Thiuram disulfides oxidative addition

Curing Systems. Polychloroprene can be cured with many combiaations of metallic oxides, organic accelerators, and retarders (114). The G family of polymers, containing residual thiuram disulfide, can be cured with metallic oxides alone, although certain properties, for example compression set, can be enhanced by addition of an organic accelerator. The W, T, and xanthate modified families require addition of an organic accelerator, often ia combination with a cure retarder, for practical cures. 

The MofVJ compounds [Mo(R2cffc)4]X can be prepared by oxidative addition of thiuram disulfide to molybdenum carbonyls or by mild oxidation of Mo(R2C fc)4. 

The very interesting [Cr(L)2 Q2 ] Cl complexes have been described (138). (L = tetraalkyl thiuram monosulfide or disulfide). The infrared and electronic spectra demonstrate convincingly that these molecules cannot be described as oxidative addition products. The thiuram ligands are placed between R2Dtc and RXant in the spectrochemical series. The electrochemistry of these complexes and, particularly, their reduction characteristics should be examined. 

The ion (113) is an oxidant in several reactions.128 In addition to the normal thiuram disulfides, isothiuram disulfide (116) also exists.129 However, no complexes are known in either a neutral or (for R = H) a deprotonated form. Numerous other ring systems exist in which a CS2 fragment is incorporated, e.g. rhodanines and thiazoles.1 They can act as neutral ligands in coordination compounds. 

Several researchers have concluded that both radical and polar mechanisms are operative and that the precise nature is dependent on the formulation. A typical accelerated sulfur system contains sulfur, accelerator, zinc oxide, and a saturated fatty acid such as stearic or lauric acid. Other additives can include stabilizers, antioxidants, and fillers such as silica or carbon black. There are also sulfur-free systems the most widely used sulfurless system is zinc oxide and tetramethyl thiuram disulfide. Additionally, other accelerators can be added to thiocarbamate systems to increase the induction time. The most widely used compound for this application is MET. Accelerated sulfur vulcanization has been found to consume the accelerator in the system at a rate far greater than the rate of cross-linking. This has led to the proposal that accelerated vulcanization proceeds through an intermediate. 

The discoloration was found to be caused by trace amounts of hydrochloric acid which form when Neoprene oxidizes on aging. This acid reacts with iron to form ferric chloride, which in turn reacts with the thiuram disulfide modifier to form a black iron sulfide compound. Acid acceptors such as magnesia and zinc oxide were found to be effective in preventing this discoloration. However, their addition tended to decrease the viscosity stability of adhesive cements even further. 

The transformation may occur via initial coordination of the thiuram disulfide complex followed by later addition of the sulfur-sulfur bond to the metal center. A limitation would appear to be the ability of the metal center to undergo a two-electron oxidation. However, Stiefel and co-workers (184, 185) reported unusual examples, whereby addition of thiuram disulfides (R = Me, Et, i-Bu) to the rhenium(VII) complex, [Et4N][ReS4], results in the reduction of the metal center. Thus, addition of 1.5 equiv of thiuram disulfide affords dimeric rhenium(lV) complexes, [Re(p-S)(S2CNR2)2]2, while with 3 equiv rhenium(V)... 

Casey and Vecchio (186,187) developed the oxidative-addition of thiuram disulfides to metals in order to produce a one-step preparation of a wide range of transition metal dithiocarbamate complexes. In this way, [Ni(S2CNMe2)2] and [Zn(S2CNMe2)2] can be prepared in near quantitative yields. With copper, only the copper(II) species are seen, and this is in keeping with Akerstrom s (188) earlier observation that copper(I) species react instantaneously with thiuram disulfides to generate the analogous copper(II) complexes. 

The iridium(IV) cations are strongly oxidizing, addition of a further equivalent of dithiocarbamate salt leading to the formation of thiuram disulfide... 

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