Cerium alkoxide

Figure 12.2 Various cerium alkoxide complexes employed for the generation of cerium surface species (layers) wt% Ce of [Ce(OR)3] MCM-41.28o (2) obtained from atomic absorption spectroscopy.

Gulino, A., Casarin, M., Conticellio, V.P. etal. (1988) Efficient synthesis, redox characteristics, and electronic structure of a tetravalent tris(cyclopentadienyl)cerium alkoxide complex. Organometallics, 1, 2360. 

The chemical reactivity of metal alkoxides towards hydrolysis and condensation depends mainly on the positive charge of the metal atom 5(M) and its ability to increase its coordination number N. As a general rule, the electronegativity of metal atoms decreases and their size increases when going from the top right of the periodic table to the bottom left. The corresponding alkoxides become progressively more reactive towards hydrolysis and condensation. Silicon alkoxides are rather stable, while cerium alkoxides are very sensitive to moisture. Alkoxides of electropositive metals must be handled with care under a dry atmosphere, otherwise precipitation occurs as water is present... 

Ceria nanoparticles can also be synthesized in organic solvents. Yan and co-workers used cerium alkoxide as a precursor to be decomposed in a high-boiling solvent (tri-n-octylphosphine oxide and dioctyl ether). The structure of the final product can be tuned by changing reaction conditions such as temperature and the concentration of the precursors. 

The same reason was given in the literature for the formation of a number of other bimetallic oxocomplexes such as Cd4Sn4(p4-O)(OCH2Bu,)10(OAc)10 [325] and [PbTi2(p4-0)(0Et)7(0Ac)]2 [320]. When the alkoxides of zirconium, hafnium, or cerium are used for this reaction, the formation of the oxocomplexes can be due to the elimination of an unsaturated hydrocarbon from the initial alkoxide [1565], leading to the formation of monometallic oxoalkox-ides as intermediates (the latter are formed already on desolvation of [M(OR)4(ROH)]2 and are always present as admixtures in the samples of des-olvated M(OR)4, where R is a primary or secondary radical [1612] (see also Section 12.12). In this case the possible sequence of transformations (taking place usually on reflux in toluene) can look as follows ... 

The ring opening of 2,2 -diphenyloxetane has been achieved using cerium(iv) ammonium nitrate (CAN) as a redox catalyst (Equation 7) <2003TL4585>. The first step involves oxidation of the oxetane by Ce(rv) to a cyclic radical cation. Equilibration to the ring-opened distonic version 42 with a stabilized cation, then quenching of the cation with methanol and reduction of the alkoxide radical by Ce(lll) to the anion, completes the catalytic cycle. 

Lanthanides in homogeneous systems As organometallics As cerium(IV) salts As coordination complexes As nitrates, chlorides, alkoxides etc. For olefin polymerization For olefin hydrogenation For free radical polymerization For Diels-Alder reactions For olefin polymerization In organic synthesis... 

It is a commonplace to say that there has been explosive growth in the use of lanthanides in organic chemistry. For many years, the use of cerium(iv) compounds as oxidants was widespread, but more recently a whole range of other compounds have made their appearance. Thus samarium(ii) compounds are now routinely used as one-electron reducing agents and the use of trifluoromethanesulfonate ( triflate ) salts of scandium and the lanthanides as water-soluble Lewis acid catalysts is widespread. Beta-diketonate complexes and alkoxides have also come into use there are even applications of mischmetal in organic synthesis. 

Evans, W.J., Deming, T.J., and Ziller, J.W. (1989) The utility of ceric ammonium nitrate-derived alkoxide complexes in the synthesis of organometallic cerium(lV) complexes. Synthesis and first X-ray crystallographic determination of a tetravalent cerium cyclopentadienide complex, (C5H5)3Ce(OCMe3). Organometallics, 8, 1581. 

The most interesting result is the formation of a transparent colloidal solution of ceria with 2 nm particles. Cerium metal tips with the superficial layers of oxide are allowed to react in 2-methoxyethanol at 250 to 300°C, and removal of coarse ceria particles originating from the superficial layers yields the colloidal solution. Addition of water to the solution does not cause any change except dilution of the color of the solution, but addition of a drop of a solution of any kind of salt immediately causes precipitation of ceria particles. - The reaction mechanism is as follows The solvent slowly dissolves the superficial layers, and when the solvent reaches the metal, rapid reaction occurs, yielding an alkoxide solution. The concentration of the ceria precursor becomes so high that a burst of nucleation occurs, yielding the colloidal solution. The reaction of cerium acetylacetonate in the same solvent yields ceria particles but does not give a colloidal solution. 

In an interesting c( nparison of the propensity for alkoxide attack on silicon, given the option of elimination in the direction of phosphorus or silicon, it is the silicon elimination that prevails, to give the vinylphosphonium salt (Scheme 3). For use with enolizable carbonyls, the basicity of the reagent can be greatly reduced by the use of cerium chloride, as discussed in Section 3.I.3.4.2. 

The transesterification of DMT to BHET requires use of homogeneously dissolved metal salts, such as the acetates or alkoxides of calcium, magnesium, manganese, cobalt, zinc, sodium, lithium, lead, cerium and cadmium, and is run in the melt at 170-220 °C [18-20]. A mechanism in which the metal salt coordinates to the carbonyl oxygen of DMT, thereby enhancing the reactivity toward nucleophilic diol, is believed to occur during transesterification [21] (see Scheme 5). 

The electronic structures of the cerium(iv) alkoxide complexes Cp3Ce(OR) have been investigated by He I and He II UV photoelectron spectroscopy combined with SCF Xa-DVM calculations.503... 

A three-step synthesis of cerpegin (118) was described by Hong and Comina (235). Lithiation of 2-methoxypyridine (361) with mesityllithium followed by the addition of N-formyl-A/,lV, 7V -trimethylethylenediamine (362) gave an a-amino alkoxide which was treated with n-butyl lithium. The resulting dianion 363 was reacted with cerium chloride followed by acetone to yield, after acidification, the lactol 364. Oxidation with PCC gave 365, which was treated with methyl iodide at 145°C to afford 118 (Scheme 43) (235). The overall yield was 34%. 

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