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.

首次成功分离出纯净的硝基苯负离子自由基（NB•或硝基苯负离子），并以碱金属盐的形式呈现。通过有意识地使用辅助配体18-冠-6和[2.2.2]杯芳烃，实现了选择性形成被称为(crown)M+NB•的接触离子对，其中M+为K+、Rb+和Cs+，以及分离的离子对K(cryptand)+NB•。这两系列结构均经精确的低温X射线晶体学、ESR分析和紫外-可见光谱学表征。分离的离子对中NB•的异常共轭结构源于N−C键的显著缩短和苯环的显著喹啉形变，以此表明与硝基取代基的完全（95%）电子共轭。另一方面，接触离子对的形成导致了与阳离子尺寸成反比（K+ > Rb+）的电子共轭的显著降低，这是由于负电荷从部分（NO2）键合到碱金属阳离子的局部化程度增加所引起的。这种电子共轭的损失（或逆向电荷转移）可能看似违背直觉，但它与ESR数据中奇电子自旋电子密度的分布以及电子光谱中特征吸收带的蓝移相一致。最重要的是，这项晶体学研究强调了离子对结构对组成离子固有性质（及其反应性）的重要性——正如本研究聚焦于硝基苯负离子。