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Chirality Transfer in Propeller‐Shaped CyclenCalcium(II) Complexes: Metal‐Coordinating and Ion‐Pairing Anion Procedures
Author(s) -
Ito Hiroshi,
Tsukube Hiroshi,
Shinoda Satoshi
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201204323
Subject(s) - cyclen , chemistry , chirality (physics) , stereochemistry , pyridine , moiety , enantiomer , circular dichroism , quinoline , carboxylate , metal ions in aqueous solution , chromophore , crystallography , diastereomer , metal , medicinal chemistry , photochemistry , organic chemistry , chiral symmetry breaking , physics , quantum mechanics , nambu–jona lasinio model , quark
Abstract A series of quadruple‐stranded Na + and Ca 2+ complexes with octadentate cyclen ligands was synthesized to produce complexes that contained four different side‐arm combinations (one triazolecoumarin group and three pyridine groups ( 1 ), four pyridine groups ( 2 ), one triazolecoumarin 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 triazolecoumarin moiety, or by replacing Na + ions with Ca 2+ 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 cyclenCa 2+ 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 ‐Ca 2+ and 3 ‐Ca 2+ , 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 Ca 2+ complexes worked as effective probes for the determination of the enantiomeric excess of the chiral anion. The cyclenCa 2+ 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 ‐Ca 2+ and 3 ‐Ca 2+ 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.