Coordination Cages Based on Bis(pyrazolylpyridine) Ligands: Structures, Dynamic Behavior, Guest Binding, and Catalysis.
Coordination Cages Based on Bis(pyrazolylpyridine) Ligands: Structures, Dynamic Behavior, Guest Binding, and Catalysis.
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DOI：
10.1021/acs.accounts.8b00261
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发表时间：
2018-08
影响因子：
18.3
通讯作者：
M. Ward;C. Hunter;Nicholas H Williams
中科院分区：
文献类型：
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作者：
M. Ward;C. Hunter;Nicholas H Williams
在这里，我们描述了一个家庭的协调笼有趣的结构，客人绑定，和催化性能。含有两个二齿吡唑基吡啶末端的柔性桥接配体与过渡金属二价阳离子组装以提供在每个顶点处含有金属离子的配位笼，桥接配体跨越每个边缘，并且金属：配体比率为2：3。这种化学计量表示为从M4 L 6四面体到M16 L24四帽截短四面体的结构，其通过在笼周边周围形成的富电子和贫电子配体片段之间形成π堆叠阵列来稳定。在某些情况下，在多个笼状结构之间发生浓度和/或温度依赖性平衡，这是由熵和焓之间的平衡引起的，熵有利于形成更多数量的较小组件，焓使较大组件中的配体间芳族堆积和疏溶剂效应最大化。笼是中空的，可以容纳客人-通常是阴离子或溶剂分子-在中央空腔。对于一个笼科来说，M8 L12物种具有近似立方体结构和大约。400 nm ~ 3的空穴，客体结合性能得到了广泛的研究。这个笼子可以容纳各种各样的中性有机客人，在水中的结合主要由疏水效应驱动，这导致结合常数高达108 M-1。大量的经验数据的积累，在水中的M8 L12笼中的客体结合提供了一个预测工具的基础上，在硅片上筛选潜在的客人使用的分子对接程序黄金，这种方法允许识别许多新的客人与准确预测的结合常数，并提供了一个变革性的新方法来探索的主机/客体化学的笼。苯并异恶唑在M8 L12笼内的结合导致肯普消除的速率显著提高-高达2 × 105倍，其中苯并异恶唑反应生成2-氰基苯酚。催化作用的产生是因为16+笼阳离子通过离子配对在表面周围积累阴离子，导致客体周围的氢氧根离子的高有效浓度，即使在本体pH值适中时也是如此。因此，催化依赖于两个正交相互作用的操作，使反应伙伴在一起：疏水客体结合在空腔中，这是内衬与CH基团的配体，和离子配对周围的高度阳离子笼表面。其结果是，在某些条件下，笼催化的肯普消除的产物（2-氰基苯酚根阴离子）本身在笼表面周围积累，并使另一个苯并异恶唑客体去质子化，以自催化方式使反应持续。因此，聚集在笼周围的不同阴离子可以作为与空腔结合的客体反应的伙伴，从而打开了M8 L12笼可以作为亲电客体与表面结合的阴离子反应的通用催化剂的可能性。
We describe here a family of coordination cages with interesting structural, guest-binding, and catalytic properties. Flexible bridging ligands containing two bidentate pyrazolylpyridine termini assemble with transition-metal dications to afford coordination cages containing a metal ion at each vertex, a bridging ligand spanning each edge, and a 2:3 metal:ligand ratio. This stoichiometry is expressed in structures ranging from M4L6 tetrahedra to M16L24 tetracapped truncated tetrahedra, which are stabilized by the formation of π-stacked arrays between electron-rich and electron-poor ligand segments that form around the cage periphery. In some cases concentration- and/or temperature-dependent equilibria between multiple cage structures occur, arising from a balance between entropy, which favors the formation of a larger number of smaller assemblies, and enthalpy, which maximizes both interligand aromatic stacking and solvophobic effects in the larger assembles. The cages are hollow and can accommodate guests-often anions or solvent molecules-in the central cavity. For one cage family, M8L12 species with an approximately cubic structure and a ca. 400 Å3 cavity, the guest binding properties have been studied extensively. This cage can accommodate a wide range of neutral organic guests, with binding in water being driven principally by the hydrophobic effect, which leads to binding constants of up to 108 M-1. The accumulation of a large amount of empirical data on guest binding in the M8L12 cage in water provided the basis for a predictive tool for in silico screening of potential guests using the molecular docking program GOLD; this methodology has allowed the identification of numerous new guests with accurately predicted binding constants and provides a transformative new approach to exploring the host/guest chemistry of cages. Binding of benzisoxazole inside the M8L12 cage results in substantial rate enhancements-by a factor of up to 2 × 105-of the Kemp elimination, in which benzisoxazole reacts to give 2-cyanophenolate. Catalysis arises because the 16+ cage cation accumulates anions around the surface by ion pairing, leading to a high effective concentration of hydroxide ions surrounding the guest even when the bulk pH is modest. Thus, the catalysis relies on the operation of two orthogonal interactions that bring the reaction partners together: hydrophobic guest binding in the cavity, which is lined with CH groups from the ligands, and ion pairing around the highly cationic cage surface. A consequence of this is that under some conditions the product of the cage-catalyzed Kemp elimination (the 2-cyanophenolate anion) itself accumulates around the cage surface and deprotonates another benzisoxazole guest, perpetuating the reaction in an autocatalytic manner. Thus, different anions accumulating around the cage can act as partners for reaction with a cavity-bound guest, opening up the possibility that the M8L12 cage can act as a general catalyst for reactions of electrophilic guests with surface-bound anions.