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It is well-established that very weak acids such as water and alcohols strongly H-bond to quinone dianions, Q2–, in aprotic solvents. This results in thermodynamic stabilization of Q2– 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 Q2–. CVs of the naturally occurring naphthoquinone Vitamin K1 in very dry 0.1 M NBu4PF6/CH2Cl2 show a reversible Q0/– 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 Q2– on CH2Cl2 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 Q2– blocks the irreversible reaction with CH2Cl2 by significantly decreasing the rate of that reaction. Further experiments sho...

Recently we introduced the “wedge scheme” as a convenient means to include H-bonded intermediates in an overall proton-coupled electron transfer (PCET) mechanism by showing that it can nicely explain the unusual solvent- and concentration-dependent voltammetry of a phenylenediamine-based urea, U(H)H. This compound undergoes an apparent 1 e– reversible oxidation in CH2Cl2 that actually corresponds to 2 e– oxidation of half of the ureas to the quinoidal cation accompanied by transfer of a H+ and deactivation of the other half of the ureas. The reversibility of the process is due to the second electron transfer and proton transfer proceeding through a H-bond complex between the oxidized urea and the dimethylamino group on another urea. In this study the effect of adding 1,8-naphthyridine, naph, which H-bonds to the U(H)H in its reduced state, is examined using cyclic voltammetry. Addition of naph causes both an increase in the current of the reversible U(H)H cyclic voltammetric wave as well as the appearance...

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.