UNUSUAL 1-2 LIGAND-METAL COMPLEX-FORMATION BETWEEN AN ANTHRAQUINONE CRYPTAND AND LI+

A new cryptand containing an integral anthraquinone unit, 2, has been synthesized. Electrochemical reduction of 2 in MeCN showed the two expected quasi-reversible waves at -1.03 and -1.40 V vs Ag/AgCl. Upon addition of 1 equiv of Li+, a total of six reduction waves could clearly be seen at -0.44, -0.69, -0.97, -1.07, -1.31, and -1.48 V. The six waves correspond to the two electron reduction processes leading to the dianionic state of each of the three forms of the ligand: the free cryptand, the 1:1 complex, and 1:2 L:Li+ complex. This is the first time that a 1:2 complex of this type has been detected by cyclic voltammetry. The existence of this 1:2 complex (2.-:2Li+) was confirmed by ESR spectroscopy and by a binding study using Li-7 NMR. The binding constant for this 1:2 complex was found to be 1.7 x 10(2), but this value was increased to 1.0 X 10(8) upon one-electron reduction and to 1.3 x 10(13) upon two-electron reduction. Even in the presence of only 1 equiv of Li+/equiv of 2.-, the ESR spectrum shows a hyperfine splitting from two equivalent Li+ cations of 0.20 G. A possible structure for this 1:2 complex is proposed, in which the two cations interact with the same carbonyl group on the anthraquinone, while they are simultaneously solvated by the enveloping crown ether. 1:2 complex formation with Na+ and K+ has also been detected by using voltammetry and ESR. Since hyperfine coupling from only one cation is resolved, an unsymmetric complex is proposed. In this structure, the cation exhibiting the hyperfine splitting is inside the cryptand cavity. The other is assumed to be interacting with the external carbonyl and to be in relatively fast exchange with the solvent. While the 2.-:Na+ complex can be dissociated by addition of cryptand [2.2.11, 2.-:K+ complex cannot be fully dissociated by addition of cryptand [2.2.2]. This implies that reduced 2 is at least as good a K+ binder as cryptand [2.2.2].