Macrocyclic receptor exhibiting unprecedented selectivity for light lanthanides.

Area of Science:

• Coordination Chemistry
• Supramolecular Chemistry
• Lanthanide Chemistry

Background:

• Macrocyclic ligands are crucial for selective metal ion complexation.
• Lanthanide ions exhibit unique properties due to their electronic structure.
• Understanding lanthanide complexation is vital for applications in catalysis, imaging, and materials science.

Purpose of the Study:

• To design and synthesize a novel macrocyclic ligand, H(2)bp18c6, for lanthanide ion complexation in aqueous media.
• To investigate the selectivity of the ligand for different lanthanide ions.
• To elucidate the structural and conformational changes of lanthanide complexes in solution.

Main Methods:

• Potentiometric titrations to determine complex stability constants (log K(ML)).
• X-ray crystallography to determine solid-state structures of Gd(III) and Yb(III) complexes.
• NMR spectroscopy (1H and 13C) and DFT calculations to study solution structures and conformations.
• Paramagnetic NMR shifts to confirm structural changes.

Main Results:

• H(2)bp18c6 exhibits unprecedented selectivity for larger lanthanide(III) ions (Ln(III)).
• Stability constants (log K(ML)) decrease significantly from Ce(III) to Lu(III) (difference of 6.9).
• X-ray structures reveal direct binding of Ln(III) to all 10 donor atoms.
• DFT calculations predict a conformational change in the ligand around the middle of the lanthanide series.
• Structural changes confirmed by NMR spectroscopy and paramagnetic shifts.

Conclusions:

• H(2)bp18c6 is a highly selective ligand for larger lanthanide ions.
• The ligand's selectivity arises from optimal steric fit with larger Ln(III) ions.
• A conformational flexibility of the macrocycle allows adaptation to different lanthanide ion sizes.
• The study provides insights into the structure-selectivity relationship in lanthanide complexation.