Decomposition of Precursor Compounds

In general, aUcoxides and carboxylates are the precursors employed in the synthesis of metal oxides. Chandler et al. [1] reviewed organometallic precursors employed in the synthesis of some perovskite oxides. It would be instructive to examine a few of the reactions involved in the synthesis of oxides from precursor compounds. Let us first examine the mode of formation of BaTiOj from the decomposition of barium titanyloxalate, which is best represented as Ba2Ti2(0)2(C20 ).  

Essentials of Inorganic Materials Synthesis, First Edition. C.N.R. Rao and Kanishka Biswas. 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

FIGURE 4.1 Distribution of two different cations (closed and open dirks) in reactantparticles and the diffusion distances in (a) the ceramic procedure and (b) in piecursorcompounds or precursor solid solutions. 

PbTi03 can be prepared by making use of carboxylate and alkoxide precursors as 

By refluxing a 1 1 mixture of the Pb and Ti precursors in alcohol, we obtain the precursor for PbTiOj. On decomposition the precnrsor gives PbTiOj. In order to prepare PbZrj Ti 03 and such oxides, the following procednre can be employed  

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Nanocrystals are zero-dimensional particles and can be prepared by several chemical methods, typical examples being reduction of salts, solvothermal synthesis and the decomposition of molecular precursors, among which the first method is the most commonly used in the case of metal nanocrystals. Metal oxide nanocrystals are generally prepared by the decomposition of precursor compounds such as metal acetates, acetylacetonates and cupferronates in appropriate solvents, often under solvothermal... 

C. Preparation of Metal Sols in Polymers by the Thermal Decomposition of Precursor Compounds... 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

The first carbene ever isolated was la, which was prepared using the most classical route to transient carbenes, namely, the decomposition of diazo compounds. The [bis(diisopropylamino)phosphino](trimethylsilyl)diazomethane precursor (la) was obtained by treatment of the lithium salt of trimethylsilyldiazomethane with 1 equiv of bis(diisopropylamino)chlorophosphine. Dinitrogen elimination occurs by photolysis (300 nm) or thermolysis (250 °C under vacuum) affording carbene la as a red oily material in 80% yield (Scheme 8.1). Carbene la is stable for weeks at room temperature and can even be purified by flash distillation under vacuum (10-2 Torr) at 75-80 °C. 

Metal catalysed decomposition of diazocarbonyl compounds in the presence of alkenes provides a facile and powerful means of constructing electrophilic cyclopropanes. The cyclopropanation process can proceed intermolecularly or intramolecularly. Early work on the topic of intramolecular cyclopropanation (mainly using diazoketones as precursors) has been surveyed31. With the discovery of powerful group VIII metal catalysts, in particular the rhodium(II) derivatives, metal catalysed cyclopropanation of diazocarbonyls is currently the most fertile area in cyclopropyl chemistry. In this section, we will review the efficiency and versatility of the various catalysts employed in the cyclopropanation of diazocarbonyls. Cyclopropanations have been organized according to the types of diazocarbonyl precursors. Emphasis is placed on recent examples. 

During the decomposition of precursors as mentioned above, additional reactions may occur. With hydrazinium compounds reduction may well take place [40] ... 

The thermal decomposition of aryl- and vinylcopper compounds is an alternative method of obtaining dimeric products from organocopper compounds. Both methods have been used extensively by Kauffmann and co-workers 153-164). Thermal decomposition of organocopper compounds of the type ZN=C(R)—CHRCu affords suitable precursors for a variety of 1,4-diketones. Thermal decomposition is not applicable to all organocopper compounds (see Section III,B,3). Various oxidations and dimerizations are listed in Table V. 

This chapter is intended to cover major aspects of the deposition of metals and metal oxides and the growth of nanosized materials from metal enolate precursors. Included are most types of materials which have been deposited by gas-phase processes, such as chemical vapor deposition (CVD) and atomic layer deposition(ALD), or liquid-phase processes, such as spin-coating, electrochemical deposition and sol-gel techniques. Mononuclear main group, transition metal and rare earth metal complexes with diverse /3-diketonate or /3-ketoiminate ligands were used mainly as metal enolate precursors. The controlled decomposition of these compounds lead to a high variety of metal and metal oxide materials such as dense or porous thin films and nanoparticles. Based on special properties (reactivity, transparency, conductivity, magnetism etc.) a large number of applications are mentioned and discussed. Where appropriate, similarities and difference in file decomposition mechanism that are common for certain precursors will be pointed out. 

