Ligand-to-Ligand Interactions That Direct Formation of <i>D</i>₂‑Symmetrical Alternating Circular Helicate

This
work demonstrates that ligand-to-ligand interactions between
achiral bis-β-diketonate (BTP) and chiral bis­(4-phenyl-2-oxazolinyl)­pyridine
[(R)- or (S)-Ph-Pybox] are successfully
directed to the fabrication of a D2-symmetrical
alternating circular helicate with the general formula [(R)- or (S)-Ph-Pybox]4(LnIII)4(BTP)6. The lanthanide­(III) LnIII assemblies (LnIII4-RRRR and LnIII4-SSSS) have a nanometer-size squarelike grid (interatomic
distances > 10 Å). X-ray structure analysis revealed that
the
circular helicate contains two double helicate LnIII2L2 units, where both show (M)-helicity
for LnIII4-RRRR and (P)-helicity for LnIII4-SSSS, where π–π stacking interaction is present between
the side arm of (R)-Ph-Pybox (Ph1) and
the adjacent BTP ligand around the EuIII metal center (dππ = 3.636 Å: the diketonate
plane···Ph1 distance). The newly obtained
circular lanthanide­(III) helicate exists as single and homochiral
diastereomers in solution (LnIII4-RRRR and LnIII4-SSSS), exhibiting
circularly dichroism (CD) and circularly polarized luminescence (CPL).
Conversely, the circular helicate favors the heterochiral arrangement
(i.e., LnIII4-RRRR/LnIII4-SSSS).