Kinetics and mechanism of the reduction of cobalt(II) 4,4',4",4"'-tetrasulfophthalocyanine by 2-mercaptoethanol under anoxic conditions

The kinetics and mechanism of reduction of cobalt(II) 4,4’,4’’,4'’’-tetrasulfophthalocyanine by 2-mercaptoethanol to yield cobalt(I) tetrasulfophthalocyanine and 2-hydroxyethyl disulfide under anoxic conditions were investigated and the following rate law was found: v = -d[Co^(II)TSP]_T/dt = k_2K_1[RSH]_T[Co^(II)TSP]_T{2(1 + ɑ_H+ /Ka_1,+ Ka_2/ɑ_H+)(l+ ɑ[RSH]_T)}, where k_2 is a rate constant for the rate-limiting electron-transfer step, and K_1 is the equilibrium constant for the complexation of a CoTSP dimer with thioethanol; K_(a1) and K_(a2) are the apparent acid dissociation constants of HOC_2H_4SH and HOC_2H_4S^-, respectively; ɑ is K_1/(1 + ɑ_(H+)/K_(a1)+ K_(a2)/ɑ_(H+)):ɑ_H+ is the hydrogen ion activity. A nonlinear least-squares fit of the experimental data to the above rate law gave k_2 = 228 ± 3.8 s^(-l) and K_1 = 117 ± 2.5 M^(-1) at 27 ºC at µ = 0.4 M