Intermolecular Proton-Coupled Electron Transfer through H-Bond Complexes in a p-Phenylenediamine-Based Urea Use of the Wedge Scheme to Describe Reactions of this Type
Authors: Smith Diane K. , Clare Laurie A. , Pham An T. 
Published: February 2020

There is a growing appreciation of the important role that H-bond complexes can play in the mechanism of proton-coupled electron transfer (PCET) reactions. Until relatively recently it had been thought that PCET reactions always proceeded step-wise with sequential electron and proton transfer. Much of the recent fundamental interest in PCET stems from the realization that a third option is available, concerted electron and proton transfer or CPET in which the electron and proton both move in a single kinetic step. This interest in the concerted process has increased awareness of H-bonding states in PCET, since the concerted reaction occurs within a H-bonded intermediate. However, even if the proton and electron transfer is not concerted, the H-bonded complex formed in the process of proton transfer can play an important role in the PCET mechanism if it is sufficiently long-lived.
Recently we have introduced a generally useful mechanistic framework with which to include H-bonding steps within an overall PCET pathway. This scheme, which for obvious reasons we call a “wedge”, is shown in Scheme 1 for the generic 1e−, 1H+ oxidation, AH + B = A + HB+ + e−. The front face in the wedge (in bold) is the standard electron transfer/proton transfer square scheme, with the two possible electron transfer reactions on the top and bottom edges, and the two possible proton transfers on the left and right edges. However, proton transfer reactions actually go through a H-bond intermediate, so a more accurate description of the proton transfer follows the dashed lines on the triangular sides of the wedge to and from the H-bond intermediates, A-H-B or A-H-B+, which are meant to represent the thermodynamically most stable H-bond complex in each oxidation state. If the H-bonded intermediate has sufficient lifetime, then electron transfer to/from the H-bond complex is also possible, represented by the rear edge of the wedge (thin solid line). If the proton moves from being more attached to A in A-H-B to being more attached to B in A-H-B+, then E° of this reaction is that of the CPET step, if the proton doesn’t move then the E° is simply that of oxidation of the H-bond complex. Either way, it is straightforward to show that E°(A-H-B0/+) has to have a value in between E°(AH0/+) and E°(A−/0). Thus the possibility of electron transfer through the H-bond complex opens up a pathway of intermediate potential for the overall reaction AH + B = A + HB+ + e−.
The usefulness of the wedge scheme is demonstrated by its ability to explain the unusual electrochemistry of the phenylenediamine-based urea, U(H)H, which we have shown undergoes a self proton transfer upon oxidation to give half equivalent of the doubly oxidized quinoidal cation and half-equivalent of the electroinactive, protonated reduced urea, Scheme 2. The reaction gives chemically irreversible voltammetry in acetonitrile as would be expected given that the quinoidal cation is harder to reduce than the initially formed radical cation. However, it gives reversible voltammetry in methylene chloride, which can be explained by the greater stability of the H-bonded intermediate in this solvent. In addition, in methylene chloride, we are able to clearly observe a concentration and scan rate dependent conversion between two different reduction pathways on the return scan. This behavior cannot be explained by a simple square scheme, but is readily explained by the wedge scheme.
In this presentation, we will report recent results on the voltammetry of U(H)H in the presence of guest molecules that H-bond to the starting, reduced state. We will show that their effect on the voltammetry can be explained in terms of two interlinked wedge schemes, one representing the electron transfer / H-bonding / proton transfer reactions of U(H)H with itself and the other representing the reactions with the added guest.