Reduction of Cysteinesulfenato and Cysteinesulfinato Derivatives of Cobalt(III) by Chromium(II)

The reductions of the (cysteinesulfenato)bis(ethylenediamine)cobalt(III), abbreviated [Co(en)₂(CysO)]²⁺, and the (cysteine-sulfinato)bis(ethylenediamine)cobalt(III), abbreviated [Co(en)₂(CysO₂)]²⁺, complexes by Cr(II) proceeded rapidly. The actual reduction of the cysteinesulfenato derivative was too fast to be measured directly but was estimated to be >3 × 10⁴ M^-1 s^-1. Two electrons were rapidly consumed by the oxidant. It was postulated that one of these was used in inner-sphere attack by Cr(II) at the sulfur atom and the other remained on the sulfur ligand producing a radical intermediate. Three fast reactions followed reduction: the first was attributed to S-O bond fission (k^25deg;C = 39.5 s^-1, ΔH^≠ = 10 ± 1 kcal/mol, ΔS^≠ = -16 ± 3 eu), the second was assigned to formation of an encounter pair from two radical ions followed by coupling to give the cystine dimer of Cr(III) (k^25°C = 5.4 ± 0.2 s^-1, ΔH≠ = 11 ± 2 kcal/mol, ΔS^≠ = -19 ± 6 eu), and the third reaction was attributed to the Cr(II)-catalyzed aquation of the dimeric Cr(III)-cystine product (k^25°C = 2.9 ± 0.4 M^-1 s^-1, ΔH^≠ = 9 ± 2 kcal/mol, ΔS^≠ = -26 ± 11 eu). The reduction of the cysteinesulfinato derivative occurred by attack of Cr(II) at sulfur to produce the Cr(III)-sulfur-bonded intermediate of cysteinesulfinic acid (k^25°C = (1.46 ± 0.07) × 10³ M^-1 s^-1, ΔH^≠ = 4.4 ± 0.3 kcal/mol, ΔS^≠ = -29 ± 1 eu). This was followed by a slow Cr(II)-catalyzed aquation of the Cr(III)-sulfinic acid product (k^25°C = (2.5 ± 0.1) × 10^-3 M^-1 s^-1, ΔH^≠ = 21 ± 2 kcal/mol, ΔS^≠ = -1 ± 5 eu). These results are compared to work done on related systems.