Equilibrium and kinetic study of imidazole binding to phthalocyaninatoiron(II) in dimethyl sulphoxide

The reaction between imidazole (Him) and phthalocyaninatoiron(II) [Fe(pc)] in dimethyl sulphoxide (dmso) has been studied, at 20 °C, from both equilibrium and kinetic points of view. In the presence of excess of Him the adduct [Fe(pc) (Him)₂] is formed via two consecutive pseudo-first-order processes. The observed rate constants have the general form: k_obs. = k_1f[Him] + k_1r with k_1f′ = (9.8 ± 0.2) × 10³ dm³ mol^-1 s^-1 and k_1f″ = 5.4 ± 0.15 dm³ mol^-1 s^-1, for the binding of the first and second Him molecules, respectively. Equilibrium experiments yield K₁′ = (6.4 ± 2.6) × 10⁵ dm³ mol^-1 and K₁″ = (7.4 ± 0.5) × 10³ dm³ mol^-1 for the stepwise formation of the [Fe(pc) (dmso) (Him)] and [Fe(pc) (Him)₂] adducts, respectively. From the values of the second-order rate constants (k_1f′ and k_1f″) and of the equilibrium constants (K₁′ and K₁″), the dissociation rate constants have been estimated to be k_1r′ = 1.5 × 10^-2 s^-1 and k_1r″ = 7.3 × 10^-4 s^-1. The results are discussed in the light of related previous studies.