Alkali metal cation-pi interactions observed by using a lariat ether modelsystem

The Na+ or K+ cation-pi interaction has been experimentally probed by using synthetic receptors that comprise diaza-18-crown-6 lariat ethers having et hylene sidearms attached to aromatic pi -donors. The side chains are 2-(3-i ndolyl)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-pentafluor ophenylethyl (12), and 2-(1-naphthyl)ethyl (13). Solid-state structures are reported for six examples of alkali metal complexes in which the cation is pi -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 counteranio n is excluded from the solvation sphere. NMR measurements in acetone-d(6) s olution 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 indo le is the pi -donor for the alkali metal cation. Cation-pi complexes were a lso observed fur 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 pentafluoro phenyl 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-pi complexation is not observed. Steric and electrostatic considerations in the pi -complexation of alkali metal cations by these lar iat ethers are thought to account fur the observed complexation behavior or lack thereof.