“Separated” versus “Contact” Ion-Pair Structures in Solution from Their Crystalline States: Dynamic Effects on Dinitrobenzenide as a Mixed-Valence Anion

Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile
solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline
(reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB-) can be prepared in
the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of
macrocyclic polyether ligands:  LC (cryptand) and LE (crown-ethers). The crystalline “separated” ion-pair
salt isolated as K(LC)+//DNB- is crystallographically differentiated from the “contact” ion-pair salt isolated
as K(LE)+DNB- by their distinctive interionic separations. Spectral analysis reveals pronounced near-IR
absorptions arising from intervalence transitions that characterize dinitrobenzenide to be a prototypical
mixed-valence anion. Most importantly, the unique patterns of vibronic (fine-structure) progressions that
also distinguish the “separated” from the “contact” ion pair in the crystalline solid state are the same as
those dissolved into THF solvent and ensure that the same X-ray structures persist in solution. Moreover,
these distinctive NIR patterns are assigned with the aid of Marcus−Hush (two-state) theory to the “separated”
ion pair in which the unpaired electron is equally delocalized between both NO2-centers in the symmetric
ground state of dinitrobenzenide, and by contrast, the asymmetric electron distribution inherent to “contact”
ion pairs favors only that single NO2-center intimately paired to the counterion. The labilities of these dynamic
ion pairs in solution are thoroughly elucidated by temperature-dependent ESR spectral changes that provide
intimate details of facile isomerizations, ionic separations, and counterion-mediated exchanges.