Department of Chemistry and Biochemistry, Ohio University, 181 Clippinger Hall, Athens, Ohio, 45701, USA.
Chemistry. 2018 Jun 18;24(34):8670-8678. doi: 10.1002/chem.201800856. Epub 2018 May 22.
To maximize Coulombic interactions, cucurbit[8]uril (CB[8]) typically forms ternary complexes that distribute the positive charges of the pair of guests (if any) over both carbonylated portals of the macrocycle. We present here the first exception to this recognition pattern. Platinum(II) acetylides flanked by 4'-substituted terpyridyl ligands (tpy) form 2:1 complexes with CB[8] in an exclusively stacked head-to-head orientation in a water/acetonitrile mixture. The host encapsulates the pair of tpy substituents, and both positive Pt centers sit on top of each other at the same CB[8] rim, leaving the other rim free of any interaction with the guests. This dramatic charge imbalance between the CB[8] rims would be electrostatically penalizing, were it not for allosteric π-π interactions between the stacked tpy ligands, and possible metal-metal interactions between both Pt centers. When both tpy and acetylides are substituted with aryl units, the metal-ligand complexes form 2:2 assemblies with CB[8] in aqueous medium, and the directionality of the assembly (head-to-head or head-to-tail) can be controlled, both kinetically and thermodynamically.
为了最大化库仑相互作用，[8]轮烷（CB[8]）通常形成三元配合物，将一对客体（如果有的话）的正电荷分布在大环的两个羰基化门控上。我们在这里介绍了这种识别模式的第一个例外。由 4'-取代的三联吡啶配体（tpy）侧翼包围的铂（II）炔烃在水/乙腈混合物中以完全堆叠的头对头取向与 CB[8]形成 2:1 配合物。主体封装了一对 tpy 取代基，并且两个正 Pt 中心位于同一 CB[8]边缘的彼此顶部，使另一侧边缘没有与客体的任何相互作用。如果不是堆叠的 tpy 配体之间的变构 π-π 相互作用，以及两个 Pt 中心之间可能的金属-金属相互作用，那么 CB[8]边缘之间的这种显著电荷不平衡将是静电惩罚性的。当两个 tpy 和炔烃都被芳基取代基取代时，金属-配体配合物在水溶液中与 CB[8]形成 2:2 组装体，并且组装体的方向性（头对头或头对尾）可以在动力学和热力学上进行控制。
