Crystallographic Distinction between “Contact” and “Separated” Ion Pairs:  Structural Effects on Electronic/ESR Spectra of Alkali-Metal Nitrobenzenides

The classic nitrobenzene anion-radical (NB^-• or nitrobenzenide) is isolated for the first time as pure crystalline alkali-metal salts. The deliberate use of the supporting ligands 18-crown-6 and [2.2.2]cryptand allows the selective formation of contact ion pairs designated as (crown)M⁺NB^-•, where M⁺ = K⁺, Rb⁺, and Cs⁺, as well as the separated ion pair K(cryptand)⁺NB^-•both series of which are structurally characterized by precise low-temperature X-ray crystallography, ESR analysis, and UV−vis spectroscopy. The unusually delocalized structure of NB^-• in the separated ion pair follows from the drastically shortened N−C bond and marked quinonoidal distortion of the benzenoid ring to signify complete (95%) electronic conjugation with the nitro substituent. On the other hand, the formation of contact ion pairs results in the substantial decrease of electronic conjugation in inverse order with cation size (K⁺ > Rb⁺) owing to increased localization of negative charge from partial (NO₂) bonding to the alkali-metal cation. Such a loss in electronic conjugation (or reverse charge transfer) may be counterintuitive, but it is in agreement with the distribution of odd-electron spin electron density from the ESR data and with the hypsochromic shift of the characteristic absorption band in the electronic spectra. Most importantly, this crystallographic study underscores the importance of ion-pair structure on the intrinsic property (and thus reactivity) of the component ionsas focused here on the nitrobenzenide anion.