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.
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.
Laurie A
LA
Clare
Lydia E
LE
Rojas-Sligh
Sonia M
SM
Maciejewski
Karina
K
Kangas
Jessica E
JE
Woods
L Jay
LJ
Deiner
Andrew
A
Cooksy
Department of Chemistry and Biochemistry, San Diego State University, San Diego, CA 92182-1030.
Diane K
DK
Smith