Cyclic Voltammetry Studies of H-Bond Complex of a p-Phenylenediamine-Based Urea with 1,8-Naphthyridine As the Proton-Coupled Electron Transfer Guest with Platinum and Glassy Carbon As Working Electrodes
Authors: Tamashiro Bryan T. , Smith Diane K. 
Published: February 2020

Reactions in which both electron and proton are transferred, called proton-coupled electron transfer (PCET) reactions, are essential to many of the fundamental chemical processes of life. While PCET reactions have been well studied, it is only relatively recently that the role of H-bonding intermediates has been examined. The role that H-bonding can play in PCET was demonstrated by studies in the Smith group with U(H)H, a p-phenylenediamine-based urea which undergoes a reversible oxidation in methylene chloride.1 The overall oxidation of U(H)H is a 1 e- transfer with a surprising 2 e-/ 1 H+ mechanism where removal of the first electron produces a radical cation with an acidic NH proton that is then removed by the dimethylamino group from another U(H)H. This leads to removal of a second electron and creates the final products: the protonated electroinactive species, HU(H)H+, and the doubly oxidized form, U(H)+. Since this corresponds to 2 e- per 2 ureas, the process appears to be a net 1 e-. The interesting observation is that on the return scan two distinct pathways are observed. At the more negative potential, one pathway proceeds via the reduction of the U(H)+.  This path dominates at low concentrations and fast scan rates. We hypothesize that the other path, which occurs at less negative potential, involves reduction of a H-bond complex formed between HU(H)H+ and U(H)+, leading to an overall electrochemically reversible process.

In this study, cyclic voltammetry of U(H)H in the presence of 1,8-naphthyridine has been examined. The addition of naphthyridine results in an increase in current height and the appearance of a second oxidation wave at a more positive potentials . The increase in current is due to competition for the acidic proton between naphthyridine and the dimethylamino group of another U(H)H. NMR titration analysis show that naphthyridine H-bonds to the reduced U(H)H meaning naphthyridine blocks the H-bond sites from another U(H)H. Interestingly, the shape and behavior of the more positive oxidation wave alters depending on the electrode material. We hypothesize that the change in the CV wave is due to electrode fouling on the platinum (Pt) and poorly polished glassy carbon (GC) electrodes. With a carefully polished GC electrode, which minimizes the effect of electrode fouling, the observed second oxidation wave shifts slightly to more negative potentials and appears to merge with the first oxidation wave as the concentration of naphthyridine increases. This behavior reflects a mechanism that is similar to that of U(H)H by itself except the dimethylamino group of the other urea is replaced with naphthyridine. Therefore, the appearance of the second oxidation wave is due to oxidation of the H-bonding complex between the radical cation urea, U(H)H+, and naphthyridine, which is accompanied by proton transfer to naphthyridine.

1. Clare, L. A.; Pham A. T.; Magdaleno, F.; Acosta, J.; Woods, J. E.; Cooksy, A. L.; Smith, D. K. J. Am. Chem. Soc., 2013, 135 (50), 18930–18941.