REDOX PROPERTIES OF DIARYL CHALCOGENIDES AND THEIR OXIDES

The redox properties of diaryl chalcogenides and their corresponding oxides were studied by means of pulse radiolysis. Diaryl sulfides, selenides, and tellurides were found to be readily (k = 10(9)-10(10) M-1 s-1) oxidized to the corresponding radical cations by a variety of one-electron oxidants (Tl2+, OH., Br2 radical anion, N3.). None of the radical cations appeared to form three-electron-bonded dimers with their corresponding chalcogenides. The radical cations of diaryl chalcogenides were also formed by one-electron reduction of their respective oxides. Among one-electron reductants tested, only the solvated electron was able to rapidly (k = (0.9-2) x 10(10) M-1 s-1) reduce diphenyl sulfoxide and diphenyl selenoxide between pH 3 and 13. Diphenyl telluroxide is present predominantly as a hydrate, (C6H5)2Te(OH)2, Which undergoes protonation/dehydration below pH 5.3 to yield (C6H5)2TeOH+. Both of these species react rapidly with the solvated electron to yield the radical cation, but only (C6H5)2TeOH+ reacts with CO2 radical anion with a measurably fast rate (k = 6 x 10(9) M-1 s-1). Upon one-electron oxidation, bis(4-hydroxymethyl) sulfide (pH > 0.5) and bis(4-hydroxyphenyl) telluride (pH > 2.5) were found to readily deprotonate to form phenoxyl radicals. Below pH 2.5, it was also possible to observe the radical cation spectrum of the organotellurium compound. One-electron reduction potentials of a variety of diaryl chalcogenide radical cations in water were obtained by bringing the chalcogenides to react with redox standards and by observing their redox equilibria by pulse radiolysis. The following E-degrees values versus NHE were determined: E-degrees((C6H5)2S.+/(C6H5)2S) = 1.54 V; E-degrees((C6H5)2Se.+/(C6H5)2Se) = 1.37 V; E-degrees((C6H5)2Te.+/(C6H5)2Te) = 1.14 V; E-degrees((4-HO-C6H4)2Te.+/(4-HO-C6H4)2Te) = 0.95 V; E-degrees((4-H2N-C6H4)2Te.+/(4-H2N-C6H4)2Te) = 0.80 V; E-degrees((4--OOCCH2O-C6H4)2S.+/(4--OOCCH2O-C6H4)2S) = 1.21 V. The two-electron redox potentials of the telluroxide/telluride redox couple were determined by means of EMF titration as a function of the pH. A value of 0.65 V was obtained for both the ((4-HO-C6H4)2Te(OH)2,2H+)/((4-HO-C6H4)2Te,2H2O) and the ((4-H2N-C6H4)2Te(OH)2,2H+)/((4-H2N-C6H4)2Te,2H2O) couples. The chalcogen-oxygen single-bond strengths in the OH adducts to diaryl chalcogenides were found to increase as one traverses the chalcogens from sulfur to tellurium. This is in contrast to the trend for the corresponding chalcogen-oxygen double bond strengths. A dissociation enthalpy of 84 kcal/mol was estimated for the Te=O bond in diaryl telluroxides. Calculated one-electron reduction potentials for diphenyl sulfoxide and dimethyl sulfoxide did not provide a thermodynamic rationale for the low reactivity of dialkyl sulfoxides toward the hydrated electron. Finally, the (C6H5)2S.+ radical cation was produced by reduction of (C6H5)2SO in a 50/50 v/v water/tert-butyl alcohol mixture. We thus propose (C6H5)2S.+ as a useful one-electron oxidant in mixed solvents.