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46191-AC4
Electrochemical Studies on the Effect of Hydrogen Bonding and Proton Transfer on the Electron Transfer Reactions of Organic Redox Couples

Diane K. Smith, San Diego State University

The goal of this grant is to develop a better understanding of the role of hydrogen bonding and proton transfer in organic redox chemistry by doing detailed cyclic voltammetry (CV) studies of organic redox couples in the presence of hydrogen donors and acceptors of various strength.  Three systems were proposed for investigation: (i) reduction of 1,4-dinitrobenzene in DMF in the presence of various substituted diarylureas, (ii) oxidation of 2,3,5,6-tetramethylphenylenediamine in the presence of different pyridines, and (iii) reduction of quinones in buffered and unbuffered solutions.  During the first year of the grant we focused attention on the second two projects. Work on the first project will be commencing in the second year.
    Oxidation of phenylenediamines.  In preliminary work we had established that addition of pyridines to phenylenediamines, H₂PD, causes a substantial negative shift in the E_1/2 of the second oxidation in acetonitrile, indicating strong stabilization of the doubly oxidized product, H₂PD²⁺.  This could be due to either hydrogen bonding, proton transfer or both.  Aqueous pKa’s suggest that even the least basic pyridine should be able to deprotonate  H₂PD²⁺, however, the oxidation remains reversible, behavior typically associated with hydrogen bonding stabilization, not proton transfer.  In our initial studies in the grant period we looked at weaker bases such as amides, that should not be able to deprotonate H₂PD²⁺.  These also cause a substantial shift in the second wave, providing clearer evidence that just as with the more well-studied quinones, hydrogen bonding can have a substantial effect on the redox chemistry of phenylenediamines.
    Our ultimate goal is to sort out the mechanism for the oxidation of H₂PD in the presence of both the amide guests and the more basic pyridines by fitting the CV’s  using computer simulations.  To do this, we first need to fit the voltammetry of H₂PD by itself.  One obvious issue that needs to be addressed is that the second CV wave is too small relative to the first to just involve simple e- transfer.  It is possible that this is due to proton transfer between outgoing H₂PD²⁺ and incoming H₂PD.  Since this is a bimolecular reaction, one of the first things we did was to lower the concentration. This should slow the bimolecular reaction, causing the second wave to increase in size.  However, the opposite effect is actually observed, and, at the lowest concentration (5 micromolar), the second wave disappears entirely!  Parallel studies with N,N,N’,N’-tetramethylphenylenediamine show the opposite effect.  The second wave gets relatively larger as the concentration is decreased.  So, needless to say, this project has got a little more complicated than originally intended.  We believe we are close to sorting out a reasonable mechanism that fits the higher concentration data, and at least provides an explanation of the low concentration data.  This explanation involves proton transfer, hydrogen bonding, and adsorption of the radical intermediates. We now expect two publications on this part of the project, one dealing primarily with H₂PD by itself and a second dealing with the behavior in the presence of added hydrogen acceptors.
    The phenylenediamine project has also been expanded to include work with a phenylenediamine derivative in which one of the amino groups is part of an urea functional group.  Because ureas are very good hydrogen bonders, we expected to see potential shifts in the first oxidation in the presence of basic guests.  However, we did not observe significant shifts until we realized that the commonly used electrolyte anion PF6– was interfering by hydrogen bonding to the urea itself.  Switching to a much larger electrolyte anion resulted in a very significant shift with a cyclic diamide guest.  This work, which has general significance to redox-dependent hydrogen bonding, was published as a JACS communication this past summer.
    Quinones in unbuffered aqueous solution.  While it is well understood that proton transfer plays an important role in the aqueous redox chemistry of quinones, the role of hydrogen bonding has been almost ignored.  It’s our belief that in order to understand the role hydrogen bonding is playing, it is useful to look at the electrochemistry without buffers present.  This makes these studies more directly comparable to the non-aqueous work, where the role of hydrogen bonding is well established.   The initial results of our work on this project were published as an article in JACS in 2007, and provide a strong qualitative argument that the reduction of quinones in unbuffered aqueous solution of neutral pH is best described as an overall two electron process resulting in the formation of a strongly hydrogen-bonded quinone dianion.   In continuing work, we are attempting to simulate the CV behavior.  One thing that is clear from the initial simulations is that the traditional nine-membered square scheme will not be able to do so.  It is likely that strong hydrogen-bonded intermediates, and possibly concerted proton-coupled electron transfer is involved.

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