Singlet oxygen and the production of sulfur oxygenates of nickel(II) and palladium(II) thiolates

The metal dithiolate [1,5-bis(2-mercaptoethyl)-1,5-diazacyclooctane]nickel(II) (Ni-1), a sterically hindered analogue (Ni-1*), and the palladium analogue Pd-l react with (1) Delta O-2 to yield a variety of stable and isolable metallosulfones (MS(O-2)R) and metallosulfoxides (MS(O)R). Singlet oxygen was generated both photochemically with the sensitizer Rose Bengal and thermally by decomposition of the 1,4-endoperoxide of 1,4-dimethylnaphthalene. Increased rates and oxygenation yields are observed upon excitation of O-2 from its ground state, (3) Sigma, to the excited state, (1) Delta. The reactions are both solvent and concentration dependent, with sulfones generally favored in acetonitrile and sulfoxides favored in methanol. There is also a ligand and metal effect. The proposed mechanistic pathways involving a persulfoxide precursor to single sulfur site O-2 addition (producing metallosulfones) and adjacent sulfur site O-2 addition (producing metallosulfoxides) are consistent with product distribution, comparison to the much studied oxygenation of organic sulfides, and previous isotopic labeling experiments.