The electrochemistry of several p-phenylenediamine derivatives, in which one of the amino groups is part of an urea functional group, has been investigated in methylene chloride and acetonitrile. The ureas are abbreviated U(R)R', where R' indicates the substituent on the N that is part of the phenylenediamine redox couple and R indicates the substituent on the other urea N. Cyclic voltammetry and UV-vis spectroelectrochemical studies indicate that U(Me)H and U(H)H undergo an apparent 1e(-) oxidation that actually corresponds to 2e(-) oxidation of half the ureas to a quinoidal-diimine cation, U(R)(+). This is accompanied by proton transfer to the other half of the ureas to make the electroinactive cation HU(R)H(+). This explains the observed irreversibility of the oxidation of U(Me)H in both solvents and U(H)H in acetonitrile. However, the oxidation of U(H)H in methylene chloride is reversible at higher concentrations and slower scan rates. Several lines of evidence suggest that the most likely reason for this is the accessibility of a H-bond complex between U(H)(+) and HU(H)H(+) in methylene chloride. Reduction of the H-bond complex occurs at a less negative potential than that of U(H)(+), leading to reversible behavior. This conclusion is strongly supported by the appearance of a more negative reduction peak at lower concentrations and faster scan rates, conditions in which the H-bond complex is less favored. The overall reaction mechanism is conveniently described by a "wedge scheme", which is a more general version of the square scheme typically used to describe redox processes in which proton transfer accompanies electron transfer.

It is well-established that very weak acids such as water and alcohols strongly H-bond to quinone dianions, Q 2− , in aprotic solvents. This results in thermodynamic stabilization of Q 2− and a shift of the formal potential of the Q −/2− couple to less negative values. This study shows that the strong H-bonding of water also results in a type of kinetic stabilization of Q 2− . CVs of the naturally occurring naphthoquinone Vitamin K1 in very dry 0.1 M NBu 4 PF 6 /CH 2 Cl 2 show a reversible Q 0/− wave but a chemically irreversible Q −/2− wave. Similar behavior is seen with anthraquinone and duroquinone. Evidence suggests that this is due to nucleophilic attack of Q 2− on CH 2 Cl 2 to give ether products. Addition of water results not only in the expected positive shift in potential of the second wave but also in an increase in the chemical reversibility. This indicates that the H-bonding of water to Q 2− blocks the irreversible reaction with CH 2 Cl 2 by significantly decreasing the rate of that reaction. Further experiments show that the kinetic stabilization by water is great enough that it can slow the reduction of Q 2− with iodomethane, a very reactive electrophile, in CH 3 CN.

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.

The kinetics of many proton-coupled electron transfer (PCET) reactions cannot be adequately described by stepwise proton and electron transfer. Concerted electron− proton transfer (CPET) is another possibility, but examples exist where stepwise mechanisms are not viable yet there is no compelling evidence for CPET. This study investigates such a reaction, the oxidation of an NH-containing phenylenediamine radical cation, H 2 PD + , in the presence of pyridines in acetonitrile, using CV and UV/vis spectroelectrochemistry. As observed previously, the E 1/2 for the radical oxidation jumps to a considerably more negative potential upon addition of 1 equiv of pyridine. The CV wave broadens but stays chemically reversible. Further addition of pyridine leads to smaller E 1/2 shifts with continued reversibility. Different explanations have been put forth for this behavior; however, this study provides strong evidence that the E 1/2 shift can be completely explained by the overall reaction being H 2 PD + + pyr − e − → HPD + + Hpyr + . Classic stepwise proton−electron transfer cannot explain the reversibility, but it can be explained by a "wedge" scheme mechanism in which electron and proton transfer occurs in a stepwise fashion within the H-bond complex formed as an intermediate in proton transfer. This result points to the important role H-bonding may play in PCET even without CPET.

Cyclic voltammetric and spectroelectrochemical evidence is presented showing that the H-bonded dimer formed from a ureidopyrimidone derivative containing a phenylenediamine redox couple can be reversibly broken apart at mM concentrations in CH2Cl2 by an electrochemically induced proton-coupled electron transfer reaction.