Photooxidation of a Pt(II) Thiolato Complex without Alpha Hydrogens Adjacent to the Thiolato Moiety: A Mechanistic Study

Singlet oxygen is very electrophilic and undergoes several organic reactions. Singlet oxygen undergoes reactions with biomolecules, including sulfur containing methionine and cysteine. There has been considerable interest in the scope and mechanism of the oxidation of metal bound sulfur. We are presenting a mechanistic study of the photooxidation of a Pt(II) monothiolato complex without any alpha hydrogens adjacent to the thiolato moiety. During the photooxidation of Pt (bipyridine)(benzene dithiolate), Pt(bpy)(bdt) by Gray and Connick, no sulfenate product was formed. In contrast, the Selke laboratory characterized the oxidation products and explored the mechanism of the reaction of a Pt monothiolate system, Pt(II) )(bipyridine) (aminoethanethiol)Cl, [Pt(II)(bpy)(aet)Cl]. This reaction led to sequential formation of sulfenate followed by a slower reaction to form sulfinite. A persulfoxide was the primary intermediate in this reaction. The persulfoxide may behave differently in aprotic and protic solvents. In addition, it was suggested that a secondary intermediate may also be involved in the formation of sulfenate. It is well established that in the photooxidation of organic sulfides, the persulfoxide is the primary intermediate, followed by the formation of the hydroperoxy sulfonium ylide as the secondary intermediate. The formation of such a hydroperoxy sulfonium ylide during the intermolecular oxidation of a Pt complex was investigated in this work. We synthesized the complex Pt(II)(bipyridine)(dimethyl aminoethanethiol), Pt(II)(bpy)(dmaet) because this complex does not have any alpha hydrogen, thereby making the formation of a hydroperoxy sulfonium ylide impossible. The complex Pt(II)(bipyridine)(dimethyl aminoethanethiol), Pt(II)(bpy)(dmaet) was synthesized from Pt(II)(bipyridine)(dichloride) and 1-amino- 2- methyl 2-propanethiol and characterized by 1H NMR, UV Vis and mass spectroscopy. Photooxidation of the complex was carried out in ultra pure water and in anhydrous acetonitrile. Product studies involving mass spectroscopy showed the formation of sulfenate and sulfinite in water but only the formation of sulfinite in anhydrous acetonitrile. The formation of a sulfenate in water in the absence of an alpha Hydrogen in the complex leads to the conclusion that a proton is abstracted from water to form a hydroperoxy thiolate which then transfers an oxygen atom. It also implies that formation of an ylide is not required for sulfenate formation. In contrast, photooxidation in anhydrous acetonitrile leads to the formation of only sulfinite, indicating that without a proton from the solvent or the complex, intramolecular oxygen atom transfer and sulfenate formation is not possible for these Pt- thiolato complexes