Electrochemical Evidence for Intermolecular Proton-Coupled Electron Transfer through a Hydrogen Bond Complex in a p-Phenylenediamine-Based Urea. Introduction of the 鈥淲edge Scheme鈥?as a Useful M
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- 作者：Laurie A. Clare ; An T. Pham ; Francine Magdaleno ; Jaqueline Acosta ; Jessica E. Woods ; Andrew L. Cooksy ; Diane K. Smith
- 刊名：Journal of the American Chemical Society
- 出版年：2013
- 出版时间：December 18, 2013
- 年：2013
- 卷：135
- 期：50
- 页码：18930-18941
- 全文大小：792K
- 年卷期：v.135,no.50(December 18, 2013)
- ISSN：1520-5126
文摘
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鈥? where R鈥?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鈥搗is spectroelectrochemical studies indicate that U(Me)H and U(H)H undergo an apparent 1e鈥?/sup> oxidation that actually corresponds to 2e鈥?/sup> 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 鈥渨edge scheme鈥? which is a more general version of the square scheme typically used to describe redox processes in which proton transfer accompanies electron transfer.