Precursors solubility

Several aspects should be considered in the selection of appropriate Pt precursors. Solubility of precursor is important as this parameter determines how well a precursor can be dissolved in a given solvent so as to facilitate the nucleation and growth. Reduction potential is another parameter, which governs the ease with which the precursor can be reduced to Pt metal. Finally, the thermal stability of the precursor partly determines the reaction temperature for the formation of Pt nanoparticles. 

SupercriUcal Fluid DeposiUon (SFD) Metal films may be grown from precursors that are soluble in CO2. The SFD process yields copper films with fewer defects than those possible by using chemical vapor deposition, because increased precursor solubility removes mass-transfer hmitations and low surface tension favors penetration of high-aspect-ratio features [Blackburn et al.. Science, 294, 141-145 (2001)]. 

Kelly and Brown51 describe a method for the preparation of concentrated solutions of carbocations ( 1 M) in SbFs-S02ClF. This method, which employs a syringe technique, allows quantitative conversion of precursors soluble in SO2CIF at —78°C into the corresponding carbocations. 

Uses Inexpensive Precursors Soluble precursors are used instead of expensive, high vapor pressure organometallics. 

Heat is generally released when a solid/gas interface is replaced by a solid/liquid interface. This might influence the quality of impregnation if the precursor solubility decreases when the temperature increases or if detrimental reactions involving the support take place due to a temperature rise. These drawbacks can be avoided by exposing the support to water vapor before impregnation and thus stabilizing the interface with a hydrous film. 

If we consider metals from the Mn—Zn series, chlorides and sulfates are readily soluble, but chloride ions may act as poisons for the active phases [89] and sulfate ions do not decompose at moderate temperatures. However, their presence is sometimes searched for due to the acidity they can induce on the support [90]. The most used precursors are metal nitrates sold under the dualistic formula M(N03)x.yH20, which associate a high solubility (respectively, 8.5 and 4.6 mol per 1 of water at room temperature for nickel or cobalt, respectively) to the possibility of decomposing nitrate ions by calcination between 250 and 350°C [91]. Carboxylates such as formates or acetates have been sometimes used, thou they are more than three times less soluble than nitrates [92]. These anions also have coordinative properties toward the cations in solution or in the crystals, as is the case for the acetylacetonate anion, which gives precursors soluble in organic solvents. 

The mechanism for the fine-particle formation in supercritical water is discussed as follows (Figure 7) The solubility of metal oxides in subcritical water is higher than that at supercritical conditions, as discussed above. Thus, after nucleation, inclusion of precursors (soluble intermediates) takes place to grow crystals. On the other hand, in supercritical water hydrothermal synthesis reaction proceeds faster than that in subcritical water due to the higher temperature and the lower dielectric constant, as expected from Eq. (2). The solubility... 

The first step in designing a precursor synthesis is to pick precursor molecules that, when combined in organic solvents, yield the bulk crystalline solid. For metals, a usual approach is to react metal salts with reducing agents to produce bulk metals. The main challenge is to find appropriate metal salts that are soluble in an organic phase. 

Dimethyl acetylenedicarboxylate (DMAD) (125) is a very special alkyne and undergoes interesting cyclotrimerization and co-cyclization reactions of its own using the poorly soluble polymeric palladacyclopentadiene complex (TCPC) 75 and its diazadiene stabilized complex 123 as precursors of Pd(0) catalysts, Cyclotrimerization of DMAD is catalyzed by 123[60], In addition to the hexa-substituted benzene 126, the cyclooctatetraene derivative 127 was obtained by the co-cyclization of trimethylsilylpropargyl alcohol with an excess of DMAD (125)[6l], Co-cyclization is possible with various alkenes. The naphthalene-tetracarboxylate 129 was obtained by the reaction of methoxyallene (128) with an excess of DMAD using the catalyst 123[62],... 

The diazonium salts precursors can be aniline, o- and p-toluidine, o-and p-anisidine, o- and p-phenetidine. or 3-naphthy]amine. The resulting formazans are crystalline and inlensel> colored. They are soluble in organic solvents, giving a red-violet coloration that darkens to blue. Dehydrogeneration gives the corresponding tetrazolium salts, which are isolated as perbromides (Scheme 51. Table X-13). 

Cholesterol is biosynthesized in the liver trans ported throughout the body to be used in a va riety of ways and returned to the liver where it serves as the biosynthetic precursor to other steroids But cholesterol is a lipid and isn t soluble in water How can it move through the blood if it doesn t dis solve in if The answer is that it doesn t dissolve but IS instead carried through the blood and tissues as part of a lipoprotein (lipid + protein = lipoprotein) The proteins that carry cholesterol from the liver are called low density lipoproteins or LDLs those that return it to the liver are the high-density lipoproteins or HDLs If too much cholesterol is being transported by LDL or too little by HDL the extra cholesterol builds up on the walls of the arteries caus mg atherosclerosis A thorough physical examination nowadays measures not only total cholesterol con centration but also the distribution between LDL and HDL cholesterol An elevated level of LDL cholesterol IS a risk factor for heart disease LDL cholesterol is bad cholesterol HDLs on the other hand remove excess cholesterol and are protective HDL cholesterol IS good cholesterol... 

