A cyclic voltammetric and stopped-flow study of five coordinate hemins in noncoordinating solvent: evidence for electron transfer through the axial ligand

Abstract

A series of 29 penta-coordinate, high spin hemin complexes were studied by cyclic voltammetry. Ten of these complexes were also studied by a stopped-flow method. These compounds differed only in the identity of the ligand occupying the fifth coordination site. E 1 2 for the Fe(III) → Fe(II) reduction was found to increase linearly with the p K a of the axial ligand over the p K a range 0.7–10 but no correlation between the heterogeneous rate constant, k 0, and the p K a of the axial ligand was observed. The dependence of the heterogeneous electron transfer rate constant on axial ligand structure was interpreted to indicate that electrochemical electron transfer occurs through the coordinated axial ligand. Stopped-flow measurements of the hydrazine reduction of a selection of these hemin complexes showed no correlation between the electrochemical reaction rate and the homogeneous reaction rate although the homogeneous reaction rate was dependent on the axial ligand.