REDOX PROPERTIES OF DIARYL CHALCOGENIDES AND THEIR OXIDES

The redox properties of diaryl chalcogenides and their corresponding o xides were studied by means of pulse radiolysis. Diaryl sulfides, sele nides, 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 rad ical cations appeared to form three-electron-bonded dimers with their corresponding chalcogenides. The radical cations of diaryl chalcogenid es were also formed by one-electron reduction of their respective oxid es. Among one-electron reductants tested, only the solvated electron w as able to rapidly (k = (0.9-2) x 10(10) M-1 s-1) reduce diphenyl sulf oxide and diphenyl selenoxide between pH 3 and 13. Diphenyl telluroxid e is present predominantly as a hydrate, (C6H5)2Te(OH)2, Which undergo es protonation/dehydration below pH 5.3 to yield (C6H5)2TeOH+. Both of these species react rapidly with the solvated electron to yield the r adical cation, but only (C6H5)2TeOH+ reacts with CO2 radical anion wit h a measurably fast rate (k = 6 x 10(9) M-1 s-1). Upon one-electron ox idation, bis(4-hydroxymethyl) sulfide (pH > 0.5) and bis(4-hydroxyphen yl) telluride (pH > 2.5) were found to readily deprotonate to form phe noxyl radicals. Below pH 2.5, it was also possible to observe the radi cal cation spectrum of the organotellurium compound. One-electron redu ction potentials of a variety of diaryl chalcogenide radical cations i n water were obtained by bringing the chalcogenides to react with redo x standards and by observing their redox equilibria by pulse radiolysi s. The following E-degrees values versus NHE were determined: E-degree s((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-C6H 4)2Te) = 0.80 V; 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 strengt hs in the OH adducts to diaryl chalcogenides were found to increase as one traverses the chalcogens from sulfur to tellurium. This is in con trast to the trend for the corresponding chalcogen-oxygen double bond strengths. A dissociation enthalpy of 84 kcal/mol was estimated for th e Te=O bond in diaryl telluroxides. Calculated one-electron reduction potentials for diphenyl sulfoxide and dimethyl sulfoxide did not provi de a thermodynamic rationale for the low reactivity of dialkyl sulfoxi des toward the hydrated electron. Finally, the (C6H5)2S.+ radical cati on was produced by reduction of (C6H5)2SO in a 50/50 v/v water/tert-bu tyl alcohol mixture. We thus propose (C6H5)2S.+ as a useful one-electr on oxidant in mixed solvents.