We describe the use of two series of aryl-extended calix[4]pyrrole receptors bearing two and four electronically tunable phenyl groups, respectively, in their meso-positions as model systems for the quantification of CH-π interactions in solution. The “four-wall” and the “two-wall” receptors formed thermodynamically stable 1:1 complexes in acetonitrile solution with both trimethylamine N-oxide and trimethylphosphine P-oxide as guests. The complexes were mainly stabilized by the formation of four convergent hydrogen bonds between the oxygen atom of the guests and the pyrrole NHs of the host. In general, the N-oxide produced thermodynamically more stable hydrogen bonding interactions than the P-oxide. Upon guest binding, the receptors adopted the cone conformation and the methyl groups of the included guests engaged in CH-π interactions with the aromatic walls. We show that the modification of the electronic properties of the aromatic surfaces, in any of the receptor series, did not have a significant impact in the measured binding affinities for a given guest. However, the larger binding affinities determined for the “four-wall” receptors in comparison to the “two-wall” counterparts supported the importance of CH-π interactions on guest complexation. The strength of the CH-π interactions present in the inclusion complexes was quantified employing the octamethyl calix[4]pyrrole as reference. We determined an average magnitude of ~1 kcal·mol−1 for each CH-π interaction. The CH-π interactions featured a reduced electrostatic nature and thus dispersion forces were assigned as main contributors of their strength.
我们介绍了使用两个系列的芳基扩展
钙并[4]
吡咯受体作为模型系统,这两个受体的中位分别带有两个和四个电子可调苯基,用于定量溶液中的 CH-π 相互作用。四壁 "和 "两壁 "受体在
乙腈溶液中与作为客体的
三甲胺 N-氧化物和
三甲基膦 P-氧化物形成热力学稳定的 1:1 复合物。这些络合物主要通过客体的氧原子与宿主的
吡咯 NH 之间形成四个会聚氢键而稳定。一般来说,N-氧化物产生的氢键相互作用在热力学上比 P-氧化物更稳定。与客体结合后,受体呈锥形构象,客体中的甲基与芳香壁发生 CH-π 相互作用。我们的研究表明,在任何受体系列中,改变芳香族表面的电子特性都不会对特定客体的结合亲和力产生显著影响。然而,与 "双壁 "受体相比,"四壁 "受体的结合亲和力更大,这证明了 CH-π 相互作用对客体复合的重要性。我们以八甲基 calix[4]pyrrole 为参照物,对包合物中 CH-π 相互作用的强度进行了量化。我们确定每个 CH-π 相互作用的平均值为 ~1 kcal-mol-1。CH-π 相互作用的特点是静电性质减弱,因此分散力被认为是其强度的主要贡献者。