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

Abstract

The reaction between pyridine (py) and phthalocyaninatoiron(II), [Fe(pc)], in dimethyl sulphoxide (dmso) has been studied from the kinetic and the equilibrium viewpoints at 20 ± 0.5 °C. In the presence of a large excess of pyridine the adduct [Fe(pc)(py)₂] is formed via pseudo-first-order kinetics. The observed rate constants have the general form k_obs.=k_1f[py]+k_1r, with k_1f′≈ 1.5 × 10⁴ dm³ mol^–1 s^–1 and k_1f″= 35.5 ± 0.7 dm³ mol^–1 s^–1 and k_1r″= 0.15 ± 0.02 s^–1, for the binding and dissociation of the first and second py molecules, respectively. From the values of the equilibrium constant [K₁′=(3.9 ± 0.1)× 10⁵ dm³ mol^–1] and of the second-order rate constant (k_1f′) for the binding of the first py molecule, the dissociation rate constant has been estimated to be k_1r′≈ 4 × 10^–2 s^–1. The value of the equilibrium constant for the binding of the second py (K₁″= 300 ± 28 dm³ mol^–1), determined directly, is in good agreement with the corresponding value (240 ± 40 dm³ mol^–1) obtained from the ratio k_1f″/k_1r″). The data obtained in the present study are discussed in the light of related previous studies.