Lithium isotope discrimination by the anion radical of an anthraquinone-15-crown-5 lariat ether

Abstract

The reduction potentials, or solution electron affinities, of the ⁶Li⁺ and the ⁷Li⁺ complexes of (9,10-dioxoanthracen-1-yloxymethyl)-15-crown-5 have been measured in acetonitrile, and in mixtures of acetonitrile and dichloromethane, to include 50 and 75%, by volume, of the latter cosolvent. While in acetonitrile the reduction potentials are the same within experimental error, in the presence of 50 and 75% dichloromethane, the ⁶Li⁺ complex is anodically shifted relative to its ⁷Li⁺ counterpart, by 7 ± 3 and 12 ± 3 mV, respectively. The preceding differences in the reduction potentials suggest equilibrium constant values of ca. 1.3 ± 0.1 and 1.6 ± 0.2, in turn, for electron exchange between ⁷Li⁺ and ⁶Li⁺ complexes of the macrocyclic donor, whereby the anionic charge resides preferentially in the latter electrophore.