Kinetic metal chelates

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

Kinetics and mechanism of metal chelation processes via solvent extraction techniques. H. Freiser, Acc. Chem. Res., 1984,17,126-131 (40). 

The regioselectivity of Michael additions of thiolates to 2,4-dienones can be altered drastically by variation of the reaction conditions and addition of Lewis acids to the reaction mixture. Lawton and coworkers examined the reaction of 2-mercaptoethanol with l-(3-nitrophenyl)-2,4-pentadien-l-one and observed a high regioselectivity in favor of the 1,6-addition product at 45 °C (equation 42)123,124. Lowering of the reaction temperature caused an increase in the amount of 1,4-adduct, and at —40°C, a product ratio of 40 60 was found. These events suggest that kinetic control favors the 1,4-addition product whereas the 1,6-adduct is thermodynamically more stable. If, however, the reaction was carried out with a complex of the dienone and titanium tetrachloride, only the 1,4-adduct was isolated after hydrolytic workup123. Obviously, this product is trapped as a metal chelate which prevents formation of the 1,6-adduct by retro-Michael/Michael addition. In the absence of the chelating Lewis acid, the 1,4-addition product can indeed be converted... 

R3 R2 and R2 Ri gauche interactions however, for the same set of substituents, an increase in the steric requirements of either Rj or R3 will influence only one set of vicinal steric interactions (Rj R2 or R3 R2). Some support for these conclusions has been cited (eqs. [6] and [7]). These qualitative arguments may also be relevant to the observed populations of hydrogen- and nonhydrogen-bonded populations of the aldol adducts as well (see Table 1, entries K, L). Unfortunately, little detailed information exists on the solution geometries of these metal chelates. Furthermore, in many studies it is impossible to ascertain whether the aldol condensations between metal enolates and aldehydes were carried out under kinetic or thermodynamic conditions. Consequently, the importance of metal structure and enolate geometry in the definition of product stereochemistry remains ill defined. This is particularly true in the numerous studies reported on the Reformatsky reaction (20) and related variants (21). 

Examples for and have been observed under certain experimental conditions for reactive and/or strained chiral oxiranes which were separated by complexation gas chromatography (Figure 21)133. The first eluted peak was diminished in the separation of racemic 2-methyl-3-phenylo.xirane. In this case two enantioselective processes are mediated by the chiral metal chelate, i.e., chromatographic resolution and kinetic resolution (in favor of the first eluted enantiomer). Since two enantioselective processes are involved, the elution profile will be the same svhen the chirality of the metal chelate is inverted. 

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). 

Polyphenols as Metal Chelators Thermodynamics Polyphenols as Metal Chelators Kinetics... 

In the macrocydic structure the metal binding site within the ligand is more encapsulated and the entropy is decreased upon metal incorporation. As a result the stability of the majority of macrocydic metal chelates is higher than that of acyclic complexes (Table 3) [27,28]. Generally the macrocydic complexes exhibit a higher kinetic stability. 

Raspor, B., Nurnberg, H.W., Valenta, P. and Branica, M., 1980. Kinetics and mechanism of trace metal chelation in seawater. 3. Electroanal. Chem., 115 293-308. 

The technical and economic aspects of wet flue gas simultaneous desulfurization and denitrification systems are presented so that their practicality for utilization by utility industry can be assessed. The emphasis is on the kinetics of the systems based on the employment of ferrous chelates to promote the solubility of NO and the reactivity of NO with SO2 in scrubbing liquors. Analytical techniques are developed for characterizing reaction intermediates and products. Alternative approaches and novel ideas that could develop into a more efficient and cost-effective scrubber system employing metal chelate additives are discussed. 

The o-character of flavin-metal contacts is more easily accepted a priori, since kinetic stability of metal-N,0-ligation need not be apprehended. Metal chelates (bidentate or polydentate structures) may, however, exhibit enough thermodynamic advantages to preclude dissociation and thereby, electron transfer. Flavin, however, is established being a... 

The peak shapes of metal chelating analytes are often poor because metal impurities in the stationary phase behave as active sites characterized by slowo desorption kinetics and higher interaction energies compared to reversed phase ligand sites. This phaiomaion is typical of silica-based stationary phases [31] ultrapure silicas were made commercially available to reduce it. However, styrene-divinylbenzene-based chromatogripliic packings suffer from the same problem and it was hypothesized that metals may be present in the matrix at trace conditions because they were used as additives in the polymerization process they may have been c tured via Lewis acid-base interactions between the aromatic ring n electrons and impurities in the mobile phase [32]. 

This paper calls attention to the need for new ions in coordination chemistry—ions that would permit more detailed physico-chemical studies to be made, ions that would facilitate studies of less familiar metals and of less familiar coordination numbers, and ions that would help studies of chemical bonding and reaction mechanisms. Organometallic ions of the type RmM+ are such ions, and these form metal-chelate compounds of the type RmM Ch) . Three aspects of the chemistry of organometallic-chelate compounds are described (1) equilibria of compound formation ( ) kinetic and mechanistic studies of three types of reactions (a) reactions of the coordinated ligand, (b) substitution at the 4-, 5-, or 6-coordinate metal atom, and (c) reactions of the organic moiety and (3) studies of stereochemistry and chemical bonding. 

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