Volatility properties, transition metal

Several patents dealing with the use of volatile metal amidinate complexes in MOCVD or ALD processes have appeared in the literature.The use of volatile amidinato complexes of Al, Ga, and In in the chemical vapor deposition of the respective nitrides has been reported. For example, [PhC(NPh)2]2GaMe was prepared in 68% yield from GaMes and N,N -diphenylbenzamidine in toluene. Various samples of this and related complexes could be heated to 600 °C in N2 to give GaN. A series of homoleptic metal amidinates of the general type [MIRCfNROilnl (R = Me, Bu R = Pr, BuO has been prepared for the transition metals Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La. The types of products are summarized in Scheme 226. The new compounds were found to have properties well-suited for use as precursors for atomic layer deposition (ALD) of thin films. 

For catalytic application where a transition metal catalyst is dissolved in the ionic liquid or the ionic liquid itself acts as the catalyst two additional aspects are of interest. Firstly, the special solubility properties of the ionic liquid enables a biphasic reaction mode in many cases. Exploitation of the miscibility gap between the ionic catalyst phase and the products allows, in this case, the catalyst to be isolated effectively from the product and reused many times. Secondly, the non-volatile nature of ionic liquids enables a more effective product isolation by distillation. Again, the possibility arises to reuse the isolated ionic catalyst phase. In both cases, the total reactivity of the applied catalysts is increased and catalyst consumption relative to the generated product is reduced. For example, all these advantages have been convincingly demonstrated for the transition metal catalysed hydroformylation [17]. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

The r/ -acyl derivatives of transition metals are usually compounds with covalent properties and moderate to good solubilities in common organic solvents, and sometimes with sufficiently good volatility that they can be isolated and/or purified by recrystallization or by sublimation under reduced pressure. Sometimes care has to be taken since the acyl or aroyl complexes may be or may become thermodynamically unstable with respect to the corresponding alkyl or aryl compounds. Heating to elevated temperatures can trigger the reverse of reaction (b) to become a kinetically important path, when the compounds have been prepared by route (a) or when the carbonyl insertion (b) has been carried out at low temperature. 

Although the transition metal ehaleogenides usually are quite refractory, direct reaction is feasible in many cases. In some cases (e.g., W and Mo), the oxide is volatile, making the surface at least accessible to reaction. In addition, the metals often have high rates of diffusion in the compounds, thus reducing the surface passivation effect of compound formation. This is probably because diffusion jumps are more probable in the presence of elements that can change their charge states. This property can be helpful in conversion of an oxide to a sulfide via H2S or CS2. 

Most widely used are N,N -dialkyhrnidazohum salts, since they are easily prepared. Ionic liquids have been used as solvents for numerous reactions. Their physical and chemical properties vary with the combination of cation and anion. This allows a degree of tuning of their properties. Since they are highly polar solvents, ionic liquids can dissolve many inorganic salts and transition metal complexes, and often form biphasic mixtures with non-polar organic solvents. Thus, organic products can be extracted from ionic liquids, while ionic transition metal catalysts are immobilized. Volatile products can be easily distilled off from ionic liquids, since the latter show no volatility [17]. 

Evaluating the experimental results, the high activity of metal (oxy)chloride based catalysts in the oxidation of soot is induced by a high mobility or volatility of the metal chlorides. Chlorine modification of transition metal oxides might also induce beneficial oxygen activation properties and/or transfer of activated ojgfgen to the soot surface. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal catalyzed reactions. Besides the engineering advantage of their exhemely low volatility, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations allows a systematic optimization of the... 

This systematic classification of recycling methods can be related directly to the solubility properties of the organometallic catalysts. The majority of these are poorly soluble in CO2, which therefore acts as an anti-solvent for solutions containing such species. However, either substrates or products may act as entrainers which serve to enhance the CO2 solubility, so in some cases additional catalyst modification may be necessary to render them sufficiently C02-phobic for efficient separation. In the other two approaches, the catalysts need to be C02-philic to ensure sufficient solubility in the C02-based media under the reaction conditions. This behavior is exhibited by certain volatile and nonpolar complexes such as transition metal carbonyl complexes, and also by metal complexes containing suitably modified ligands (e.g., containing perfluoroalkyl groups). 

Many reactions are known in which a volatile ligand is removed from the coordination sphere and replaced by an anion. The volatile ligands involved in these anation reactions have included water, ammonia, pyridine, ethylenediamine, 2,2 -bipyridine and dimethylformamide. Any complex cation containing one or more volatile ligands might be expected to undergo a solid-state anation reaction. The recent book by Wendlandt and Smith on the thermal properties of transition-metal complexes contains numerous examples. ... 

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