Kinetics and mechanisms 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^IITSP]_T/dt = {k₂k₁[RSH]_T[Co^IITSP] _T}/{2(1 + a_H+/K_a1 + K_a2/a_H+)(1 + α[RSH]_T)}, where k₂ is a rate constant for the rate-limiting electron-transfer step, and K₁ 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 HOC₂H₄SH and HOC₂H₄S^-, respectively; α is K₁/(1 + a_H+/K_a1 + K_a2/a_H+); a_H+ is the hydrogen ion activity. A nonlinear least-squares fit of the experimental data to the above rate law gave k₂ = 228 ± 3.8 s^-1 and K₁ = 117 ± 2.5 M^-1 at 27°C at μ = 0.4 M.