Unconventional redox properties of hydroquinones with intramolecular OH−N hydrogen bonds

Abstract

The redox (chemical and electrochemical) properties of several hydroquinones are reported in which the OH protons are engaged in intramolecular hydrogen bonds to a nitrogen-based acceptor (pyridine or amine). The 1,4-hydroquinones generally undergo reversible oxidation to quinones in which both OH protons have transferred to the pendant bases; the oxidation processes are generally chemically and electrochemically reversible, in stark contrast with normal hydroquinones, which are oxidized irreversibly (via proton loss) to quinones. The oxidation processes, believed to occur in concerted proton/electron transfer steps, are at much lower potentials for the hydrogen-bonded derivatives relative to unsubstituted derivatives. In contrast, isomeric 1,3-hydroquinones (resorcinols) are oxidized irreversibly at relatively high potentials. The stability of some of the 1,4-hydroquinone oxidized species permits their isolation and characterization both spectroscopically and structurally. Somewhat surprisingly, in the oxidized species in which the proton is now located on the nitrogen base, the characterization data indicate that there is no NH−O hydrogen bond. Relationships between the particulars of the redox properties of the hydroquinones (potentials, reversibility/stability) and molecular structure are discussed.Résumé : Les propriétés redox (chimiques et électrochimiques) de plusieurs hydroquinones sont décrites, dans lesquelles les protons OH participent à des liaisons hydrogènes intramoléculaires avec un accepteur à base d'azote (pyridine ou amine). Les 1,4-hydroquinones subissent en général une oxydation réversible qui produit des quinones dans lesquelles les deux protons OH ont été transférés aux bases correspondantes. Les processus d'oxydation sont généralement chimiquement et électrochimique-ment réversibles, à l'opposé des hydroquinones normales qui sont oxydées de manière irréversible (par perte de proton) sous la forme de quinones. Les processus d'oxydation, dont on pense qu'ils se produisent selon des étapes de transfert concerté de proton/électron, présentent des potentiels beaucoup plus bas dans le cas des dérivés comportant des liaisons hydrogènes que dans celui des dérivés non substitués. En revanche, les 1,3-hydroquinones isomériques (résorcinols) sont oxydées de manière irréversible à des potentiels relativement élevés. La stabilité de quelques-unes des 1,4-hydroquinones oxydées permet de les isoler et de les caractériser structurellement et par spectroscopie. De façon quelque peu surprenante, les données de caractérisation indiquent que les espèces oxydées, dans lesquelles le proton est désormais situé sur la base azotée, ne possèdent pas de liaisons hydrogènes NH−O. Les liens entre les propriétés redox détaillées des hydroquinones (potentiels, réversibilité/stabilité) et la structure moléculaire sont analysés. [Traduit par la Rédaction]

Scalable Synthesis and Characterisation of a Liquid 2,3,5,6-tetraallylbenzene-1,4-diol Quinone

“…In this case, the voltammogram (Supporting Information Section S7.5) shows an irreversible QH 2 behavior as reported in the literature for hydroquinone in non-aqueous solvent. [26][27][28][29][30][31][32] The oxidation process of the quinone dianion and the hydroquinone complex, leading to subsequent dissociation into free species, appears to be a plausible mechanism. However, in the absence of a strong base the concentration of dianion should be minimal.…”

Exploring Eutectic Mixing of Quinones for Engineering High Energy Density Electrolytes

“…CV of DENQ and DIPNQ at 10 mM concentration reveals a single pair of redox peaks for each species, shown in Figure . This feature corresponds to the 2 e – , 2H + reduction of the quinone (oxidized state) to the hydroquinone (reduced state). − The redox features for DENQ and DIPNQ occur in close proximity, with E 1/2 values equal to −573 and −619 mV vs Fc/Fc + , respectively. The large peak splitting, characteristic of a kinetically slow redox process, has been reported for benzoquinones in similar electrolyte environments due to slow proton transfer from the protonated NSPIL cations …”

Exploring Eutectic Mixing of Quinones for Engineering High Energy-Density Electrolytes

“…This feature corresponds to the 2 e − , 2H + reduction of the quinone (oxidized state) to the hydroquinone (reduced state). [55][56][57] The redox features for DENQ and DIPNQ occur in close proximity, with E 1/2 values equal to -573 mV and -619 mV vs F c/F c + , respectively. The large peak splitting, characteristic of a kinetically slow redox process, has been reported for benzoquinones in similar electrolyte environments due to slow proton transfer from the protonated NSPIL cations.…”