Strukturen und Molekül‐Eigenschaften ladungsgestörter Moleküle. 52. Mitteilung. 2, 3‐Diphenylchinoxalin‐Radikalanionen in Lösung und im Kristall

Abstract

Structures and Molecular Properties of Charge‐Pertubed Molecules. 2, 3‐Diphenylquinoxaline Radical Anions in Solution and in CrystalsThe Na⊕ and K⊕ radical‐ion salts of 2, 3‐diphenylquinoxaline 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 accompanied by an overcrossing of the CV line, can be rationalized by an ‘ECE‐DISP’ mechanism via a dianion redox disproportionation. 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 \documentclass{article}\pagestyle{empty}\begin{document}$[2,3{\rm - diphenylquinoxaline}^{{.} \ominus} {\rm Met}^ \oplus ({\rm DME})]^.$\end{document} 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.