Methods for investigating the mechanistic and kinetic role of ligand exchange reactions in coordination electrochemistry

The two successive reductions of chloroiron(III)tetraphenylporphyrin in dimethylformamide provide a model example illustrating the use of cyclic voltammetry for investigating the role of ligand exchange reactions in the electrochemistry of coordination compounds. The Cl−-DMF association constant for FeII is determined spectrometrically. The Cl−-DMF association constant for FeIII is then derived from the standard potential difference between the [FeIII (DMF)2]+/[FeIIDMF] and [FeIIICl]/[FeIICl]− redox couples. Starting from the values thus obtained, the kinetics of the Cl−-DMF ligand exchange reaction at the FeIII and FeII oxidation levels are investigated using cyclic voltammetry. Both the reduction and oxidation of the FeII complexes into FeI and FeIII respectively can be used for this purpose, under pseudo-first order conditions in the first case and second order conditions in the second. Development and application of the theory for a CEC square scheme under second order conditions allowed the role of the ligand exchange reaction in the reduction of the FeIII complexes to be unraveled.