Molecular Recognition with Lanthanide(III) Tris(β-diketonate) Complexes:  Extraction, Transport, and Chiral Recognition of Unprotected Amino Acids

Lipophilic lanthanide complexes of fluorinated β-diketonate ligands were demonstrated to bind unprotected phenylalanine, leucine, and other amino acids under neutral conditions. Extraction experiments supported that these lanthanide tris(β-diketonates) formed highly coordinated 1:1 complexes with amino acids, although they were electrically neutralized. NMR and CD spectroscopic studies further suggested that the lanthanide tris(β-diketonates) bound amino acids at two points. Their extraction, transport, and chiral recognition behaviors were significantly controlled by a combination of central lanthanide cation and coordinating ligand: Chiral ytterbium complex offered good enantioselectivity in the extraction of unprotected amino acids, and the related praseodymium complex provided their efficient membrane transport. Thus, these lipophilic lanthanide tris(β-diketonates) were confirmed to be a new class of receptors for amino acids.