Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
Materials and Process Simulation Center, California Institute of Technology , 1200 California Boulevard, Pasadena, California 91125, United States.
J Am Chem Soc. 2017 Mar 22;139(11):3986-3998. doi: 10.1021/jacs.6b09892. Epub 2017 Mar 8.
Persistent π-radicals such as MV (MV refers to methyl viologen, i.e., N,N'-dimethyl-4,4'-bipyridinum) engage in weak radical-radical interactions. This phenomenon has been utilized recently in supramolecular chemistry with the discovery that MV and [cyclobis(paraquat-p-phenylene)] (CBPQT) form a strong 1:1 host-guest complex [CBPQT⊂MV]. In this full paper, we describe the extension of radical-pairing-based molecular recognition to a larger, square-shaped diradical host, [cyclobis(paraquat-4,4'-biphenylene)] (MS). This molecular square was evaluated for its ability to bind an isomeric series of possible diradical cyclophane guests, which consist of two radical viologen units that are linked by two ortho-, meta-, or para-xylylene bridges to provide different spacing between the planar radicals. UV-Vis-NIR measurements reveal that only the m-xylylene-linked isomer (m-CBPQT) binds strongly inside of MS, resulting in the formation of a tetraradical complex [MS⊂m-CBPQT]. Titration experiments and variable temperature UV-Vis-NIR and EPR spectroscopic data indicate that, relative to the smaller trisradical complex [CBPQT⊂MV], the new host-guest complex forms with a more favorable enthalpy change that is offset by a greater entropic penalty. As a result, the association constant (K = (1.12 ± 0.08) × 10 M) for [MS⊂m-CBPQT] is similar to that previously determined for [CBPQT⊂MV]. The (super)structures of MS, m-CBPQT, and [MS⊂m-CBPQT] were examined by single-crystal X-ray diffraction measurements and density functional theory calculations. The solid-state and computational structural analyses reveal that m-CBPQT is ideally sized to bind inside of MS. The solid-state superstructures also indicate that localized radical-radical interactions in m-CBPQT and [MS⊂m-CBPQT] disrupt the extended radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly, the formation of [MS⊂m-CBPQT] was probed by cyclic voltammetry, demonstrating that the radical states of the cyclophanes are stabilized by the radical-pairing interactions.
持久性 π-自由基，如 MV（MV 是指甲紫精，即 N,N'-二甲基-4,4'-联吡啶），会发生弱自由基-自由基相互作用。最近，这种现象在超分子化学中得到了应用，因为人们发现 MV 和 [环双（对醌-对亚苯基）]（CBPQT）形成了一个强的 1:1 主客体配合物 [CBPQT⊂MV]。在这篇全文中，我们描述了基于自由基对的分子识别扩展到一个更大的正方形双自由基主体，[环双（对醌-4,4'-联苯）]（MS）。我们评估了这种分子正方形结合一系列可能的双自由基环芳烃客体的能力，这些客体由两个自由基紫精单元组成，它们通过两个邻位、间位或对位的亚甲基桥连接，以提供平面自由基之间的不同间距。UV-Vis-NIR 测量表明，只有间位亚甲基连接的异构体（m-CBPQT）强烈结合在 MS 内，形成一个四自由基配合物 [MS⊂m-CBPQT]。滴定实验和变温 UV-Vis-NIR 和 EPR 光谱数据表明，与较小的三自由基配合物 [CBPQT⊂MV]相比，新的主客体配合物形成的焓变更有利，但熵罚更大。因此，[MS⊂m-CBPQT]的结合常数（K =（1.12 ± 0.08）×10^M）与之前确定的 [CBPQT⊂MV]相似。通过单晶 X 射线衍射测量和密度泛函理论计算研究了 MS、m-CBPQT 和 [MS⊂m-CBPQT]的结构。固态和计算结构分析表明，m-CBPQT 大小适中，可结合在 MS 内。固态超结构还表明，m-CBPQT 和 [MS⊂m-CBPQT]中的局部自由基-自由基相互作用破坏了其他紫精自由基阳离子晶体中常见的扩展自由基对相互作用。最后，通过循环伏安法探测 [MS⊂m-CBPQT]的形成，证明了环芳烃的自由基态通过自由基对相互作用得到稳定。
