One-Electron Oxidation of Hydrogen Sulfide by a Stable Oxidant: Hexachloroiridate(IV)

Detailed reports on the oxidation of aqueous H2S by mild one-electron oxidants are lacking, presumably because of the susceptibility of these reactions to trace metal-ion catalysis and the formation of turbid sulfur sols. Here we report on the reaction of [IrCl6](2-) with H2S in acetate buffers. Dipicolinic acid (dipic) is shown to be effective in suppressing metal-ion catalysis. In the presence of dipic the reaction produces [IrCl6](3-) and polysulfides; turbidity develops primarily after the Ir-IV oxidant is consumed. Water-soluble phosphines are shown to prevent the development of turbidity; in the case of tris-hydroxymethylphosphine (THMP) the product is the corresponding sulfide, THMP=S. THMP diminishes the rates of reduction of Ir-IV, and the rate law with sufficient THMP is first order in [Ir-IV] and first order in [HS-]. The rate-limiting step is inferred to be electron transfer from HS- to Ir-IV with kappa(et) = 2.9 x 10(4) M-1 s(-1) at 25.0 degrees C and mu = 0.1 M. The kinetic inhibition by THMP is attributed to its interception of a polysulfide chain elongation process.