Decomposition of Metal Chelates

In using the procedures described in the following sections it must be emphasized that small changes in reaction conditions i.e. pH, OH/Fe, Fe concentration, etc. may prevent the desired product from being obtained, hence absolute accuracy in following the preparative procedures is essential. 

Goethite may be synthesized from either Fe or Fe systems. In the Fe system goethite can form over a wide pH range. Methods of synthesis in both acidic and alkaline media are provided in the following section. In alkaline media, synthesis involves holding ft-eshly prepared ferrihydrite (obtained by neutralizing a Fe salt solution with alkali) in KOH at pH 12 for several days. This method was first described by Bohm in 1925. 

Tbp right. Polydomainic star-like twin produced in 0.3 M KOH at 70 °C (Courtesy P. Weidler). 

Bottom left. Rafts consisting of short needles lying with their needle (6-) axis either parallel (a) or perpendicular ( ihombic cross section) (b) to the plane of the paper. They were produced at RT from a partially neutralized Fe(N03)3 solution at an initial pH of 1.7 (Courtesy S. Glasauer) (Method 5.2.2). 

Bottom right. Subrounded crystals produced from 2-line ferrihydrite in the presence of cysteine (Method 5.2.3 for particle size distribution see Fig. 5-6). 

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Decomposition of Metal Chelates. The basic procedure involves aging a solution of Fe salt in alkaline media (pH 12) in the presence of triethanolamine (TEA) at 250°C for 1 hour (Sapiesko and Matijevic, 1980). 

Sapieszko, R. S. and Matijevic, E. (1980) Preparation of well defined colloidal particles by thermal decomposition of metal chelates. I Iron oxides. J. Coll. Interface Sci. 74 405-422. 

It is believed [1135,1136] that the decomposition of metal complexes of salicyaldoxime and related ligands is not initiated by scission of the coordination bond M—L, but by cleavage of another bond (L—L) in the chelate ring which has been weakened on M—L bond formation. Decomposition temperatures and values of E, measured by several non-isothermal methods were obtained for the compounds M(L—L)2 where M = Cu(II), Ni(II) or Co(II) and (L—L) = salicylaldoxime. There was parallel behaviour between the thermal stability of the solid and of the complex in solution, i.e. Co < Ni < Cu. A similar parallel did not occur when (L—L) = 2-indolecarboxylic acid, and reasons for the difference are discussed... 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

The reaction of metal diketonates with NO (x = 1, 2) has been reported . NO2 reacts with substituted M(acac) derivatives of Fe(III), Mn(II) and Cu(II) affording the iminoxy radical 111. The radicals presumably are formed after decomposition of the chelate complex, in view of the fact that the EPR parameters of the free radical are not influenced by the metal ion and that the same radical is also obtained by reaction of the corresponding free ligand and N02 . 

Increasing the reaction temperature not only increases conversions to monoadduct but also decreases catalyst lifetime and isomerizes the product. Under favorable conditions, more than 1300 grams product/ gram BzNa were obtained. Aging the catalyst, iso-HMTT-BzNa, at 110°C for four hours results in an inactive catalyst. Presumably the catalyst slowly decomposes via metalation and decomposition of the chelating agent as was shown for the chelated alkyllithium catalysts. That the decrease in alpha/intemal unsaturation with increasing tern-... 

The development of reaction GC methods for analysing metals has led to drastic changes in metal analysis because they are simpler and more sensitive for some substances than the traditionally used spectral methods (Table 8.2). Chelates of metals feature a number of advantages over other derivatives (1) quantitative formation of chelates is usually simple and does not require highly skilled personnel (2) most metal chelates are stable and can be vaporized without decomposition (3) metal chelates are readily soluble in organic solvents (4) chelates are easily sensed by electron-capture and other ionization detectors and (5) most chelates can be separated by GC. 

The chelates showed a similar thermal behavior in air atmosphere as summarized in Table VI. The sample were run under a purge of stream of air in order to evaluate the inorganic residue, metal oxide, which is obtained as a result of complete decomposition of the chelates at 7S0 C. 

