Size-Matched Radical Multivalency
Size-Matched Radical Multivalency
复制标题
DOI：
10.1021/jacs.6b09892
复制
发表时间：
2017-03-22
影响因子：
15
通讯作者：
Stoddart, J. Fraser
中科院分区：
文献类型：
--
作者：
Lipke, Mark C.;Cheng, Tao;Stoddart, J. Fraser
持久性π-自由基如MV+中心点（MV是指甲基紫精，即，N，N '-二甲基-4，4'-联吡啶）参与弱的自由基-自由基相互作用。这种现象最近已被用于超分子化学中，发现MV+中心点和[环双（百草枯-对亚苯基）]（2（+中心点））（CBPQT（2（+中心点）形成强的1：1主-客体络合物[CBPQTCIVIV]（3（+中心点））。在这篇全文中，我们描述了基于自由基配对的分子识别扩展到更大的方形双自由基主体，[cydobis（paraquat-4，4 'biphenylene）]（2（+center dot））（MS 2（+center dot））。这种分子广场进行了评估，其能力，以结合可能的双自由基环蕃客人，其中包括两个自由基紫精单元，由两个邻，Meta，或对亚二甲苯桥连接，以提供不同的平面自由基之间的间距的异构系列。UV-Vis-NIR测量结果表明，只有间亚二甲苯基连接的异构体（m-CBPQT（3（+中心点）与MO 2（+中心点）内部强烈结合，导致形成四自由基复合物[MSCm-CBPQT]（4（+中心点））。滴定实验和变温UV-Vis-NIR和EPR光谱数据表明，相对于较小的三自由基复合物[CBPQTcMV]（3（+中心点）），新的主体-客体复合物形成具有更有利的焓变，其被更大的熵罚抵消。因此，[MSC m-CBPQT]（4（+中心点））的结合常数（K-a =（1.12 +/- 0.08）× 10（5）M-1）与先前测定的[CBPQTCMV]（3（+中心点））相似。通过单晶X射线衍射和密度泛函理论计算研究了MS 2（+center dot），m-CBPQT（2（+center dot））和[MScm-CBPQT]（4（+center dot））的超结构.固态和计算结构分析表明，m-CBPQT（2（+中心点））的理想尺寸为结合在MS 2（+中心点）内部。固态超结构还表明，m-CBPQT（2（+中心点））和[MSc m-CBPQT]（4（+中心点））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配对相互作用。最后，通过循环伏安法探测了[MSCm-CBPQT]（4（+中心点））的形成，表明环番的自由基状态通过自由基配对相互作用而稳定。
Persistent pi-radicals such as MV+center dot (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+center dot and [cyclobis(paraquat-p-phenylene)](2(+center dot)) (CBPQT(2(+center dot))) form a strong 1:1 host-guest complex [CBPQTCIVIV](3(+center dot)). In this full paper, we describe the extension of radical-pairing-based molecular recognition to a larger, square-shaped diradical host, [cydobis(paraquat-4,4'biphenylene)](2(+center dot)) (MS2(+center dot)). 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-xylylenelinked isomer (m-CBPQT(3(+center dot))) binds strongly inside of MO2(+center dot), resulting in the formation of a tetraradical complex [MSCm-CBPQT](4(+center dot)). Titration experiments and variable temperature UV-Vis-NIR and EPR spectroscopic data indicate that, relative to the smaller trisradical complex [CBPQTcMV](3(+center dot)), 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-a = (1.12 +/- 0.08) x 10(5) M-1) for [MSC m-CBPQT](4(+center dot)) is similar to that previously determined for [CBPQTCMV](3(+center dot)). The (super)structures of MS2(+center dot), m-CBPQT(2(+center dot)), and [MScm-CBPQT](4(+center dot)) were examined by single-crystal X-ray diffi-action measurements and density functional theory calculations. The solid-state and computational structural analyses reveal that m-CBPQT(2(+center dot)) is ideally sized to bind inside of MS2(+center dot). The solid-state superstructures also indicate that localized radical-radical interactions in m-CBPQT(2(+center dot)) and [MSc m-CBPQT](4(+center dot)) disrupt the extended radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly, the formation of [MSCm-CBPQT](4(+center dot)) was probed by cyclic voltammetry, demonstrating that the radical states of the cyclophanes are stabilized by the radical-pairing interactions.