Electrochemical Evidence for Intermolecular Proton-Coupled Electron Transfer through a Hydrogen Bond Complex in a p-Phenylenediamine-Based Urea. Introduction of the “Wedge Scheme” as a Useful Means To Describe Reactions of This Type

Abstract

The electrochemistry of several p-phenylenediamine derivatives, in which one of the amino groups is part of an urea functional group, has been investigated in methylene chloride and acetonitrile. The ureas are abbreviated U(R)R', where R' indicates the substituent on the N that is part of the phenylenediamine redox couple and R indicates the substituent on the other urea N. Cyclic voltammetry and UV-vis spectroelectrochemical studies indicate that U(Me)H and U(H)H undergo an apparent 1e(-) oxidation that actually corresponds to 2e(-) oxidation of half the ureas to a quinoidal-diimine cation, U(R)(+). This is accompanied by proton transfer to the other half of the ureas to make the electroinactive cation HU(R)H(+). This explains the observed irreversibility of the oxidation of U(Me)H in both solvents and U(H)H in acetonitrile. However, the oxidation of U(H)H in methylene chloride is reversible at higher concentrations and slower scan rates. Several lines of evidence suggest that the most likely reason for this is the accessibility of a H-bond complex between U(H)(+) and HU(H)H(+) in methylene chloride. Reduction of the H-bond complex occurs at a less negative potential than that of U(H)(+), leading to reversible behavior. This conclusion is strongly supported by the appearance of a more negative reduction peak at lower concentrations and faster scan rates, conditions in which the H-bond complex is less favored. The overall reaction mechanism is conveniently described by a "wedge scheme", which is a more general version of the square scheme typically used to describe redox processes in which proton transfer accompanies electron transfer.

Use of a Wedge Scheme to Describe Intermolecular Proton-Coupled Electron Transfer through the H-bond Complex Formed Between a Phenylenediamine-Based Urea and 1,8-Naphthyridine

“…In addition there is increased irreversibility in wave I, with peak Ic′ growing in as the naph concentration increases. This behavior is extremely similar to what was previously observed with pyridine additions to U(H)H. 28 While the behavior shown in Figure 3 clearly shows the conversion from an overall 1 e − process to a 2 e − one, indicating that naph successfully competes with another urea for the acidic H + , there are also indications of electrode fouling. Notably, the currents start to decrease and the irreversibility of wave I continues to increase as the concentration of naph increases beyond what is shown in the figure.…”

“…3,25−27 Recently we introduced what we think is a generally useful framework for explicitly including H-bonding steps in a PCET mechanism, which we termed the "wedge" scheme for obvious reasons. 28 This is illustrated in Scheme 1b for AH + B = A + HB + + e − . The wedge scheme arises from the square scheme by explicitly considering the formation and dissociation of the Hbonded complexes that are intermediates in the proton transfer reactions.…”

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“…1-(4-(dimethyl-amino)phenyl)-3-(p-tolyl)urea ( Urea-N , see Supplementary Figs. 1, 46, 47) was selected as an ideal electroacid due to its ability of proton release after the reversible two-electron oxidation as a urea derivative 21 to stimulate the pH-sensitive rhodamine B derivative 3′,6′-bis(diethylamino)-3-oxospiro[isoindoline-1,9′-xanthen]-2 yl-acetate ( Rh-M , see Supplementary Figs. 2, 3, 48, 49 and Supplementary Table 1) 22 to realize electrochromism.…”

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…However, even if a proton cannot transfer (i.e., move from one atom to another) in the same step, movement in the relative position of a proton within a hydrogen-bonding interface is documented to influence the rate of electron transfer in PCET [167][168][169][170][171]. Provided an appropriate hydrogen-bond acceptor is present, the potentials of indoles can be attenuated.…”