Alkali Metal Cation−π Interactions Observed by Using a Lariat Ether Model System

The Na+ or K+ cation−π interaction has been experimentally probed by using synthetic receptors that comprise diaza-18-crown-6 lariat ethers having ethylene sidearms attached to aromatic π-donors. The side chains are 2-(3-indolyl)ethyl (7), 2-(3-(1-methyl)indolyl)ethyl (8), 2-(3-(5-methoxy)indolyl)ethyl (9), 2-(4-hydroxyphenyl)ethyl (10), 2-phenylethyl (11), 2-pentafluorophenylethyl (12), and 2-(1-naphthyl)ethyl (13). Solid-state structures are reported for six examples of alkali metal complexes in which the cation is π-coordinated by phenyl, phenol, or indole. Indole-containing crown, 7, adopts a similar conformation when bound by NaI, KI, KSCN, or KPF6. In each case, the macroring and both arenes coordinate the cation; the counteranion is excluded from the solvation sphere. NMR measurements in acetone-d6 solution confirm the observed solid-state conformations of unbound 7 and 7·NaI. In 7·Na+ and 7·K+, the pyrrolo, rather than benzo, subunit of indole is the π-donor for the alkali metal cation. Cation−π complexes were also observed for 10·KI and 11·KI. In these cases, the orientation of the cation on the aromatic ring is in accord with the binding site predicted by computational studies. In contrast to the phenyl case (11) the pentafluorophenyl group of 12 failed to coordinate K+. Solid-state structures are also reported for 7·NaPF6, 10·NaI, 11·NaI, 13·KI, 13·KPF6, and 9·NaI, in which cation−π complexation is not observed. Steric and electrostatic considerations in the π-complexation of alkali metal cations by these lariat ethers are thought to account for the observed complexation behavior or lack thereof.