Collect. Czech. Chem. Commun. 1999, 64, 1001-1012

Cation-π Interactions in the Solid State: Crystal Structures of M+(benzene)2CB11Me12- (M = Tl, Cs, Rb, K, Na) and Li+(toluene)CB11Me12-

Benjamin T. King, Bruce C. Noll and Josef Michl*

University of Colorado, Department of Chemistry and Biochemistry, Boulder, Colorado 80309-0215, U.S.A.


In these crystal structures, the relatively weak electrostatic interactions between the bulky CB11Me12- anion and the title cations permit cation-π interactions in the solid state. In all cases, single-crystal X-ray diffraction analysis reveals η6-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.

Keywords: Crystal structure determination; Noncovalent interactions; Cation-π interactions; Carborane anions; Boranes; Carboranes; Sandwich complexes; Arenes; Alkali metal cations.