Racemic Atropisomeric N,N-Chelate Ligands for Recognizing Chiral Carboxylates via Zn(II) Coordination: Structure, Fluorescence, and Circular Dichroism

Two racemic atropisomeric N,N′-chelate ligands, bis{3,3′-[N-Ph-2-(2′-py)indolyl]} (1) and bis{3,3′-N-4-[N-2-(2′-py)indolyl]phenyl-2-(2′-py)indolyl} (2), have been found to be able to distinguish the enantiomers of Zn((R)-BrMeBu)₂ and Zn((S)-BrMeBu)₂ where BrMeBu = O₂CCH(Br)CHMe₂, with a distinct and intense CD spectral response at approximately the 10 μM concentration range. Computational studies established that the (R)-1-Zn((R)-BrMeBu)₂ or (S)-1-Zn((S)-BrMeBu)₂ diastereomer is more stable than (R)-1-Zn((S)-BrMeBu)₂ or (S)-1-Zn((R)-BrMeBu)₂. In addition, computational studies showed that the CD spectra of (S)-1-Zn((S)-BrMeBu)₂ and (S)-1-Zn((R)-BrMeBu)₂ are similar. ¹H NMR spectra confirmed that these two diastereomers exist in solution in about a 2:1 ratio for both complexes of 1 and 2. The distinct CD response of the racemic ligands 1 and 2 toward the chiral zinc(II) carboxylate is therefore attributed to the preferential formation of one diastereomer. The binding modes of the zinc(II) salt with ligands 1 and 2 were established by the crystal structures of the model compounds 1-Zn(tfa)₂ and 2-Zn(tfa)₂ (tfa = CF₃CO₂⁻), where the Zn^II ion is chelated by the two central pyridyl groups in the ligand. Fluorescent titration experiments with various zinc(II) salts showed that the fluorescent spectrum of the atropisomeric ligand displays an anion-dependent change. The zinc(II) binding strength to the N,N′-chelate site of the atropisomeric ligand has been found to play a key role in the selective recognition of different chiral zinc(II) carboxylate derivatives by the racemic atropisomeric ligands.