Mondal Pritam, Sarkar Sabyasachi, Rath Sankar Prasad
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208016, India.
Chemistry. 2017 May 23;23(29):7093-7103. doi: 10.1002/chem.201700577. Epub 2017 Apr 21.
Three cyclic zinc(II) bis-porphyrins (CB) with highly flexible linkers are employed as artificial molecular containers that efficiently encapsulate/coordinate various aromatic aldehydes within their cavities. Interestingly, the arrangements of guests and their reactivity inside the molecular clefts are significantly influenced by the cavity size of the cyclic containers. In the presence of polycyclic aromatic aldehydes, such as 3-formylperylene, as a guest, the cyclic bis-porphyrin host with a smaller cavity (CB1) forms a 1:1 sandwich complex. Upon slightly increasing the spacer length and thereby the cavity size, the cyclic host (CB2) encapsulates two molecules of 3-formylperylene that are also stacked together due to strong π-π interactions between them and CH-π interactions with the porphyrin rings. However, in the cyclic host (CB3) with an even larger cavity, two metal centers of the bis-porphyrin axially coordinate two molecules of 3-formylperylene within its cavity. Different arrangements of guest inside the cyclic bis-porphyrin hosts are investigated by using UV/Vis, ESI-MS, and H NMR spectroscopy, along with X-ray structure determination of the host-guest complexes. Moreover, strong binding of guests within the cyclic bis-porphyrin hosts support the robust nature of the host-guest assemblies in solution. Such preferential binding of the bis-porphyrinic cavity towards aromatic aldehydes through encapsulation/coordination has been employed successfully to catalyze the Knoevenagel condensation of a series of polycyclic aldehydes with active methylene compounds (such as Meldrum's acid and 1, 3-dimethylbarbituric acid) under ambient conditions. Interestingly, the yields of the condensed products significantly increase upon increasing spacer lengths of the cyclic bis-porphyrins because more substrates can then be encapsulated within the cavity. Such controllable cavity size of the cyclic containers has profound implications for constructing highly functional and modular enzyme mimics.
三种带有高度柔性连接基的环状锌（II）双卟啉（CB）被用作人工分子容器，它们能在其腔内有效地封装/配位各种芳香醛。有趣的是，客体在分子裂隙内的排列及其反应性受到环状容器腔大小的显著影响。当存在多环芳香醛如3-甲酰基苝作为客体时，具有较小腔的环状双卟啉主体（CB1）形成1:1夹心配合物。稍微增加间隔长度从而增大腔尺寸后，环状主体（CB2）封装两分子的3-甲酰基苝，由于它们之间强烈的π-π相互作用以及与卟啉环的CH-π相互作用，这两分子也堆叠在一起。然而，在具有更大腔的环状主体（CB3）中，双卟啉的两个金属中心在其腔内轴向配位两分子的3-甲酰基苝。通过紫外可见光谱、电喷雾电离质谱和核磁共振光谱以及主客体配合物的X射线结构测定，研究了环状双卟啉主体内客体的不同排列。此外，客体在环状双卟啉主体内的强结合支持了溶液中主客体组装体的稳健性质。双卟啉腔通过封装/配位对芳香醛的这种优先结合已成功用于在环境条件下催化一系列多环醛与活性亚甲基化合物（如丙二酸亚异丙酯和1,3-二甲基巴比妥酸）的Knoevenagel缩合反应。有趣的是，随着环状双卟啉间隔长度的增加，缩合产物的产率显著提高，因为此时更多的底物能够被封装在腔内。环状容器这种可控制的腔尺寸对于构建高功能和模块化的酶模拟物具有深远意义。
