Qualitative extension of the EC′ Zone Diagram to a molecular catalyst for a multi-electron, multi-substrate electrochemical reaction

Abstract

The EC′ Zone Diagram, introduced by Savéant and Su over 30 years ago, has been used to classify voltammetric responses for electrocatalytic systems. With a single H₂-evolving catalyst, Co(dmgBF₂)₂(CH₃CH)₂ (dmgBF₂ = difluoroboryl-dimethylglyoxime), and a series of para-substituted anilinium acids, experimental conditions were carefully tuned to access to each region of the classic zone diagram. Close scrutiny revealed the extent to which the kinetic (λ) and excess (γ) factors could be experimentally controlled and used to access a variety of waveforms for this ECEC′ catalytic system. It was found that most of the tunable experimental parameters (such as catalyst concentration, scan rate, and substrate concentration) predicted in the EC′ Zone Diagram could be extended to a multi-electron system and produced similarly-shaped waveforms with some deviations. Tuning of a single catalyst across every region of the classic zone diagram has previously been prevented due to the seven orders of magnitude that need to be traversed across the kinetic parameter; however, the cobalt catalyst in this study provided unique control of this parameter. By varying the acids used as the proton source, the rate constants for protonation were tuned via a pK_a-dependent linear free energy relationship.