Elektronentransfer und Ionenpaar-Bildung, 18 [1 -3] / Electron Transfer and Ion Pairing, 18 [ 1 - 3]

Abstract

The redox behaviour of aza-substituted naphtho- and anthraquinones, which offer O=C–C=N– chelate tongs for an advantageous five-membered ring metal cation complexation, is investigated by a combination of cyclovoltammetric and ESR/ENDOR spectroscopic measurements. The formation of paramagnetic contact ion pairs like [(quinoline-5,8-semiquinone)·⊖Me⨁]·, with Me⨁ = Li⨁, Na⨁, Tl⨁, or of triple ion radical cations like [(1,4-diazo-9,10-anthrasemiquinone)eMe⨁]·⨁, with Me⨁ = Li⨁, Na⨁ is corroborated both by shifts of the second reduction potential of up to 0,67 V for e.g. quinoline 5,8-quinone upon addition of Li⨁[B(C6H5)4]⊖ to its DMF solution and by the observation of ESR/ENDOR metal couplings.

The aza-analogues of 1,4-naphthoquinones are potent substrates and inhibitors of plasmodial thioredoxin and glutathione reductases and of human erythrocyte glutathione reductase

“…The reduction of aza-analogues of 1,4-naphthoquinone and menadione has been studied using cyclic voltammetry in organic and aqueous electrolytes. In agreement with a previous study, 59 it appears that the quinoline-5,8-dione is easier to reduce to its radical anion form than the 1,4-naphthoquinone, (E 1/2 = −0.57 and −0.66 versus SCE in DMF electrolyte, respectively). It is also known that the quinoxaline-5,8-dione is reduced at a more positive potential than the 5,8-quinolinedione in organic electrolytes.…”

Multi‐electron Reduction Capacity and Multiple Binding Pockets in Metal–Organic Redox Assembly at Surfaces

“…To test proposed electron storage capabilities of TAAQ, redox behavior was characterized using cyclic voltammetry (CV) (Figure ). Consistent with previous electrochemical investigations of N‐heteroquinones, TAAQ shows two reversible reductions (Figure ). These processes are characterized as single‐electron reductions assigned to the formation of the singly charged semiquinone radical anion, then the doubly charged catecholate dianion.…”

“…The irreversible nature of these last two reduction processes signifies instability of the triply and quadruply charged anionic species in this non‐interacting solvent environment. Previous work has demonstrated shifts towards more positive potentials of the Q − /Q 2− redox couple of quinones in the presence of polar protic solvents or when accompanied by alkali metal cation supporting electrolytes resulting from stabilization of the anionic species . These CV studies demonstrate the feasibility of the four‐electron reduction of free TAAQ, making it an appealing redox‐active ligand for augmenting the reductive capacity of its metal complexes.…”

“…TAAQ was synthesized through a Gabriel synthesis to give tetra‐aminobenzoquinone (TABQ), and then followed by a symmetric Schiff base condensation to generate the ligand in good yield . (Supporting Information Scheme S1) The ethyl substituents were added to enhance solubility for easier purification, characterization, and solution reactivity.…”

“…To fully explore the metal–ligand redox behavior and resulting electronic structure, molecular analogues of the TAAQ coordination species were synthesized. To further simplify the system, in addition to TAAQ, the analogous diethyldi‐aza‐anthraquinone (DAAQ) ligand (Scheme ) was used to limit the number of metal binding sites while maintaining the same redox properties . In place of iron, titanium‐based starting materials containing two cyclopentadienyl (Cp) ligands were used to allow for the synthesis of molecular complexes rather than polymeric chains via the two Cp group's ability to satisfy the metal's coordination needs, thus inhibiting chain growth.…”