Electrochemically Controlled Hydrogen Bonding. o-Quinones as Simple Redox-Dependent Receptors for Arylureas

Abstract

9,10-Phenanthrenequinone and acenaphthenequinone are shown to act as simple redox-dependent receptors toward aromatic ureas in CH(2)Cl(2) and DMF. Reduction of the o-quinones to their radical anions greatly increases the strength of hydrogen bonding between the quinone carbonyl oxygens and the urea N-hydrogens. This is detected by large positive shifts in the redox potential of the quinones with no change in electrochemical reversibility upon addition of urea guests. Cyclic voltammetric studies with a variety of possible guests show that the effect is quite selective. Only guests with two strong hydrogen donors, such as O-H bonds or amide N-H bonds, that are capable of simultaneously interacting with both carbonyl oxygens give large shifts in the redox potential of the quinones. The electronic character and conformational preference of the guest are also shown to significantly affect the magnitude of the observed potential shift. In the presence of strong proton donors the electrochemistry of the quinone becomes irreversible indicating that proton transfer has taken place. Experiments with compounds of different acidity show that the pK(a) of the protonated quinone radical is about 15 on the DMSO scale, >4 pK(a) units smaller than that of 1,3-diphenylurea. This is further proof that hydrogen bonding and not proton transfer is responsible for the large potential shifts observed with this and similar guests.

Mechanistic Study on the Electrochemical Reduction of 9,10-Anthraquinone in the Presence of Hydrogen-bond and Proton Donating Additives

“…[1][2][3][4][5] Hydrogen-bonding and proton transfer are of key importance for controlling the reduction potential and reaction path. [5][6][7][8][9][10][11][12][13][14][15] In well-buffered aqueous media, quinone-hydroquinone couples provide familiar, reversible two-electron redox systems in which half-wave reduction potentials vary with the pH in a straightforward Nernstian manner. 16 On the other hand, in dry, neutral aprotic media, quinones typically show two cathodic waves corresponding to reversible formation of the radical anion and the dianion.…”

“…It is well documented that thermodynamic stabilization of these ions by hydrogen donors significantly affects the electrochemical behavior of quinones. [6][7][8][9][10][11][12][13][14][15] Clearly different types of behavior are observed for the electrochemistry of quinones in the presence of various kinds of additives. Electrochemical and spectroelectrochemical investigations on redox systems composed of the quinone-additive pair in aprotic media, involving electron transfer coupled with hydrogenbonding and proton transfer, give much information concerning the effect of the molecular structure and environment on these basic processes.…”

“…Electrochemical and spectroelectrochemical investigations on redox systems composed of the quinone-additive pair in aprotic media, involving electron transfer coupled with hydrogenbonding and proton transfer, give much information concerning the effect of the molecular structure and environment on these basic processes. [6][7][8][9][10][11][12][13][14][15]17 Much attention has been paid to the characteristics of the redox behavior of quinones by hydrogen-bonding with weakly interacting proton donors, such as CH3OH and C2H5OH, [6][7][8][9][10][11][12][13][14][15] which is implicated in controlling both intra-and intermolecular structures in biological systems and biological function as an active site of quinoenzymes. [3][4][5] Such studies demonstrate that the first and second reduction waves of quinones move towards less-negative potential values as the concentration of the proton donor is increased, with no loss of reversibility.…”

“…7 Similar models were used to get a deeper insight into the characterization of the electron-transfer coupled with hydrogen-bond formation in quinone redox systems. [10][11][12][13][14][15] It is well-recognized that the presence of a strongly interacting donor, such as benzoic acid (BA), is characterized by disappearance of the second wave and the appearance of new waves prior to the first wave, together with a new anodic wave. 18,19 The presence of a stronger acid than BA gives similar cyclic voltammograms.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19] These would result from stabilization by hydrogen-bonding or protonation of the anionic products, either of the first or second stage of reduction, attributed to the basicity of the anionic products and the acidity of the additives. Although there is a long history of investigations of the effect of proton donors on the reduction of quinones, little systematic research has been done.…”