Spin pairing, electrostatic and dipolar interactions influence stacking of radical anions in alkali salts of 4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile (DDQ)

Abstract

A series of five novel air-stable alkali salts of the 4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile radical anion (DDQ) were prepared and structurally characterised. Four of them (Li, Na and two Cs salts) involve Peierls-distorted π-stacks of radicals, while the K salt-structure comprises equidistant stacks of radicals. Different orientations of radicals in the stacks and different directions of the offset have been observed. For an unambiguous description of the π-stacking geometry of quinones and semiquinone radicals, the geometric parameters calculated by the program PLATON are not sufficient. Therefore, two additional geometric parameters are introduced: δ (torsion angle between molecular axes OC⋯CO of contiguous rings) and ε (angle between the molecular axis OC⋯CO and direction of the offset). On the basis of these geometric parameters, the optimal stacking orientations can be quantified and correlated with the polarity and steric effects.