlier, fairly successful conjectures (147, 148) about the structural features of homo- as well as heterometallic alkoxides were made earlier on the basis of physicochemical studies in solution. Even as late as 1989, Bradley (72) pointed out the special difficulties in the X-ray structural elucidation of metal alkoxide complexes. Such difficulties were reemphasized by Caul-ton and Hubert-Pfalzgraf (4), particularly the heterometallic alkoxides, which in many cases are obtained in a highly plastic (deformable) state even when crystallizable, which renders them unsuitable for single-crystal X-ray analysis. Therefore, the process of X-ray crystallographic structural characterization was gathering considerable momentum (4, 23, 28, 38, 39) and even with the availability of other more sophisticated techniques, publications were being limited mainly to derivatives characterized crystallographically. 

Another promising result of X-ray structural elucidation of metal alkoxide complexes was the discovery of the presence of inter- and intramolecular O—H -0 hydrogen bonding (cf. Section III.B.3). 

Metal alkoxide complexes with related alcohols are apparently edge-sharing bioctahedral dimers, on the evidence of the X-ray structure of the isopropyl complex. Some stable mixed alkoxides of the type Q[M3(OR)g] (Q = Li, Na, K, NH4, Ca/2) have been reported they distill in vacuo without decomposition. The X-ray structures of several bimetallic alkoxides obtained byCaulton,QZr2[0()-Pr)]g,Q = Li[HO(/-Pr)], K(DME), Ba[0(t-Pr)], show a similar triangular structure with two /X3 and three /x-OR bridges. The thallium salt of composition Tl2Zr(O R)e obtained by the reaction shown in equation (18) has a distorted octahedral structure stabilized by six T1 F contacts. 

Many of the typical reactions that form transition metal-heteroatom bonds can be used to prepare late transition metal alkoxides. - For example, metathetical exchange between late metal halides and alkali metal salts of the corresponding alcohols often forms late metal alkoxides. Complexes that are more reactive than metal halides, such as metal acetates or triflates, are sometimes used when exchanges with halides are slow or reversible (Equation 4.66). Late metal fluorides can also be used for these exchanges, as shown in Equation 4.67. ... 

In 1998, the anodic oxidation of molybdenum and tungsten in alcohols in the presence of LiCl (as electroconductive additive) was found to yield a variety of interesting oxo-metal alkoxide complexes, some of which have been authenticated by singlecrystal X-ray crystaUograpy. 

This procedure, sometimes referred to as transmetallation or (metathesis or salt-elimination) reaction, is by far the most versatile synthetic method for a wide range of d-and p-block metal alkoxide complexes. The alkali metal (usually sodium or potassium) alkoxide is treated in the presence of excess alcohol with the corresponding metal(loid) halide either in a hydrocarbon (generally benzene) or an ether solvent (Eq. 2.41) ... 

As NMR spectroscopy has become a well-established technique for elucidation of structural features of organic componnds, it has also been employed qnite snccessfiilly for throwing light on the stmctural features of metal alkoxides. AU the early NMR work involved proton NMR, which continues to be one of the most nsefiil techniqnes for the investigation of metal alkoxides. However, metal alkoxide complexes have also been studied by and metal NMR. 

Such a delocalized active complex might be expected to be more stable and give a diene polymer having a high molecular weight. But the range of solvents in which stable sodium-metal alkoxide complex solutions could be obtained was severely limited by solubility and decomposition problems, especially in hydrocarbon media. The application of dicyclohexyl-18-crown-6 or DCHE (ll) to complex the sodium metal as reported by Schire has... 

The nanocrystalline metal oxides can be prepared by the hydrolysis of chelate metal alkoxide complexes at elevated temperature in solutions of the parent chelating alcohols [321]. This method allows the preparation of nonaggregated nanocrystals with variable size and composition and high crystallinity. The surface of the isolated nanoparticles is coated with a labile layer of the solvent and remains chemically active and available for further derivatization. Nanocrystals in this state are capable of forming stable aqueous colloids without using capping ligands or surfactants. The... 

Scheme 2 (a) Chain-end stereocontrol mechanism, (b) Enantiomorphic site stereocontrol mechanism, (c) Tactic polymers via chain-end control mechanism, (d) Isotactic polymers via enantiomorphic site control mechanism. L pM-OR is an enantiomerically pure metal alkoxide complex that prefers R-monomer L n is an enantiomerically pure, chiral ligand. 

The innovative catalysts involve the use of mixed metal alkoxide complexes that incorporate the dual functions of base, giving rise to a chiral metal enolate, and Lewis acid, activating the aldehyde component. A large famOy of these systems has been examined by Shibasaki in a host of different transformations for complex molecule synthesis. The lanthanum binaphthoxide complexes LLB (213) are some of the earliest of these and were shown to give aldol adducts with high selectivities and yields. This process has been utilized elegantly in the kinetic resolution of aldehyde ( )-215 in the enantioselective syntheses of epothilones A and B (217/218, Scheme 4.24) [107]. 

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