Graduate Thesis Or Dissertation
 

Reactions and stereochemistry of thiolato and sulfenato cobalt (III) complexes

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  • This thesis describes a study of the synthesis, stereochemistry, and reactivity of cobalt(III) cysteinato (CyS) and related thiolato complexes. The preparation of [Co(en)₂CyS]⁺ resulted in a product solution containing a mixture of diastereomers in the ratio 2.3:1 (Δ:A). The isomers were separated by ion exchange chromatography, and their absolute configurations were assigned on the basis of their CD spectra and relative stabilities. Interconversion of the two isomers was shown to occur under the proper conditions. Oxidation of A-[Co(en)₂CyS]⁺ by H₂O₂ led to a 2.9:1 (S:R) mixture of AS- and AR- [Co(en)₂CyS]⁺, the S and R indicating configuration at sulfur. Both of these sulfenato isomers underwent further oxidation by H₂O₂ to form the same product, A-[Co(en)₂(CySO₂)]⁺. The S and R sulfenato isomers were stable in solution if kept in the dark, but exposure to room light caused a rapid interconversion of the isomers, which led in 1-3 hr to an equilibrium 76:24 (AS:AR) mixture. A slower decomposition reaction also occurred upon exposure to light. Similar behavior toward H₂O₂ and light was observed in the Δ system. The synthesis of the analogous penicillaminato complexes produced a ca. 2:1 mixture of A-[ Co(en)₂PenS]⁺. Both isomers underwent oxidation by H₂O₂ to form sulfenato and sulfinato complexes. The complexes [ Co(CyS)₃]³⁻ and [Co(PenS)₂]⁻ also reacted readily with H₂O₂, producing sulfenato and sulfinato complexes. The bis(cysteinato) complex previously formulated in the literature as [ Co(CyS)₂(H₂O)₂]⁻ did not react with H₂O₂. Based on this and other observations, a new trinuclear structure was proposed in which six sulfur ligands form bridges between three cobalt(III) ions. The kinetics of oxidation by H₂O₂ were measured for several of the thiolato and sulfenato complexes. The resulting rate constants were analyzed in terms of the steric and electronic characteristics of the various isomeric species. Other reactions briefly examined include oxidation of [ Co(en)₂CyS]⁺ and [ Co(en)₂(CySO)]⁺ by Br₂, and reaction of [ Co(en)₂(CySO)]⁺ with hydroxylamine-O-sulfonic acid.
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