The cobalt catalyst can be introduced into the reactor in any convenient form, such as the hydrocarbon-soluble cobalt naphthenate [61789-51 -3] as it is converted in the reaction to dicobalt octacarbonyl [15226-74-17, Co2(CO)g, the precursor to cobalt hydrocarbonyl [16842-03-8] HCo(CO)4, the active catalyst species. Some of the methods used to recover cobalt values for reuse are (11) conversion to an inorganic salt soluble ia water conversion to an organic salt soluble ia water or an organic solvent treatment with aqueous acid or alkah to recover part or all of the HCo(CO)4 ia the aqueous phase and conversion to metallic cobalt by thermal or chemical means. 

The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with two moles of butyUithium. Unfortunately, because of the tendency of organ olithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates (see Table 2) (33,38,44,67), attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (34,66,68—72). These precipitates are not effective initiators because of their heterogeneous initiation reactions with monomers which tend to result in broader molecular weight distributions > 1.1)... 

In addition to providing fully alkyl/aryl-substituted polyphosphasenes, the versatility of the process in Figure 2 has allowed the preparation of various functionalized polymers and copolymers. Thus the monomer (10) can be derivatized via deprotonation—substitution, when a P-methyl (or P—CH2—) group is present, to provide new phosphoranimines some of which, in turn, serve as precursors to new polymers (64). In the same vein, polymers containing a P—CH group, for example, poly(methylphenylphosphazene), can also be derivatized by deprotonation—substitution reactions without chain scission. This has produced a number of functionalized polymers (64,71—73), including water-soluble carboxylate salts (11), as well as graft copolymers with styrene (74) and with dimethylsiloxane (12) (75). 

Research and development in the field ate stiU continuing at a fast pace, particularly in the area of absorption and emission characteristics of the polymers. Several reasons account for this interest. First, the intractable polydimethyl silane [30107-43-8] was found to be a precursor to the important ceramic, siUcon carbide (86—89). Secondly, a number of soluble polysdanes were prepared, which allowed these polymers to be studied in detail (90—93). As a result of studies with soluble polymers it became cleat that polysdanes are unusual in their backbone CJ-conjugation, which leads to some very interesting electronic properties. 

Examples of polymers which form anisotropic polymer melts iaclude petroleum pitches, polyesters, polyethers, polyphosphaziaes, a-poly- -xyljlene, and polysdoxanes. Synthesis goals iaclude the iacorporation of a Hquid crystal-like entity iato the maia chaia of the polymer to iacrease the strength and thermal stabiHty of the materials that are formed from the Hquid crystal precursor, the locking ia of Hquid crystalline properties of the fluid iato the soHd phase, and the production of extended chain polymers that are soluble ia organic solvents rather than sulfuric acid. 

More recendy, molecular molybdenum-sulfur complexes and clusters have been used as soluble precursors for M0S2 in the formulation of lubricating oils for a variety of appHcations (70). Presumably, the oil-soluble molybdenum—sulfur-containing precursors decompose under shear, pressure, or temperature stress at the wear surface to give beneficial coatings. In several cases it has been shown that the soluble precursors are trifunctional in that they not only display antifriction properties, but have antiwear and antioxidant characteristics as weU. In most cases, the ligands for the Mo are of the 1,1-dithiolate type, including dithiocarbamates, dithiophosphates, and xanthates (55,71). 

Because of its volatility, the cobalt catalyst codistills with the product aldehyde necessitating a separate catalyst separation step known as decobalting. This is typically done by contacting the product stream with an aqueous carboxyhc acid, eg, acetic acid, subsequently separating the aqueous cobalt carboxylate, and returning the cobalt to the process as active catalyst precursor (2). Alternatively, the aldehyde product stream may be decobalted by contacting it with aqueous caustic soda which converts the catalyst into the water-soluble Co(CO). This stream is decanted from the product, acidified, and recycled as active HCo(CO)4. 

Squalene is also an intermediate in the synthesis of cholesterol. StmcturaHy, chemically, and biogeneticaHy, many of the triterpenes have much in common with steroids (203). It has been verified experimentally that squalene is the precursor in the biosynthesis of all triterpenes through a series of cyclization and rearrangement reactions (203,204). Squalene is not used much in cosmetics and perfumery formulations because of its light, heat, and oxidative instabiUty however, its hydrogenated derivative, squalane, has a wide use as a fixative, a skin lubricant, and a carrier of Hpid-soluble dmgs. 

CVD processing can be accompanied by volatile hot-reaction by-products such as HCl or HF, which, along with unused precursor gases, must be removed from the exhaust gas stream. This is done by scmbbing the chemicals from the gas using water to dissolve soluble products or by burning the precursor gases to form oxides. 

Attempts have also been made to reduce the odor associated with the peracid in the home laundry. Use of a precursor that generates the peracid of a fatty acid can result in an objectionable odor in the wash bath (106). This odor is exacerbated by the higher piC of the peracid versus its parent acid resulting in a greater proportion of the peracid in the unionized and therefore less water-soluble form. To mitigate this circumstance, functionalization of the fatty tail typically alpha to the carbonyl has been utilized (112). The modifications include alpha-chloro and alpha-methoxy substituents on the parent acid portion of the precursor ester. 

---