Steric effect on the axial substitution of bis(dimethyl sulphoxide)phthalocyaninatoiron(II) by substituted imidazoles

Abstract

The axial substitution reactions of (DMSO) 2FePc by ring substituted imidazoles (B) of similar basicity have been studied in solvent DMSO from both equilibrium and kinetic points of view. Under excess imidazole the substitution of the first DMSO molecule occurs with a pseudo first-order rate constant k′ obs= k′ f[B]+ k′ r, and, while k′ f is substantially independent of the incoming base B, the rate constant of the reverse reaction, k′ r, strongly increases with increasing steric hindrance of B in the order im<1-Meim<4(5)-Phim<2-Meim<1,2-Me 2im≈2-Etim. Consequently, the equilibrium constant for the first substitution step decreases about 1000 times in the same order. During the second substitution step the DMSO molecule of the complex (DMSO)FePc(B) is exchanged for a second B molecule to give FePc(B) 2. Under pseudo first-order conditions the observed rate constant follows the equation k″ obs= k″ f[B]+ k″ r. In this case the rate constant of the forward process, k″ f, increases of two orders of magnitude on going from unhindered to hindered imidazoles while the rate constant for the reverse reaction, k″ r, increases in a parallel manner of about four orders of magnitude. The experimental facts are explained in terms of a dissociative mechanism and a ground state destabilisation of the mono- and disubstituted complexes due to steric repulsion, this effect being almost additive and amounting to ca. 3 Kcal per mole of coordinated imidazole. The transition states are, instead, almost unaffected by the steric hindrance of the leaving and (if present) the trans imidazole.