Supporting Electrolyte Bulkiness as a Semiquantitative Probe for Slow Electron Transfer in Dissociative Electron Transfer Studies

An important model for the study of dissociative electron transfer (DET) is provided by the electrochemical reduction of organic halides, proceeding along either a concerted pathway, electron transfer and bond cleavage occurring in a single step, or a stepwise one, involving the intermediate formation of a radical anion. In the first case the reaction is kinetically controlled by the electron transfer, whereas in the second case two energy barriers must be taken into account. In a recent investigation on the electrochemical reduction of aryl bromides ArBr on Ag and GC electrodes, we have observed that catalytic Ag significantly modifies the reaction energy profile, making electron transfer more kinetically determining with respect to bond cleavage, and stabilising the reaction products. As a result, in many cases the DET appears to proceed along a stepwise pathway on a non-catalytic electrode such as GC, but along a concerted pathway on catalytic Ag. In the same mechanistic investigation, when testing the effect of different quaternary ammonium supporting electrolyte cations (with C2, C3, C4 and C6 alkyl chains), we realised that the cation bulkiness, all other experimental conditions being carefully kept constant, not only affects the irreversible electron transfer with the well known shielding effect (the longer the alkyl chains, the slower the electron transfer, with a linear relationship between reduction peak potentials and cation radius), but may be regarded (and, possibly, exploited) as a convenient and effective semiquantitative probe of the kinetic influence of the first electron transfer step in the overall EC process. In fact, the increase of the shielding effect with the chain bulkiness appears to regularly decrease with increasing aapp. This means that the energy barrier of the chemical step becomes increasingly more determining with respect to that of the electron transfer step. At aapp = 1, i.e. with a process entirely controlled by the chemical step, the supporting electrolyte effect becomes negligible. On the contrary, the largest supporting electrolyte effects were recorded for DET processes that are kinetically controlled by the first electrochemical step. An exhaustive series of examples in both catalytic and non-catalytic conditions shall be compared and discussed.