Ligand-to-Ligand Interactions That Direct Formation of D2-Symmetrical Alternating Circular Helicate.

Abstract

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).