STRUCTURES AND MOLECULAR-PROPERTIES OF CH ARGE-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 i n aprotic 1,2-dimethoxyethan (DME) solution at metal mirrors and the c rystallization under Ar have been preceded by cyclovoltammetric (CV) a nd ESR/ENDOR measurement. The first electron insertion at -1.63 V prov es to be reversible, whereas the irreversible second step, which is ac companied by an overcrossing of the CV line, can be rationalized by an 'ECE-DISP' mechanism via a dianion redox disproportionation. The ENDO R spectrum resolves four H-1 couplings and allows to simulate the ESR spectrum including the N-14 hyperfine splittings. Both dark-blue singl e 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 molecul e are observed in the vicinity of the N-centers bearing the highest ef fective nuclear charge. The DME-chelated metal cations coordinate al t he N electron pairs and form Met(+)(DME)-bridged polymer chains of the radical anion, which are differently ondulated in the Na+ and K+ radi cal anion salts. The take-home lesson suggests that many more N-hetero cyclic molecules might be analogously reduced under optimized conditio ns and isolated as single crystals.