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

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 ( p ­ CF 3 PAN ) , 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 ( HClO 4 ) ] , 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, ( Q • − ) , and dianion hydroquinone, ( Q 2 − ) , electrogenerated species. Due to the low acidity level of EtOH, the hydrogen bonding association process was detected only for [ EtOH ] / [ Q ] ratio > 100 ; meanwhile, with PhOH a strong hydrogen bonding process was detected, even at 0.3 equivalents of PhOH. The diagrams of average number of “ligands” ( n av ) vs. log [EtOH]/[Q] for Q • − ( EtOH ) n and Q 2 − ( EtOH ) m 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 Q • − and Q 2 − directly depends on the basicity of the corresponding quinone. With HBz, the voltamperometric behavior shows the direct protonation of the electrogenerated Q • − provoking a typical electrochemical-chemical-electrochemical mechanism, with “the chemical step” being a proton transfer. Finally, the use of a completely dissociated acid HClO 4 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.