Structures and Molecular Properties of Charge-Perturbed Molecules. 2,3-Diphenylquinoxaline Radical Anions in Solution and in Crystals

Article abstract of DOI:10.1002/hlca.19940770606

The Na⁺ and K⁺ radical-ion salts of 2,3-diphenylquinoxyline seem to be (according to a structural database search) among the first ones of N-heterocyclic radical anions in crystals.The one-electron reduction in aprotic 1,2-dimethoxyethan (DME) solution at metal mirrors and the crystallization under Ar have been preceded by cyclovoltammetric (CV) and ESR/ENDOR measurement.The first electron insertion at -1.63 V proves to be reversible, whereas the irreversible second step, which is accompained by an overcrossing of the CV line, can be rationalized by an "ECE-DISP" mechanism via a dianion redox disproportination.The ENDOR spectrum resolves four 1H couplings and allows to simulate the ESR spectrum including the 14N hyperfine splittings.Both dark-blue single crystals of the radical ion salts <2,3-diphenylquinoxaline^.- Met⁺ (DME)> show unexpected similarities for Met⁺ = Na⁺, K⁺ despite the 36-pm difference in their ionic radii.The largest structural changes inflicted by the one-electron reduction of the N-heterocyclic molecule are observed in the vicinity of the N-centers bearing the highest effective nuclear charge.The DME-chelated metal cations coordinate at the N electron pairs and form Met⁺(DME)-bridged polymer chains of the radical anion, which are differently ondulated in the Na⁺ and K⁺ radical anion salts.The take-home lesson suggests that many more N-heterocyclic molecules might be analogously reduced under optimized conditions and isolated as single crystals.