Asymmetric twisting and chirality probing properties of quadruple-stranded helicates: coordination versatility and chirality response of Na(+), Ca(2+), and La(3+) complexes with octadentate cyclen ligand.

A series of Na(+), Ca(2+), and La(3+) complexes with octadentate cyclen ligand were prepared and structurally characterized in the crystal and solution states. The employed cyclen ligand formed 6-, 7-, and 9-coordinated, crystalline complexes with Na(+), Ca(2+), and La(3+) cations, respectively, in which the parent cyclen ring and quinoline-functionalized side arms were cooperatively coordinated. These three metal cations provided the quadruple-stranded helicates in CH(3)CN-C(2)H(5)OH solutions. In each helicate, four quinoline-functionalized side arms were arranged in a propeller-like fashion to yield an enantiomer-pair of Delta- and Lambda- forms. Addition of a chiral anion to the cyclen-Ca(2+) complex solution induced circular dichroism (CD) signals around the quinoline chromophore, which indicated that 1:1 diastereomeric complexation between the Ca(2+) complex and the chiral anion imposed the stereoisomeric equilibrium. The intensity and sign of the observed CD signal were significantly dependent on both the absolute configuration and the enantiomeric purity of the added anion. The corresponding cyclen-Na(+) complex rarely induced a CD signal, while the La(3+) complex exhibited complicated anion-induced spectral changes. Thus, the octadentate cyclen ligand employed was demonstrated to form the quadruple-stranded helicate with the Ca(2+) cation in the solution state, which functioned as an effective CD probe for the determination of enantiomer excess (ee%) of the chiral anions.