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 (k25deg;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) (k25°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 (k25°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 (k25°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 (k25°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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry