848223-49-4Relevant academic research and scientific papers
Separated versus contact ion-pair structures in solution from their crystalline states: Dynamic effects on dinitrobenzenide as a mixed-valence anion
Lue, Jian-Ming,Rosokha, Sergiy V.,Lindeman, Sergey V.,Neretin, Ivan S.,Kochi, Jay K.
, p. 1797 - 1809 (2005)
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(L C)+//DNB- is crystallographically differentiated from the contact ion-pair salt isolated as K(L E)+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 NO 2-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.
