The Effect of H-Bonding and Proton Transfer on the Voltammetry of 2,3,5,6-Tetramethyl-p-phenylenediamine in Acetonitrile. An Unexpectedly Complex Mechanism for a Simple Redox Couple

Abstract

The voltammetry of 2,3,5,6-tetramethyl-p-phenylenediamine, H2PD, has been studied and compared to that of its isomer N,N,N’N’-tetramethyl-p-phenylenediamine, Me2PD. Both undergo two reversible electron transfer processes in acetonitrile that nominally correspond to 1e- oxidation to the radical cations, Me2PD+ and H2PD+, and a second 1e- oxidation at more positive potentials to the quinonediimine dications, Me2PD2+ and H2PD2+. While the voltammetry of Me2PD agrees with this simple mechanism, that of H2PD does not. The second voltammetric wave is too small. UV/Vis spectroelectrochemical experiments indicate that the second wave does correspond to oxidation of H2PD+ to H2PD2+ in solution. The fact that the second wave is not present at all at the lowest concentrations (5 µM), and that it increases at longer times and higher concentrations, indicates that H2PD+ is not the initial solution product of the first oxidation. A number of lines of evidence suggest instead that the initial product is a mixed valent, H-bonded dimer between one H2PD in the the full reduced, fully protonated state, H4PD2+, and another in the fully oxidized, fully deprotonated state, PD. A mechanism is proposed in which this dimer is formed on the electrode surface through proton transfer and H-bonding. Once desorbed into solution, it breaks apart via reaction with other H2PD’s, to give 2 H2PD+, which is the thermodynamically favored species in solution.

Easy Entry into Reduced Ar‐BIANH2 Compounds: A New Class of Quinone/Hydroquinone‐Type Redox‐Active Couples with an Easily Tunable Potential

“…An intriguing feature is that, by increasing the substrate concentration, peak IIc gradually changes into peak II′c, of similar morphology but shifted to more negative potentials by about 0.2 V (Figure 3, lower section). This transition, recalling similar cases recently evidenced in voltammetric studies of quinone99 or diimine100 systems, could be justified by assuming that, at a suitably high concentration of 1 e , the 2 e product of Ic can be stabilized by formation of a quinhydrone‐like Ar‐BIANH 2 +Ar‐BIAN aggregate, resulting in its subsequent reduction step to take place at more negative potentials.…”

Unleashing the Power and Energy of LiFePO4-Based Redox Flow Lithium Battery with a Bifunctional Redox Mediator

“…Compared with the reduction process, the oxidation current is significantly lower, presumably due to the relatively small driving force, and/or bimolecular reactions between TMPD-based reaction intermediates in solution which lowers the effective concentration of TMPD •2+ . 19,20 The electrochemical reversibility and cycling stability of TMPD were also evaluated by CV measurements ( Figure S1). The CV curves kept nearly unchanged for 500 consecutive cycles, revealing the robustness of TMPD for the condition tested.…”

Intramolecular Hydrogen Bonding/Selfprotonation Processes Modulated by the Substituent Effect in Hydroxyl‐substituted Naphthoquinones

“…This pattern is significantly different from the one expected, due to the lack of symmetry elements in the structure (a C 1 point group). Thereby, the observed spectrum could be explained considering the formation of a radical dimer interacting between the π planes of the naphthoquinone structure, thus providing symmetry in atoms HFCC values of the proposed adduct , compared with the expected asymmetrical spin density distribution (Fig. C).…”

“…Results above described indicate a variety of substituent effects related to the presence of hydroxyl functions within quinone systems. In the case of compounds 1 and 2 , even though they showed a similar voltammetric behaviour, their corresponding reduction potential values are less negative for the latter compound than for the unsubstituted 1,4‐naphthoquinone, as expected due to the formation of intramolecular hydrogen bonds, which are higher in magnitude for the reduced species and therefore, lead to a shift in the experimental reduction potentials . These hydrogen bonds are a substituent field interaction , originated from spatial electronic interactions between neighbouring groups and also manifests during the formation of dianion radical or trianion radical species, leading to the experimentally observed changes in reduction potentials of the deprotonated quinones 5 and 6 from the quinone 4 (which has no OH functions at the annellated ring, Table ).…”