Cation-pi interactions in the solid state: Crystal structures of M+(benzene)(2)CB11Me12- (M = Tl, Cs, Rb, K, Na) and Li+(toluene)CB11Me12-

In these crystal structures, the relatively weak electrostatic interactions between the bulky CB,,Me;, anion and the title cations permit cation-pi in teractions in the solid state. In all cases, single-crystal X-ray diffracti on analysis reveals eta(6)-arene-cation interactions within 10% of the expe cted van der Waals distance. The Tl+, Cs+, Rb+, and K+ structures are isomo rphous, with the benzene molecules sandwiching the cation and four anions e quatorially disposed in a nearly square arrangement. Both the cation and th e near-square of closest anions are positioned to interact favorably with t he local dipoles of benzene. The smaller Na+ crystallizes in polymeric chai ns 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 co mplexes on inversion centers between anions. The simple van der Waals model is reasonably accurate for the cation-arene distances, only slightly under estimating the separation (2-10% deviation), with worse agreement for the s maller cations.