OMEGA-HYDROXYTHIOL MONOLAYERS AT AU ELECTRODES .5. INSULATED ELECTRODE VOLTAMMETRIC STUDIES OF CYANO/BIPYRIDYL IRON COMPLEXES

Au electrodes coated with self-assembled HO(CH2)(14)SH monolayers are used to measure the heterogeneous electron transfer reactivity of a series of structurally related outer-sphere redox complexes. These insulated monolayers act as electron transfer tunneling barriers, decreasing the electronic coupling between the electrode and solution redox molecules and allowing the measurement of electron transfer rates at a wide range of electrode potentials with greatly diminished diffusion limitations and double-layer effects. From electron transfer rate constant versus electrode potential data obtained from simple linear sweep voltammetric experiments, the reorganization energies and inherent adiabaticities of the complexes are determined. As cyano ligands are replaced by bipyridyls in the series Fe(CN)(6)(3-), Fe(bpy)(CN)(4)(1-), Fe(bpy)(2)(CN2)(1+), Fe(bpy)(3)(3+), the reorganization energy decreases monotonically. The measured reorganization energies are in good agreement with Marcus theory estimates. There is an abrupt order of magnitude decrease in the inherent adiabaticity of the complex between the cyano-containing complexes and Fe(bpy)(3)(3+). The bipyridyl ligand limits the closest approach of the complex when compared to the smaller cyano ligands, decreasing the electronic coupling between the Fe and the electrode surface. These electron transfer characterizations are extended to complexes containing bipyridyl ligands modified by methyl and sulfonate groups and to tris(bipyridyl) complexes of Ru and Os.