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

Abstract

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 ket = 2.9 × 10(4) M(-1) s(-1) at 25.0 °C and μ = 0.1 M. The kinetic inhibition by THMP is attributed to its interception of a polysulfide chain elongation process.

NO/H2S “Crosstalk” Reactions. The Role of Thionitrites (SNO–) and Perthionitrites (SSNO–)

“…A mechanistic proposal on the reaction of the inert one-electron oxidant [IrCl 6 ] 2− with H 2 S provides solid evidence on the participation of sulfide radicals for the production of [IrCl 6 ] 3− and polysulfides. 68 Again, the onset of polysulfides was found to begin after the (nearly) complete consumption of the oxidant.…”

Catalytic System for Aerobic Oxidation That Simultaneously Functions as Its Own Redox Buffer

“…Our group and those of Matveev, Kozhevnikov, ,, and Neumann − reviewed organic substrate oxidations, including ones based on O 2 as the terminal oxidant, catalyzed by POMs, including PV n Mo 12– n O 40 /Pd systems, while Misono and Mizuno − reviewed industrial POM-catalyzed oxidations some time ago. Thiol oxidations are also important in organic chemistry, physiological processes, and environmental science. − Numerous catalytic and stoichiometric systems are known to selectively oxidize thiols in context with either deodorization or synthesis, including nanoparticle systems, , POMs, ,,− metal–organic frameworks (MOFs), ,, strong stoichiometric oxidants, − and noble metals. − Most of these systems do not use O 2 as the terminal oxidant. In addition, most are slow, require elevated temperatures, and form side products.…”

“… 25 − 31 Numerous catalytic and stoichiometric systems are known to selectively oxidize thiols in context with either deodorization or synthesis, including nanoparticle systems, 32 , 33 POMs, 26 , 27 , 34 − 39 metal–organic frameworks (MOFs), 25 , 40 , 41 strong stoichiometric oxidants, 42 − 50 and noble metals. 51 − 55 Most of these systems do not use O 2 as the terminal oxidant. In addition, most are slow, require elevated temperatures, and form side products.…”

Inorganic Polysulfides in Solution: Structural Properties and Conformational Isomerism

“…A comprehensive mechanistic study describes how the mild oxidant [IrCl 6 ] 2− can oxidize H 2 S/HS − through an initial 1e − rate-determining step, leading to the formation of [IrCl 6 ] 3− and HS 2 − . 47 The latter product can lead to diverse [HS n ] − species of increasing chain-length, which are rapidly generated until [IrCl 6 ] 2− is consumed. Subsequently, a slow conversion of polysulfides to a sulfur colloid occurs.…”