A Study of C−F···M+ Interaction:  Alkali Metal Complexes of the Fluorine-Containing Cage Compound

The C−F···M+ interaction was investigated by employing a cage compound 1 that has four fluorobenzene units. The NMR (1H, 13C, and 19F) spectra and X-ray crystallographic analyses of 1 and its metal complexes showed clear evidence of the interaction. Short C−F···M+ distances (C−F···K+, 2.755 and 2.727 Å; C−F···Cs+, 2.944 and 2.954 Å) were observed in the crystalline state of K+ ⊂ 1 and Cs+ ⊂ 1. Furthermore, the C−F bond lengths were elongated by the interaction with the metal cations. By calculating Brown's bond valence, it is shown that the contribution of the C−F unit to cation binding is comparable or greater than the ether oxygen in the crystalline state. Representative spectroscopic changes implying the C−F···M+ interaction were observed in the NMR (1H, 13C, and 19F) spectra. In particular, 133Cs−19F spin coupling (J = 54.9 Hz) was observed in the Cs+ complex.