Transition metal alkoxide

Metal-Oxygen Compounds. Clear examples of the addition of transition metal alkoxides to acetylenes are not known however, the addition of trialkyltin alkoxides has been reported. Triethyltin methoxide, for example, reacts with dimethyl acetylenedicarboxylate to give the vinyl tin derivative XVI (63). 

The use of supported metal complexes in transesterification reactions of TGs is not new. An earlier patent claimed that supported metals in a hydroxylated solid could effectively catalyze transesterification. The catalyst preparation used an inert hydrocarbon solvent to attach transition metal alkoxide species to the support surface. The reaction, however, was carried out in the presence of water. The author claimed that water was essential in preparing materials with good catalytic activity. Among the metals employed, titanium catalysts showed the best activity. However, it was not clear from the preparation method if reproducibility could be easily achieved, an important requirement if such catalysts were to be commercially exploited. 

Reviews appeared on the synthesis of nanocomposite organoceramics [126], on hybrid nanocomposite glass-organopolymers [127], and on the sol-gel processing of transition-metal alkoxides for electronics [128],... 

Double alkoxides with alkali metals A[M(OR)6] (Table 8) were formed from the reaction of the pentaalkoxides with alkali metal alkoxides. The double alkoxides of Mg, Ca, Sr and Ba with Nb and Ta have been synthesized in the presence of MgCl2 as a catalyst.169 Refluxing M(OPri)5 and A(OPri)3 (A = A1 or Ga) in isopropyl alcohol afforded double isopropoxides of the type [MA(OPr )8J and [MA2(OPr1)n]170. [NbTa(OMe)10] appears to be the first mixed transition metal alkoxide isolated.171 NMR showed it to be in dynamic equilibrium with the symmetrical M2(OMe)i0 dimers in solution, with close to random distribution of the three species. 

We could show that the modification of transition metal alkoxides is a versatile tool to adjust the reactivity of precursors for the needs in lyotropic crystalline templating processes. In case of high surfactant concentrations where the liquid crystalline template is formed prior to the addition of the precursor the use of a modifier may become unnecessary. The synthesis of nanostructured rhenium dioxide and the utilization of MTO as precursor for this purpose clearly shows that in some cases the use of unusual specialized compounds is imperative. First promising results in the synthesis of nanostructured chromium oxide surfactant composites have been displayed although hydrolysis of the precursor seems to be still uncompleted within the nanostructure. The possibility of tailoring the d-values in a desired way besides the synthesis of certain particle morphologies encourages for further work in the future. 

The reduction of transition metal alkoxides and phenoxides with alkali metals can generate mixed metal derivatives which may or may not be stable, for example equation (48).154... 

The hydrolysis of early transition metal alkoxides is an extremely facile reaction that makes the manipulation of these materials non-routine. The typical insolubility and high molecular complexity... 

An interesting variant is the in situ preparation of transition metal alkoxides from the corresponding halogenides and subsequent reaction with an azolium salt to form the NHC transition metal complex [69]. This works particularly well with rhodium, iridium and ruthenium where [(ii -cod)MCl]j (M = Rh, Ir) and [Cp RuCl]2 are readily available [57,58,71]. 

Figure 1 Molecular structure of some transition metal alkoxides Ti(OPr )4 monomer (1) [Zr(OPF)4(PFOH)]2 dimer (2) [Ti(OEt)4]3 trimer (3)...

Metal alkoxides act as cross-linking agents between organosilicon units, increasing the hardness of the materials. Tetrafunctional silicon alkoxides are more commonly employed, but transition metal alkoxides M(OR) (M = Ti, Zr, etc.) can also be used. They not only serve as cross-linking reagents, but can also increase the refractive index or catalyze condensation. 

Komiya, S., Taneichi, S., Yamamoto, A., Yamamoto, T. Transition metal alkoxides. Preparation and properties of bis(aryloxy)iron(ll) and bis(alkoxy)iron(ll) complexes having 2,2 -bipyridine ligands. Bull. Chem. Soc. Jpn. 1980, 53, 673-679. 

More recently (1994), discrete early transition metal (Ti, V, Nb, orTa) al-koxide complexes containing homochiral trialkanolamine ligands (392) were prepared and their usefulness as highly enantioselective catalysts was demonstrated. It is noteworthy that earlier work on the reactions of tetradentate triethanolamine with transition metal alkoxides was reported to yield insoluble products (6). 

One major problem in producing gels containing homogeneous mixtures of a variety of oxides is that the precursors may not all hydrolyze at the same rate. In particular, transition metal alkoxides hydrolyze much more rapidly than silicon alkoxides. The controlled hydrolysis of low-molecular-weight homometallic species described in the previous section can be adapted to prepare mixed alkoxides. For example, pre-hydrolysis of metal alkoxide followed by reaction with the silicon alkoxide gives a mixed dimeric species such as ... 

Silicon alkoxides exhibit very slow hydrolysis and condensation reactions compared with other alkoxides of aluminum, titanium or zirconium generally used for membrane preparation. Accordingly, acid or basic catalysts are used in the case of silicon alkoxides while methods for the control of hydrolysis are advisable with transition metal alkoxides [36,37]. 

For silica gels a number of parameters have been demonstrated to have a large effect on the evolution of porosity and subsequently on the resulting silica materials [1]. Almost dense, micro- or mesoporous silica materials can be obtained depending on the experimental conditions in which hydrolysis and condensation reactions of silicon alkoxides are carried out. This is not the case for transition metal alkoxides which are very sensitive to hydrolysis. They do not cillow the adaptation of sol-to-gel transition in order to obtain controlled porous textures. Some years ago special attention was paid to the utilization of amphiphilic systems as reactive media to control hydrolysis and condensation kinetics with transition metal alkoxides [37]. In a more recent work Ayral et al. 

J. Livage, M. Henry and C. Sanchez, Sol-gel chemistry of transition metal alkoxides. Prog. Solid State Chem., 18 (1988) 259-341. 

Synthesis Under Low pH Conditions. The reactivity of transition metal alkoxides M(OR) can be controlled... 

In the early eighties, a new method for the deposition of transition metal ion monolayer on the surfaces of inorganic oxides was developed in our laboratory [3]. The essential step of this method is the selective reaction of the surface hydroxyl groups with transition metal alkoxide molecules [3,4]. The adopted procedure allows the design of catalysts containing a strictly determined number of transition metal monolayers or sandwich-type catalysts with different metals monolayers anchored to the surface of the carrier. 

It is noteworthy that the anchoring of subsequent layer of zirconium compound is not possible due to the lack of OH groups on the surface of the support modified with transition metal alkoxide. 

In order to prove the adopted mechanism of the reaction between transition metal alkoxide and surface hydroxyls, it was necessary to determine the concentrations of primary and secondary OH groups and the concentration of transition metal bonded to the support surface. These results are presented in Table 1. 

Alkoxides.9 These have been much studied and are generally typical of other transition-metal alkoxides, such as those of Hf, Ce, V, Nb, Fe and U, which will not be discussed in detail. The compounds can be obtained by reactions such as... 

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