Oxygen versus sulfur: Structure and reactivity of substituted arsine oxides and arsine sulfides

Although arsenic in its inorganic forms is a well know toxic agent, biotransformations in the environment and in the human body can produce organoarsenic compounds that are generally of much lower toxicity. Foremost among these products is a range of dimethylated arsine oxides and their analogous sulfides, which are crucial to the arsenic detoxification process. We have investigated the formation and interconversion of substituted and unsubstituted arsenicals (R²₂As(=Z)R¹, R² = CH₃, R¹ = CH₂CH₂OH, CH₂COOH; Z = S or O) with density functional theory (DFT)/B3LYP. Formation of isomers including a cyclic hydrogen bonded conformer is observed for the ethanol and acetate derivatives. Furthermore, investigating the reaction of arsine oxide with hydrogen sulfide revealed the formation of arsine sulfide via pentacoordinated trigonal bipyramidal intermediates. A tetragonal pyramidal transition state was located enabling exchange of equatorial and axial positions in the trigonal bipyramidal species. The reaction was proven exothermic for all studied substituents (ΔE(rxn) -50 to -80 kJ/mol). This fundamental study shows that H₂S easily leads to the formation of thio-organoarsenicals. Conversion of arsine sulfides into their corresponding arsine oxides is experimentally accomplished with hydrogen peroxide, which could also be rationalized by means of ab initio calculations showing high exothermicity (ΔE(rxn) ca. -550 kJ/mol). Reactions are considered at different levels of theory (i.e., DFT, second and fourth order Møller-Plesset (MP) perturbation theory) including two solvation models for DFT, which show good agreement for resulting geometries and reaction energies. Hence, the widely used B3LYP/6-31G** combination is a suitable method for the description of molecular organoarsenicals.