Size-Matched Radical Multivalency

作者：Lipke Mark C; Cheng Tao; Wu Yilei; Arslan Hasan; Xiao Hai; Wasielewski Michael R; Goddard William A III; Stoddart J Fraser^*

来源：Journal of the American Chemical Society, 2017, 139(11): 3986-3998.

DOI：10.1021/jacs.6b09892

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.

• 出版日期2017-3-22
• 单位西北大学