Date of Award

2-5-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Evguenii I. Kozliak

Abstract

Binding and kinetic studies of the thiolates/thioacid salts complexation with cobalt tetrasulfophthalocyanine (CoIITSPc) in aqueous solutions, dimethylformamide (DMF), and other polar aprotic solvents were conducted using UV-vis, 1H NMR, and ESR methods under anaerobic conditions. In DMF thiolates react with CoIITSPc with 1:1 stoichiometry. Aliphatic thiolates, regardless of their basicity, reduce Co IITSPc to CoITSPc. Low-basicity thiolates form unstable (RS-)CoIITSPe complexes as dead-end products. Indirect kinetic evidence was obtained for the electron transfer from the axial ligand to metal via the phthalocyanine equatorial ligand. 1H NMR and binding studies revealed sulfur-cobalt interactions in the CoITSPc product, indicating axial ligand attachment to CoITSPc. Low-basicity aromatic thiolates form stable (RS-)CoIITSPc complexes due to π-stacking. In aqueous solutions stepwise binding of two aliphatic thiolates to CoTSPc was observed. 1:1 binding constants increase with thiolate basicity and drop when ligand's pKa is less than 3.5. 2:1 constants are nearly one-order of magnitude smaller, and their dependence on thiolate basicity is less pronounced. 2:1 binding occurs only if the resulting complex is oligomeric and is accompanied by greater CoII--Co I reduction. Steric effects observed in 2:1 binding indicate ligand-ligand interactions in stacked oligomers. Based on kinetic curves, thiolate binding appears to be a multi-step process with lag periods prior to axial binding, pointing to intermediate formation of outer-sphere complexes. Evidence was obtained that the rate-limiting step is electron transfer from sulfur to cobalt. Hydrophobic effects were found in CoTSPc-xanthogenate binding and kinetics using alkyl xanthogenates [ROC(S)S-] with similar p Kas (2.5 ± 0.1): 1:1 binding constants nearly double upon every increase of the ligand alkyl chain (R) by two CH2 groups. The duration of lag periods decrease and the rates of inner-sphere complexation increase with elongation of R. In DMF, where hydrophobic interactions are not a factor, binding and kinetic constants are comparable. In the Merox process (CoTSPc-catalyzed aerobic thiolate autoxidation), the rate incrementally increases upon elongation of R by two CH2 groups. The values of reciprocal Michaelis constants for the Merox process are similar to the anaerobic 2:1 binding constants, indicating quasiequilibrium and the significance of dithiolate-CoTSPc complexes in catalysis.

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