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Keywords:

  • calcium;
  • chirality;
  • cyclens;
  • helical structures;
  • ion pairs

Abstract

A series of quadruple-stranded Na+ and Ca2+ complexes with octadentate cyclen ligands was synthesized to produce complexes that contained four different side-arm combinations (one triazole[BOND]coumarin group and three pyridine groups (1), four pyridine groups (2), one triazole[BOND]coumarin group and three quinoline groups (3), and four quinoline groups (4)). X-ray crystallographic analysis revealed that no significant changes occurred in the stereostructure of these complexes upon replacing one pyridine group with a triazole[BOND]coumarin moiety, or by replacing Na+ ions with Ca2+ ions, although the coordination number of the complexes in the solid state decreased when pyridine groups were replaced by quinoline groups. In solution, all of the side arms were arranged in a propeller-like pattern to yield an enantiomer pair of Δ and Λ forms in each metal complex. The addition of a tert-butoxycarbonyl (Boc)-protected amino acid anion, that is, a coordinative chiral carboxylate anion, to the cyclen[BOND]Ca2+ complex induced circular dichroism (CD) signals in the aromatic region by forming a 1:1 mixture of diastereomeric ternary complexes with opposite complex chirality, whilst the corresponding Na+ complexes rarely showed any response. In complexes 1-Ca2+ and 3-Ca2+, this chirality-transfer process was efficiently followed by considering the induction of the CD signals at two different wavelengths, that is, the coumarin-chromophore region and the aza-aromatic region. The sign and intensity of the CD signal were significantly dependent on both the nature of the aza-aromatic moiety and the enantiomeric purity of the external anion. These Ca2+ complexes worked as effective probes for the determination of the enantiomeric excess of the chiral anion. The cyclen[BOND]Ca2+ complexes also interacted with the non-coordinative Δ-TRISPHAT anion through an ion-pairing mechanism to achieve chirality transfer from the anion to the metal complex; both complexes 1-Ca2+ and 3-Ca2+ clearly showed induced CD signals in the coumarin-chromophore region, owing to ion-paring interactions with the Δ-TRISPHAT anion. Thus, the proper combination of an octadentate cyclen ligand and a metal center demonstrated effective chirality transfer.