Reductive chalcogenation

Electrochemical methods may also be used in the synthesis of chalcogen-nitrogen compounds. For example, the electrochemical reduction of salts of the [SsNs]" cation (Section 5.3.9) in SO2 or CH2CI2 at low temperatures produces microcrystals of the superconducting polymer (SN). ... 

The chalcogen-capped clusters M M2Co(/(3-S)(CO)s()/ -CsH5) (M M2 = MoEc, MoRu, WFe) and MoFeCo(/r ,-Se)(CO)H()) -CsH3) underw ent a one-electron, quasi-reversible reduction. Addition of an electron proceeded more readily for the clusters w ith the lighter metals and for the selenium capped cluster relative to its sulfur analogue. 

Because of their multiple oxidation states, the chalcogens, particularly sulfur, can engage in numerous redox couples participating in acid-base, oxidation-reduction, precipitation, and complexation equilibria. 

The electrochemical preparation of metal chalcogenide compounds has been demonstrated by numerous research groups and reviewed in a number of publications [ 1-3]. For the most part, the methods that have been used comprise (a) cathodic co-reduction of the metal ion and a chalcogen oxoanion in aqueous solution onto an inert substrate (b) cathodic deposition from a solvent containing metal ions and the chalcogen in elemental form (the chalcogens are not soluble in water under normal conditions, so these reactions are carried out in non-aqueous solvents) (c) anodic oxidation of the parent metal in a chalconide-containing aqueous electrolyte. 

The induced co-deposition concept has been successfully exemplified in the formation of metal selenides and tellurides (sulfur has a different behavior) by a chalcogen ion diffusion-limited process, carried out typically in acidic aqueous solutions of oxochalcogenide species containing quadrivalent selenium or tellurium and metal salts with the metal normally in its highest valence state. This is rather the earliest and most studied method for electrodeposition of compound semiconductors [1]. For MX deposition, a simple (4H-2)e reduction process may be considered to describe the overall reaction at the cathode, as for example in... 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

With respect to non-noble and non-Ru catalysts, transition metal chalcogenides with spinel and pyrite structures have been investigated and shown that these can also be active to oxygen reduction processes. The motivation in the present case is that chalcogen addition might enhance the stability and activity toward the ORR... 

The chemical properties of trichalcogen dications are, in many ways, analogous to the properties of dichalcogen dications, but are considerably less studied. Hydrolysis of chalcogenurane dications occurs preferentially at the onium chalcogen atom. Similar to the usual dichalcogen dications, trichalcogen dications also display oxidative properties. For example, reduction of selenu-rane dication 135 (X = Y = Se) is observed in reactions with Sm(II) salts, triphenylphosphine or thiophenol (Scheme 54).135... 

Figure 3.2. Film formation using a dimensional reduction approach involves three steps 1) breaking up the insoluble extended inorganic framework (a) into more soluble-isolated anionic species, which are separated by some small and volatile cationic species (b). 2) Solution-processing thin films of the precursor (b). 3) Heating the precursor films such that the cationic species and corresponding chalcogen anions are dissociated, leaving behind the targeted inorganic semiconductor (c).

The oxidations and reductions of organochalcogen compounds all involve the lone-pairs of electrons associated with the chalcogen atoms. These lone-pairs are stereochemically active, which provides well-defined geometries for many of the intermediate species described herein. Oxidized organochalcogen compounds also are stabilized by lone-pair donation from neighboring heteroatoms, which again leads to unusual structures with well-defined geometries as described herein. 

Several trends emerge in these data (1) The reductive elimination of bromine is 6-13kJmol more facile than reductive elimination of chlorine in similar structures, which is consistent with weaker chalcogen-bromine bonds relative to chalcogen-chlorine bonds.(2) The reductive elimination of chlorine is accelerated by the presence of a chloride counterion as opposed to a less nucleophilic counterion such as hexafluorophosphate. (3) The rate of reductive elimination is accelerated by the presence of a more polar solvent (acetonitrile) relative to tetrachloroethane, which is consistent with development of charge in the rate-determining step. These observations suggest mechanisms for oxidative... 

Once the thiol is introduced to the coordination sphere of the selenoxide or telluroxide, a second slower reaction occurs. This step is associated with reduction of the chalcogen(IV) oxidation state to the chalcogen(II) oxidation state, which was demonstrated with dihydroxy telluranes 52 and 53. In the tellurium(IV) oxidation state of 52 and 53, the 5p orbital of tellurium is involved in the three-center, four-electron bond and cannot interact with the carbon 7r-framework. Long-wavelength absorption maxima for 52 and 53 are found at 510 and 500 nm, respectively in water. Reductive elimination generates a tellurium(II) atom, whose 5p orbital can now... 

The corresponding selenium(IV) derivatives are less stabilized by the three-center, four-electron bonds in the chalcogen(rV) complex than the tellurium(lV) derivatives. As a consequence, the selenium(lV) derivative is more easily reduced than the tellurium(lV) derivative. Diphenylselenium(lV) dibromide (1) undergoes a two-electron reduction with Ep at - - 0.40 V (vs. SCE) while diphenyltellurium(lV) dibromide (2) undergoes a two-electron reduction with at-1-0.05 V (vs. SCE). ... 

Most of the catalysts employed in PEM and direct methanol fuel cells, DMFCs, are based on Pt, as discussed above. However, when used as cathode catalysts in DMFCs, Pt containing catalysts can become poisoned by methanol that crosses over from the anode. Thus, considerable effort has been invested in the search for both methanol resistant membranes and cathode catalysts that are tolerant to methanol. Two classes of catalysts have been shown to exhibit oxygen reduction catalysis and methanol resistance, ruthenium chalcogen based catalysts " " and metal macrocycle complexes, such as porphyrins or phthalocyanines. ... 

The practical significance of the reaction depicted in Scheme 7.43 consists of the development of novel antioxidants carrying chalcogen atoms. Divalent organochalcogen compounds react readily with many types of oxidants (peroxide, peroxyl radicals, peroxynitrite, singlet oxygen, and ozone). The tetravalent organylchalcogenides formed are reduced by many mild reductants. Therefore, compounds of this sort have the potential to act as catalytic antioxidants. 

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