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

Abstract

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.