STRUCTURES AND MOLECULAR-PROPERTIES OF CHARGE-PERTURBED MOLECULES .52. 2,3-DIPHENYLQUINOXALINE RADICAL-ANIONS IN SOLUTION AND IN CRYSTALS

The 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 H-1 couplings and allows to simulate the ESR spectrum including the N-14 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 al 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.