Comparison of the self-exchange and interfacial charge-transfer rate constants for methyl- Versus tert-butyl-substituted Os(III) polypyridyl complexes

Differences in the self-exchange and interfacial electron-transfer rate constants have been evaluated for a relatively unhindered Os(III/II) redox system, osmium(III/II) tris(4,4′-di-methyl-2,2′-bipyridyl), [Os(Me₂bpy)₃]^3+/2+, relative to those of a relatively hindered system, osmium(III/II) tris(4,4′-di-tert-butyl-2, 2′-bipyridyl), [Os(t-Bu₂bpy)₃]^3+/2+. In contrast to the predicted increase in rate constant by a factor of 2-3 due to the difference in reorganization energy of the two complexes, introduction of the tert-butyl functionality decreased the self-exchange rate constant, as measured by NMR line-broadening techniques, by a factor of ∼50 as compared to that of the analogous methyl-substituted osmium complex. Steady-state current density versus potential measurements, in conjunction with differential capacitance versus potential measurements, were used to compare the interfacial electron-transfer rate constants at n-type ZnO electrodes of [Os(t-Bu ₂bpy)₃]^3+/2+ and [Os(Me₂bpy) ₃]^3+/2+. The interfacial electron-transfer rate constant for the reduction of [Os(t-Bu₂bpy)₃]³⁺ was 100 times smaller than that for [Os(Me₂bpy)₃]³⁺. The results indicate that the tert-butyl group can act as a spacer on an outer-sphere redox couple and significantly decrease the electronic coupling of the electron-transfer reaction in both self-exchange and interfacial electron-transfer processes.