Ceda-based oxides can be obtained by the decomposition of some compound precursor, such as hydroxide, nitrate, halides, sulfates, carbonates, formates, oxalates, acetates, and citrates.For example, nanosize or porous cerium oxide particles have been prepared at low temperatures by pyrolysis of amorphous citrate," which is prepared by the evaporation of the solvent from the aqueous solution containing cerium nitrate (or oxalate) and citric acid. In the case of mixed oxides, the precursor containing some cations in the same solid salts is prepared. In the same manner of ceria particles, the precursors complexing some cations with citrates are useful to synthsize ceria-zirconia mixed oxides and their derivatives. Also. Ce02-Ln203 solid solutions, where Ln = La. Pr, Sm. Gd. and Tb, have been synthesized from the precursors obtained by the evaporation of nitrate solutions at 353 K in air from an intimate mixture of their respective metal nitrates. The precursors are dried and then heated at 673 K to remove niU ates, followed by calcination at 1073 K for 12h. 

Vinylidenes have been transferred from a variety of precursors to olefins to produce methylenecyclopropanes . Because of ready intramolecular hydrogen shifts to give terminal acetylenes, the addition of vinylidene to olefins is rather limited to 2,2-disubstituted species. The methodologies so far developed include (1) gem-dibromides, 7, with MeLi, (2) vinyl halides " or vinyl triflates, 8 " with r-BuOK, (3) the fluoride ion promoted decomposition of vinylsilanes, 92 2,243 4 thermolysis of mercuric derivatives, 10, at 250 (5) decomposition of vinylazo compounds, 11, at 25 (6) the alkaline... 

The alkyl groups of two identical carboxylic acids can be coupled to symmetrical dimers in the presence of a fair number of functional groups (equation 1). Since free radicals are the reactive intermediates, polar substituents need not be protected. This saves the steps for protection and deprotection that are necessary in such cases when electrophilic or nucleophilic C—C bond-forming reactions are involved. Furthermore, carboxylic acids are available in a wide variety from natural or petrochemical sources, or can be readily prepared from a large variety of precursors. Compared to chemicd methods for the construction of symmetrical compounds, such as nucleophilic substitution or addition, decomposition of azo compounds or of diacyl peroxides, these advantages make the Kolbe electrolysis the method of choice for the synthesis of symmetrical target molecules. No other chemical method is available that allows the decarboxylative dimerization of carboxylic acids. 

Carbenoid insertions. The species generated from decomposition of diazo compounds in the presence of rhodium carboxylates are capable of inserting into various X-H bonds. Thus, N-C bond formation has been exploited for the preparation of precursors of indoles, oxazoles, and peptides. ... 

So it is not possible to determine whether the Murchison values are consistent with an abiotic source as far as N isotopes are concerned. However, the N sources of a-aminoisobutyric acid and sarcosine are clearly distinct. Glycine is a common decomposition product of other amino acids, so its 15N-depletion may reflect a kinetic isotope effect during decomposition of precursor amino acids. Alternatively, gylcine, which is a relatively simple compound, may have formed in interstellar space (Snyder 1997) from isotopically distinct precursor(s) prior to the synthesis of more complex amino acids on the Murchison parent body. At present there are insufficient data to conclude whether the stable isotopic composition of amino acids in the Murchison meteorite reflects the range associated with biosynthetic processes on Earth. 

The thermal decomposition of organic compounds can also be employed to generate small carbon clusters or atoms. The borderline with chemical vapor deposition (CVD) as presented in the next section is not really fix. In both cases, the method is based on the thermal decomposition of organic precursors. Processes both with and without catalyst have been reported. Contrary to the chemical vapor deposition, however, the catalyst (if applied) is not coated onto a substrate, but the substance or a precursor is added directly to the starting material ( floating catalyst ). The resulting mixture is then introduced into the reactor either in solid or in liquid state by a gas stream. From this point of view the HiPCo-process could also be considered a pyrolytic preparation of SWNT, but due to its importance it is usually regarded as autonomous method. 

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