Reaction Mechanisms of One- and Two-Electron Oxidations of Alkanesulfinates in Aqueous Media

Abstract

The one-electron oxidation reactions of L-cysteinesulfinic acid (CSA) and methanesulfinate (MSA) with bis(1,4,7-triazacyclononane)nickel(III) and tris(1,10- phenanthroline)osmium(III) were investigated in aqueous media. Reaction kinetics are observed at 25 °C, μ = 0.1 M (NaCl), under anaerobic as well as aerobic conditions. The reactions are slower under aerobic conditions. Under pseudo-first order reaction conditions both the oxidation reactions with Ni(III) as well as CSA oxidation by Os(III) show first-order dependence on oxidant and alkanesulfinate concentration with mild inhibition by the reduced form of the oxidant. In Os(III) reaction with MSA, strong inhibition by Os(II) is observed requiring the presence of a spin trap to obtain good first-order fits under pseudo-first-order reaction conditions. In all four reactions, the corresponding alkanesulfonyl radical is formed during the first electron transfer step, consequently yielding the corresponding sulfonate as the major sulfur containing product. The empirical rate-law for the reaction of [Ni(tacn)2]3+ with CSA in aqueous media is first-order in the alkanesulfinate and Ni(III), and takes the form -d[Ni(III)]/dt = kOBS[Ni(III)], where kOBS = (Ka1[CSA]TOT) /( (1 (k1Ka2 + k2 [H+]) + ((k’/ Ka2) [[Ni(tacn)3]2+])([H+ ]^2 + [H+ ]Ka1 + Ka1Ka2)) with k1 = (2.70 ± 0.08) × 10 M-1 s -1 , k2 = (9.6 ± 0.7) M-1 s -1 and k’ = (4.7 ± 0.5) × 10 s. For the reaction of [Ni(tacn)2]3+ with MSA, the empirical rate-law is -d[Ni(III)]/dt = kOBS[Ni(III)], where kOBS = k1Ka[MSA]TOT/((1 + k’[[Ni(tacn)3]2+])(Ka+[H+ ])) with k1 = (1.90 ± 0.05) × 10^2 M-1 s -1, and k’ = (2.7 ± 0.2) × 10^3 M-1 . Oxidation of CSA by tris(1,10-phenanthroline)osmium(III) yields the empirical rate law -d[Os(III)]/dt = kOBS[Os(III)] where kOBS = k[CysSO2H]TOT/[Os(II)] in the pH range 3.5-5.5 with k = (7.53 ± 0.07) × 10^-3 s -1. The empirical rate law for MSA oxidation by tris(1,10-phenanthroline)osmium(III) is -d[Os(III)]/dt = kOBS[Os(III)]^2 with kOBS = k[CH3SO2H]/[Os(II)], where k = (2.4 ± 0.4) ×10^3 M-1 s -1 .

Methanesulfonyl iodide is produced in aqueous solutions from the reaction of triiodide with methanesulfinate. Dichroic crystals of (CH3SO2I)4 •KI3 •2I2 are formed from KI/I2 solutions with high concentrations of CH3SO2 – , while dichroic crystals of (CH3SO2I)2 •RbI3 are formed from RbI/I2 solutions. Xray crystallography of these two compounds shows that the CH3SO2I molecules coordinate through their oxygen atoms to the metal cations and that the S–I bond length is 2.44 Å. At low concentrations of CH3SO2 – the solutions remain homogeneous and the sulfonyl iodide is formed in a rapid equilibrium: CH3SO2 – + I3 ↔ CH3SO2I + 2I– , KMSI = 1.07 ± 0.01 M at 25 °C (μ = 0.1 M, NaClO4 ). The sulfonyl iodide solutions display an absorbance maximum at 309 nm with a molar absorptivity of 667 M–1 cm–1. Stopped-flow studies reveal that the equilibrium is established within the dead time of the instrument (≈ 2 ms). Solutions of CH3SO2I decompose slowly to form the sulfonate: CH3SO2I + H2O → CH3SO3 – + I– + 2H+ , (khyd). In dilute phosphate buffer this decomposition occurs with khyd = 2.0 × 10–4 s –1; the decomposition rate shows an inverse-squared dependence on [I– ] because of the KMSI equilibrium. .