Solvent effects on reactivity trends for iron (II) complexes of tridentate imine ligands in acidic media

Abstract

Kinetics of acid aquation of Fe(II) complexes of tridentate imine ligands in aqueous methanol mixtures have been studied. Tridentate imine ligands are derived from 2-pyridinecarboxaldehyde or 2-acetylpyridine or 2-benzoylpyridine and ethylenediamine. An aquation mechanism and rate law are proposed. The difference in behaviour between these tridentate imine ligand complexes and other tridentate ligand complexes like [Fe(terpy)₂]²⁺ and [Fe (tptZ)₂]²⁺ are established. Solvent effects on reactivity trends have been analysed into initial state and transition state components. These are determined from transfer chemical potentials of the reactants and kinetic data. The transfer chemical potentials δmμ^θ for complex cations are derived from solubility data of perchorate salts. Method of calculating the transfer chemical potentials of single ions are based on δmμ^θ (Ph₄P⁺, Ph₄As⁺) = δmμ^θ (Ph₄B^-). Kinetic data lead to Gibbs free energy of activation (the change of activation barrier δm△<em>G^≠</em>). The decrease in the rate constant of aquation of Fe(ll) complexes of tridentate imine ligands, as the proportion of methanol increases, is determined by the greater effect of solvent on the initial state.