Mezzina Elisabetta, Cruciani Francesca, Pedulli Gian Franco, Lucarini Marco
Department of Organic Chemistry A. Mangini, University of Bologna, Via San Giacomo 11, 40126 Bologna, Italy.
Chemistry. 2007;13(25):7223-33. doi: 10.1002/chem.200601830.
EPR spectroscopy was used for the first time to explore the binding properties of cucurbit[7]uril (CB7), a representative member of the cucurbituril family. Evidence for the formation of a complex between nitroxide radicals and the host system in an aqueous solution was provided by large changes in the nitrogen hyperfine splitting, attributed to the different polar environments experienced by the included radical. In the presence of alkali cations, the EPR spectra of benzyl tert-butyl nitroxide were characterised by new signals attributed to the radical hosted in the CB7 cavity in which one metal cation is in close contact with the nitroxidic oxygen. The formation of the coordination complex results in a substantial increase in the electron spin density on the nitrogen in inverse order with respect to the size of the cation owing to increased localisation of negative charge on the oxygen atom from bonding to the alkali cation. The EPR spectra showed selective line-broadening effects as a result of metal exchange between bulk water and the coordination complex. Analysis of the EPR linewidth variations allowed us to measure the corresponding kinetic rate constants for the first time. NMR spectroscopy showed that this behaviour is not peculiar to nitroxides but is also exhibited by the related carbonyl compounds. These data allowed us to quantify the template effect and to reach the conclusion that, in the presence of a guest having a coordinating lone pair, the formation of ternary metal-guest-CB complexes must be taken into account when discussing the complexation behaviour of cucurbituril derivatives in the presence of salts.
电子顺磁共振光谱首次被用于探索葫芦脲家族的代表性成员葫芦[7]脲（CB7）的结合特性。在水溶液中，氮超精细分裂的巨大变化提供了氮氧自由基与主体体系形成复合物的证据，这种变化归因于被包合自由基所经历的不同极性环境。在碱金属阳离子存在下，苄基叔丁基氮氧自由基的电子顺磁共振光谱的特征是出现了新的信号，这些信号归因于位于CB7空腔中的自由基，其中一个金属阳离子与氮氧基团的氧原子紧密接触。配位复合物的形成导致氮上的电子自旋密度大幅增加，且与阳离子大小呈反比，这是由于与碱金属阳离子键合时，氧原子上负电荷的局域化增加所致。电子顺磁共振光谱显示，由于大量水与配位复合物之间的金属交换，出现了选择性线宽展宽效应。对电子顺磁共振线宽变化的分析使我们首次能够测量相应的动力学速率常数。核磁共振光谱表明，这种行为并非氮氧自由基所特有，相关羰基化合物也表现出这种行为。这些数据使我们能够量化模板效应，并得出结论：在存在具有配位孤对的客体时，在讨论葫芦脲衍生物在盐存在下的络合行为时，必须考虑三元金属-客体-CB复合物的形成。
