H. Bock et al., STRUCTURES AND MOLECULAR-PROPERTIES OF CH ARGE-PERTURBED MOLECULES .52. 2,3-DIPHENYLQUINOXALINE RADICAL-ANIONS IN SOLUTION AND IN CRYSTALS, Helvetica Chimica Acta, 77(6), 1994, pp. 1505-1519
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.