Proton coupled electron transfer kinetics of redox centers attached to self-assembled monolayers on electrodes.

摘要

Proton-coupled electron transfer (PCET) reactions play an important role in many biochemical systems and have been focus of great interest recently. These reactions can be represented by a general equation: Ox + ne - + hH+ ↔ Red; Traditionally, these reactions have been studied by applying predictions from the stepwise model developed by E. Laviron in the early 1980s. This model is based upon treating the proton transfer step and electron transfer step as discrete steps. Proton transfer is assumed to be at equilibrium under all conditions. However, there is also evidence suggesting a concerted mechanism in which the electron and proton are transferred simultaneously.; In this study, the predictions of the two models are tested by tethering an osmium complex, OsII(bpy)2(py)(OH2), (bpy = bipyridine and py = 4-Aminomethylpyridine) to an electrode using self assembled monolayers. Data analysis is carried out using Cyclic Voltammetry. Results obtained show that the osmium system follows the thermodynamic model closely. However, kinetically, the system deviates substantially from predictions of the stepwise model. The standard rate constant and the transfer coefficient are weakly dependent on pH. Tafel plots are asymmetrical at all pHs. The transfer coefficient at zero overpotential is consistently less than the 0.5 value expected for simple electron transfer. Comparison of results from this study to earlier work by Haddox reveals that the standard rate constant decreases by a factor of 10 when the diluent chain length is increased by four methylene groups. The stepwise model cannot explain these observations.; The concerted mechanism is tested by measuring kinetic parameters in deuterated electrolytes, to investigate the kinetic isotope effect. The weak, but noticeable dependence of the standard rate constant on pH is interpreted in terms of a concerted mechanism, with short proton tunneling distance. The reorganization energy of the OsII species is higher than that of the OsIII form. This is contrary to expectations. Based on electrostatic arguments, the opposite would be expected since the higher oxidation state has higher bond vibration frequency. Another striking observation is the sudden break in the plot of standard rate constant vs pD. A third striking observation is the inverse correlation of the standard rate constant with the reorganization energy of OsII. An attempt to explain the results using the concerted model was limited by the absence of a fully developed theoretical model.