In these crystal structures, the relatively weak electrostatic interactions between the bulky CB 11 Me 12 - anion and the title cations permit cation-π interactions in the solid state. In all cases, single-crystal X-ray diffraction analysis reveals η 6 -arene-cation interactions within 10% of the expected van der Waals distance. The Tl + , Cs + , Rb + , and K + structures are isomorphous, with the benzene molecules sandwiching the cation and four anions equatorially disposed in a nearly square arrangement. Both the cation and the near-square of closest anions are positioned to interact favorably with the local dipoles of benzene. The smaller Na + crystallizes in polymeric chains with a nearly tetrahedrally coordinated cation in van der Waals contact with two anions and two benzene molecules in a tilted-sandwich arrangement. The Li + structure possesses two motifs, a simple van der Waals sandwich of a toluene molecule and an anion, and chains of half-occupied toluene-Li complexes on inversion centers between anions. The simple van der Waals model is reasonably accurate for the cation-arene distances, only slightly underestimating the separation (2-10% deviation), with worse agreement for the smaller cations.
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King et al. (1999) studied this question.