Interactions of cobalt tetrasulfophthalocyanine with alkyl xanthogenates: evidence for hydrophobic effects in the Merox process

Abstract

Abstract Anaerobic complexation of cobalt tetrasulfophthalocyanine (CoTSPc) with alkyl xanthogenates [ROC(S)S−] has been studied by UV-Vis spectroscopy in aqueous and N,N-dimethyl formamide (DMF) solutions. In aqueous solutions, lag periods are observed in the kinetic curves for all tested xanthogenates before the axial complex is formed. The duration of this lag period decreases with elongation of the ligand’s alkyl chain, thus indicating initial xanthogenate outer-sphere binding to CoTSPc. Rates of inner-sphere complexation, conversely, increase dramatically when using “longer” alkyl xanthogenates. CoTSPc–xanthogenate binding constants nearly double upon every increase of the ligand’s alkyl chain by two CH2 groups, from ethyl to n-octyl xanthogenate. In the aerobic Merox process (auto-oxidation of alkyl xanthogenates in the presence of CoTSPc in aqueous solutions), the rate also incrementally increases upon elongation of the substrate’s alkyl chain by two CH2 groups. The magnitude of this effect is similar to that observed for the CoTSPc–xanthogenate binding constants. In DMF, where hydrophobic interactions are not a factor, the alkyl xanthogenates display comparable kinetic and binding constants. Since the pKa values of alkyl xanthogenic acids are similar (2.5±0.1), the observed effects may be ascribed to hydrophobic interactions rather than electron donation.