Solvent Effects upon Guest Binding and Dynamics of a FeII4L4 Cage

Solvent-dependent host–guest chemistry and favoring of otherwise disfavored conformations of large guests has been achieved with an adaptive, self-assembled FeII4L4 coordination cage. Depending on the counterion, this face-capped tetrahedral capsule is soluble either in water or in acetonitrile and shows a solvent-dependent preference for encapsulation of certain classes of guest molecules. Quantitative binding studies were undertaken, revealing that both aromatic and aliphatic guests bind in water, whereas only aliphatic guests bind in acetonitrile. The flexibility of its subcomponent building blocks allows this cage to expand or contract upon guest binding, as studied by VT-NMR, thereby ensuring strong binding of both small and large guests. Upon encapsulation, large guest molecules can adopt conformations which are not thermodynamically favored in the free state. In addition, the chirotopic inner phase of the cage renders enantiotopic guest proton signals diastereotopic in specific cases.

借助自适应自组装的FeII4L4配位笼（FeII4L4 coordination cage），本研究成功实现了溶剂依赖型主客体化学（host–guest chemistry），并稳定了大型客体分子在游离状态下原本热力学上难以占据的构象。该面封盖四面体胶囊（face-capped tetrahedral capsule）可通过调整抗衡离子（counterion）种类实现在水或乙腈中的溶解，并对特定类别客体分子的包合表现出溶剂依赖的选择性偏好。通过定量结合实验发现，芳香族与脂肪族客体分子均可在水中与该配位笼结合，而在乙腈中仅脂肪族客体可实现结合。变温核磁共振（VT-NMR）研究显示，该配位笼的亚组分组装单元具有柔性，可在客体结合过程中发生膨胀或收缩，从而确保对大小型客体均具备高强度结合能力。客体被包合后，大型客体分子可采取在游离状态下热力学上不占优势的构象。此外，该配位笼的手性内部微环境会在特定情况下使原本对映异位的客体质子信号转化为非对映异位信号。