Size-Matched Radical Multivalency

Persistent π-radicals such as <b>MV</b>^<b>+•</b> (<b>MV</b> refers to methyl viologen, i.e., <i>N</i>,<i>N</i>′-dimethyl-4,4′-bipyridinum) engage in weak radical–radical interactions. This phenomenon has been utilized recently in supramolecular chemistry with the discovery that <b>MV</b>^<b>+</b>• and [cyclobis­(paraquat-<i>p</i>-phenylene)]^{2(<b>+</b>•)} (<b>CBPQT</b>^{<b>2(+•)</b>}) form a strong 1:1 host–guest complex <b>[CBPQT⊂​​MV]</b>^{<b>3(+•)</b>}. 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)]^{2(<b>+</b>•)} (<b>MS</b>^{<b>2(+•)</b>}). 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 <i>ortho-</i>, <i>meta-</i>, or <i>para-</i>xylylene bridges to provide different spacing between the planar radicals. UV–Vis–NIR measurements reveal that only the <i>m</i>-xylylene-linked isomer (<i><b>m-</b></i><b>CBPQT</b>^{<b>2(+•)</b>}) binds strongly inside of <b>MS</b>^{<b>2(+•)</b>}, resulting in the formation of a tetraradical complex <b>[MS⊂​</b><i><b>m</b></i><b>-CBPQT]</b>^{<b>4(+•)</b>}. Titration experiments and variable temperature UV–Vis–NIR and EPR spectroscopic data indicate that, relative to the smaller trisradical complex <b>[CBPQT⊂​MV]</b>^{<b>3(+•)</b>}, 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 (<i>K</i>_a = (1.12 ± 0.08) × 10⁵ M^–1) for <b>[MS⊂​</b><i><b>m</b></i><b>-CBPQT]</b>^{<b>4(+•)</b>} is similar to that previously determined for <b>[CBPQT⊂​MV]</b>^{<b>3(+•)</b>}. The (super)­structures of <b>MS</b>^{<b>2(+•)</b>}, <i><b>m-</b></i><b>CBPQT</b>^{<b>2(+•)</b>}, and <b>[MS⊂​</b><i><b>m</b></i><b>-CBPQT]</b>^{<b>4(+•)</b>} were examined by single-crystal X-ray diffraction measurements and density functional theory calculations. The solid-state and computational structural analyses reveal that <i><b>m-</b></i><b>CBPQT</b>^{<b>2(+•)</b>} is ideally sized to bind inside of <b>MS</b>^{<b>2(+•)</b>}. The solid-state superstructures also indicate that localized radical–radical interactions in <i><b>m-</b></i><b>CBPQT</b>^{<b>2(+•)</b>} and <b>[MS⊂​</b><i><b>m</b></i><b>-CBPQT]</b>^{<b>4(+•)</b>} disrupt the extended radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly, the formation of <b>[MS⊂​</b><i><b>m</b></i><b>-CBPQT]</b>^{<b>4(+•)</b>} was probed by cyclic voltammetry, demonstrating that the radical states of the cyclophanes are stabilized by the radical-pairing interactions