A third school, led by Wiesener, proposed that the Co or Fe ions of the adsorbed N4 chelates promote the decomposition of the chelate upon thermal treatment followed by the formation, at high temperature, of a special form of carbon that would be the true catalyst. In this scenario the metal is only an intermediate and has no active role in the electroreduction of oxygen. In a later publication, they concluded that nitrogen is involved in the electrocatalytic active group on carbon. 

Nanao and Eguchi [228] describe a general scheme whereby metal alkoxides and metal chelates are combined to form a gel. Thermal decomposition of the chelates produces composite multilayer films with applications as sensors, conductive or piezoelectric layers, or opto-electronic materials. Adair et al. [226] discuss a novel process based on Liesegang periodic precipitation to produce a layered Si02/Cu composite potentially useful for fabrieation of insulator-conductor multilayers for electronic packaging. 

Metal chelates afford a better initiating system as compared to other redox systems since the reactions can be carried out at low temperatures, thus avoiding wastage reactions due to chain transfer. Homopolymer formation is also minimum in these systems. It was observed by Misra et al. [66,67] that the maximum percentage of grafting occurs at a temperature much below the decomposition temperature of the various metal chelates indicating that the chelate instead of undergoing spontaneous decomposition receives some assistance either from the solvent or monomer or from both for the facile decomposition at lower temperature. The solvent or monomer assisted decomposition can be described as ... 

The rate of peroxide decomposition and the resultant rate of oxidation are markedly increased by the presence of ions of metals such as iron, copper, manganese, and cobalt [13]. This catalytic decomposition is based on a redox mechanism, as in Figure 15.2. Consequently, it is important to control and limit the amounts of metal impurities in raw rubber. The influence of antioxidants against these rubber poisons depends at least partially on a complex formation (chelation) of the damaging ion. In favor of this theory is the fact that simple chelating agents that have no aging-protective activity, like ethylene diamine tetracetic acid (EDTA), act as copper protectors. 

Co (I I) complex formation is the essential part of copper wet analysis. The latter involves several chemical unit operations. In a concrete example, eight such operations were combined - two-phase formation, mixing, chelating reaction, solvent extraction, phase separation, three-phase formation, decomposition of co-existing metal chelates and removal of these chelates and reagents [28]. Accordingly, Co (I I) complex formation serves as a test reaction to perform multiple unit operations on one chip, i.e. as a chemical investigation to validate the Lab-on-a-Chip concept. 

The patent describes the formation of complex metal chelates by treatment of the ketoester simultaneously with an alcohol and a metal to effect trans-esterification and chelate formation by distilling out the by-product ethanol [1], This process was being applied to produce the zinc chelate of 2-tris(bromomethyl)ethyl acetoacetate, and when 80% of the ethanol had been distilled out (and the internal temperature had increased considerably), a violent decomposition occurred [2], This presumably involved interaction of a bromine substituent with excess zinc to form a Grignard-type reagent, and subsequent exothermic reaction of this with one or more of the bromo or ester functions present. 

The reaction, formally speaking a [3 + 2] cycloaddition between the aldehyde and a ketocarbene, resembles the dihydrofuran formation from 57 a or similar a-diazoketones and alkenes (see Sect. 2.3.1). For that reaction type, 2-diazo-l,3-dicarbonyl compounds and ethyl diazopyruvate 56 were found to be suited equally well. This similarity pertains also to the reactivity towards carbonyl functions 1,3-dioxole-4-carboxylates are also obtained by copper chelate catalyzed decomposition of 56 in the presence of aliphatic and aromatic aldehydes as well as enolizable ketones 276). No such products were reported for the catalyzed decomposition of ethyl diazoacetate in the presence of the same ketones 271,272). The reasons for the different reactivity of ethoxycarbonylcarbene and a-ketocarbenes (or the respective metal carbenes) have only been speculated upon so far 276). 

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