Ligand-to-Ligand Interactions That Direct Formation of D-2-Symmetrical Alternating Circular Helicate

This work demonstrates that ligand-to-ligand interactions between achiral bis-beta-diketonate (BTP) and chiral bis(4-phenyl-2-oxazolinyl)pyridine [(R)- or (S)-Ph-Pybox] are successfully directed to the fabrication of a D-2-symmetrical alternating circular helicate with the general formula [(R)- or (S)-Ph-Pybox](4)(Ln(III))(4)(BTP)(6). The lanthanide(III) Ln(III) assemblies (Ln(4)(III)-RRRR and Ln(4)(III)-SSSS) have a nanometer-size squarelike grid (interatomic distances > 10 A). X-ray structure analysis revealed that the circular helicate contains two double helicate Ln(2)(III)L(2) units, where both show (M)-helicity for Ln(4)(III)-RRRR and (P)-helicity for Ln(4)(III)-SSSS, where pi-pi stacking interaction is present between the side arm of (R)-Ph-Pybox (Ph-1) and the adjacent BTP ligand around the Eu-III metal center (d(pi pi) = 3.636 A: the diketonate planePh(1) distance). The newly obtained circular lanthanide(III) helicate exists as single and homochiral diastereomers in solution (Ln(4)(III)-RRRR and Ln(4)(III)-SSSS), exhibiting circularly dichroism (CD) and circularly polarized luminescence (CPL). Conversely, the circular helicate favors the heterochiral arrangement (i.e., Ln(4)(III)-RRRR/Ln(4)(III)-SSSS).