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

摘要

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:L_c(cryptand)and L_E(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 NO_2-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.