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: kobs. = k1f[Him] + k1r with k1f? = (9.8 ± 0.2) × 103 dm3 mol-1 s-1 and k1f? = 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 (k1f? and k1f?) and of the equilibrium constants (K1? and K1?), the dissociation rate constants have been estimated to be k1r? = 1.5 × 10-2 s-1 and k1r? = 7.3 × 10-4 s-1. The results are discussed in the light of related previous studies.