Kinetic and binding studies of the thiolate–cobalt tetrasulfophthalocyanine anaerobic reaction as a subset of the Merox process

Abstract

Binding and kinetic studies of the complexation of thiolates/thioacid salts with 4,4′,4″,4′″-cobalt tetrasulfophthalocyanine (Co IITSPc) in aqueous solutions were conducted using UV–vis spectrophotometry under anaerobic conditions. Stepwise binding of two thiolate equivalents by one CoTSPc equivalent was observed. One-to-one thiolate binding constants increase along with thiolate basicity and drop when the ligand's p K a is less than 3.5. Binding constants for the second thiolate molecule are nearly one-order of magnitude smaller than those for the first one, and their dependence on thiolate basicity is less pronounced. Two-to-one thiolate binding occurs only if the resulting complex is oligomeric (as for the original Co IITSPc in aqueous media) and is accompanied by greater Co II–Co I reduction. Significant steric effects observed in 2:1 thiolate–CoTSPc binding indicate ligand–ligand interactions in stacked dimers (oligomers). Based on kinetic curves, thiolate binding appears to be a multi-step process with lag periods prior to the axial binding of both the first and second ligands, thus pointing to intermediate formation of outer-sphere complexes. Evidence was obtained that the rate-limiting step is electron transfer from sulfur to cobalt. The values of reciprocal Michaelis constants for the Merox process (CoTSPc-catalyzed aerobic autoxidation of the same substrates) are similar to those of the anaerobic 2:1 binding constants, thus indicating quasiequilibrium and the significance of dithiolate cobalt phthalocyanine complexes in catalysis.