Kinetics and mechanism of water substitution in the low-spin Fe(ii) complex of 4-octasulfophenylpyrazinoporphyrazine

Abstract

The substitution reaction of the axial-coordinated water by pyridine, pyrazine and 4-CN-pyridine in the low-spin Fe(II) complex of octasulfophenyltetrapyrazinoporphyrazine was studied. Kinetic and thermodynamic parameters for the different reaction steps of the process were determined. On the basis of NMR data and spectrophotometric titrations, a pronounced non-equivalence of the two coordinated N-donor ligands was observed. The substitution of water by pyridine and 4-CN-pyridine is shown to include the formation of a precursor outer-sphere complex, whereas substitution by pyrazine follows a limiting dissociative mechanism.