Evolution from Hydrogen Bond to Proton Transfer Pathways in the Electroreduction of α-NH-Quinones in Acetonitrile

Abstract

On the basis of voltammetric behaviors obtained for the electrochemical reduction of a series of α-NH-quinones (Q) of increasing basicity (5H-benzo[b]carbazole-6,11-dione (BCD), 2-{[4'-(trifluoromethyl)phenyl]amine}-1,4-naphthalenedione 2-(phenylamine)-1,4-naphthalenedione (PAN), 2-[(4'-methoxyphenyl)amine]-1,4-naphthalenedione (p-MeOPAN), and 2,5-di(α-methylbenzylamine)-1,4-benzoquinone (DMeBABQ)), in the presence of additives with increasing acidity [ethanol (EtOH), phenol(PhOH), benzoic acid (HBz), perchloric acid it was possible to identify some of the species present in the diffusion kinetic layer, allowing to control the protonation steps and/or hydrogen bond formation prior to and/or following the electron transfer processes. EtOH and PhOH act as hydrogen bonding donors with the corresponding semiquinone radical anion, and dianion hydroquinone, electrogenerated species. Due to the low acidity level of EtOH, the hydrogen bonding association process was detected only for meanwhile, with PhOH a strong hydrogen bonding process was detected, even at 0.3 equivalents of PhOH. The diagrams of average number of "ligands" vs. log [EtOH]/[Q] for and complexes were constructed using the successive association constants obtained from the experimental half-wave potential displacements. These diagrams show that the maximum number of molecules of EtOH hydrogen bonded to and directly depends on the basicity of the corresponding quinone. With HBz, the voltamperometric behavior shows the direct protonation of the electrogenerated provoking a typical electrochemical-chemical-electrochemical mechanism, with "the chemical step" being a proton transfer. Finally, the use of a completely dissociated acid in acetonitrile solution, allowed to observe the successive interaction of protons with each of the different species appearing in the quinone/hydroquinone systems, including mono- and di-protonation of the neutral quinones. © 2004 The Electrochemical Society. All rights reserved.