Reactions of Coordinated Sulfur Ligands

Metal compounds have found some application as desulfurization catalysts, and a novel reaction of N-phenylthiourea has been reported. The reaction of PhNCSNH2 with ethanolic Cu(OAc)2 in the presence of 2,2 -bipyridine results in the formation of the phenylcarbodiimido complex [(bipy)Cu(/i-PhN=C=N)2Cu(bipy)]. The same group have also described a related partial desulfurization of 3-methylimidazoline-2(l if)-thione to yield di( 1-methyl-imidazolyl)sulfide (41) upon reaction with copper(II) salts. These reactions are mechanistically obscure. 

A more common reaction involves oxidation of a coordinated thiolate to disulfide or an S—O compound. The former reaction may occur with or without the involvement of an oxygen transfer reagent (or dioxygen) while the latter requires such a species. The oxidation of the thiolate complex (42) by dioxygen shown in Eq. (9) is first order in (42) and dioxygen, and it is proposed that the sulfenate (43) arises by attack of a noncoordinated thiolate on coordinated dioxygen.  

The aerobic oxidation of cysteine, HSCH2CH(NH2)C02H, to H02C(NH2)CHCH2SSCH2CH(NH2)C02H is catalyzed by metal ions. The reaction proceeds in two stages, Eqs. (10) and (11). The first reaction is catalyzed by 

The anaerobic oxidation of a range of dithiocarbamates and xanthates by inert metal complexes has been shown to proceed by an outer-sphere process, in which the key step involves the formation of a sulfur-centered radical. The standard electrode potential for the Et2NCS2/Et2NCSJ couple is estimated to be 0.425 0.33 V vs. SCE. 

An interesting displacement of thiolate in nickel(II) and palladium(ll) complexes by secondary amines, Eq. (13), has been investigated. The reaction is 

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