Thiols metal complexes

Benzoquinone, 3,5-di-t-butyl-cobalt complexes, 398 Bcnzoselenophene, 2-(2-pyridyl)-metal complexes, 807 Benzothiazole-2-thiol metal complexes, 802 Benzotriazoles metal complexes, 78 P a, 77... 

The activation of persulfates by various reductant viz. metals, oxidizable metals, metal complexes, salts of various oxyacid of sulfur, hydroxylamine, hydrazine, thiol, polyhydric phenols, etc. has been reported [36-38]. Bertlett and Colman [39] investigated the effect of methanol on the decomposition of persulfates and proposed the following mechanism. 

Recent reports on transition metal complexes of 2-heterocyclic thiosemicar-bazones suggest that stereochemistries adopted by these complexes often depend upon the anion of the metal salt used and the nature of the N-substituents. Further, as indicated previously, the charge on the ligand is dictated by the thione-thiol equilibrium which in turn is influenced by the solvent and pH of the preparative medium. Many of the reported complexes have been prepared in mixed aqueous solvents, often with bases added. However, there are few reports in which workers have varied the nature of their preparations to fully explore the potential diversity of these ligands. 

The authors confirmed the formation of vinyl Ru-complex 21 by the reaction of [Cp Ru(SBu-t)]2 with methyl propiolate (Eq. 7.15). To my knowledge, this is the first observation of the insertion of an alkyne into the M-S bond within a catalytically active metal complex. In 2000, Gabriele et al. reported the Pd-catalyzed cycloisomerization of (Z)-2-en-4-yne-l-thiol affording a thiophene derivative 22 (Eq. 7.16) [26]. 

As shown in Figure 4.1, the initial step of the conversion of an ylide into a carbene complex is an electrophilic attack at the ylide. Reactions of this type will, therefore, occur more readily with increasing nucleophilicity of the ylide and increasing electrophilicity of the metal complex L M. Complexes which efficiently catalyze the decomposition of even weakly nucleophilic ylides can easily react with other nucleophiles also, such as amines or thiols. This has to be taken into account if reactions with substrates containing such strongly nucleophilic functional groups are to be performed. 

Abstract This review describes recent results in the field of poly(aryleneethynylene)s (PAEs) that contain metal ions in the polymer backbone, or in the polymer side chain. This work is focused primarily on polymers possessing ligands of metal complexes as part of the aryle-neethynylene chain. PAEs with porphyrinylene in the backbone have also been addressed. Synthetic routes toward the polymers, as well as their photochemical, photophysical, and electrochemical properties, are presented. Monodisperse oligo(phenyleneethynylene)s with terminal metal complexes or with a ferrocene and thiol at each end are mentioned. 

Although the oxidation of thiols to disulfides in the presence of a catalyst is a reaction of commercial interest, it is only comparatively recently that the marked effects of impurities on the system has been realized. Wallace and co-workers (13, 14) have studied the metal-catalyzed oxidation of some thiols in the presence of a few metal ions and complexes under comparable conditions, and they have suggested a general mechanism for the reaction, based on Reactions 1, 4, 5, 6, and 7. The rate of reaction was found to depend on the chemical nature and the physical state of the catalyst. The reaction was suggested to involve metal complexes in the solid state (13). 

In addition to the above-mentioned reactions, metal complexes catalyze decarboxylation of keto acids, hydrolysis of esters of amino acids, hydrolysis of peptides, hydrolysis of Schiff bases, formation of porphyrins, oxidation of thiols, and so on. However, polymer-metal complexes have not yet been applied to these reactions. 

Breccia, P., Cacciapaglia, R.. Mandolini. L. and Scorsini, C. (1998) Alkaline-earth metal complexes of thiol pendant crown ethers as turnover catalysts of ester cleavage. J. Chem. Soc., Perkin Trans., 2, 1257. 

The chapter by C. J. Swan and D. L. Trimm, which also emphasizes the effect on catalytic activity of the precise form of a metal complex, shows too that, depending on the metal with which it is associated, the same ligand can act either as a catalyst or inhibitor. The model reaction studied was the liquid-phase oxidation of ethanethiol in alkaline solution, catalyzed by various metal complexes. The rate-determining step appears to be the transfer of electrons from the thiyl anion to the metal cation, and it is shown that some kind of coordination between the metal and the thiol must occur as a prerequisite to the electron transfer reaction (8, 9). In systems where thiyl entities replace the original ligands, quantitative yields of disulfide are obtained. Where no such displacement occurs, however, the oxidation rates vary widely for different metal complexes, and the reaction results in the production not only of disulfide but also of overoxidation and hydrolysis products of the disulfide. 

The effects of adding various metal ions and metal complexes on the rate of a model oxidation reaction have been studied in some detail The model reaction chosen—the oxidation of ethanethiol in aqueous alkaline solution in the presence of metal-containing catalysts—involves the transfer of an electron from the thiol anion to the metal The catalytic activity of additives depends upon the solubility of the particular metal complex and varies according to the nature of the ligand attached to the metal ion. In conjunction with different metals, the same ligand can act either as a catalyst or as an inhibitor. The results are discussed in the light of proposed reaction mechanisms. 

In general, the electron transfer reaction (Reaction 2) controls the over-all rate of reaction, and the nature of the catalyst has a profound effect on the kinetics of oxidation (23). Thus Wallace et al. (23) have compared the catalytic efficiencies of metal phthalocyanines with metal pyrophosphates, phosphates, phosphomolybdates, and phosphotungstates. The activity of metal pyrophosphates was ascribed to the ease of electron transfer through the metal coordination shell, the reaction being suggested to occur at the solid pyrophosphate-liquid interface. On the other hand, the catalytic effectiveness of a series of metals, added to solution as simple salts, has been explained in terms of their ability to form soluble complexes containing thiols (13). It was not clear whether the high rates of oxidation were caused by the solubility of metal complexes or by the peculiar nature of the thiol complexes. 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

In the light of the above discussion, it is necessary to redefine the criteria useful for describing catalytic activity. The coordination atmosphere of any given metal may be expected to affect the catalytic activity by influencing the solubility of the metal. If the metal complex, added to the reactant solution, can be replaced by thiyl entities, colored metal-thiol complexes may be produced, and the rate of reaction in all cases should correspond to Case 3 for adding simple metal salts (Table II). If the metal complex cannot be replaced, the rate of reaction may be quite different and will depend on the ease with which an electron... 

The catalytic action of a metal complex thus depends on the stability and solubility of the complex present in the reactant solutions. Provided that the coordination atmosphere of a metal ion is not displaced by thiol entities, the nature of the ligand affects the rate of electron transfer and the kinetics of the over-all oxidation. 

Systematic study of the stability of metal complexes in nonaqueous systems and their reactivity with thiols, free radicals, etc., is required for a fuller understanding of their complex effects on the co-oxidation reaction. 

Pioneering work in this area was aimed at using specific metal ligand interactions to induce and stabilize secondary structures. This has been achieved by Ghadiri et al. for a-helical structures through the formation of transition metal and Ru(II) inert complexes with two imidazoles of His or one thiol of Cys and one imidazole of His in i, i + 3 or i, i + 4 relationships.1[37,38 In almost all cases the helix content and stability increased upon metal complexation, especially with i, i + 4 peptides. This work resembles the stabilization of helical structures using metal complexation by EDTA-like side chains discussed in Section 9.4.6. 

Characteristic of the heavy metal complexes is their carbon disulfide elimination, yielding mixed thiol-thioxanthato complexes with trimeric structures and thiol bridges.139 The facile nucleophilic substitution of RS by R2N is interesting.113114... 

---