Synthesis, X-ray, and Spectroscopic Study of Dissymmetric Tetrahedral Zinc(II) Complexes from Chiral Schiff Base Naphthaldiminate Ligands with Apparent Exception to the ECD Exciton Chirality

Bidentate enantiopure Schiff base ligands, (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminate (R- or S-N^O), diastereoselectively provide λ- or δ-chiral-at-metal four-coordinated Zn(R- or S-N^O)2 Ar = C6H5; Zn-1R or Zn-1S and p-C6H4OMe; Zn-2R or Zn-2S. Two R- or S-N^O-chelate ligands coordinate to the zinc(II) in a tetrahedral mode and induce λ- or δ-configuration at the zinc metal center. In the solid state, the R- or S-ligand diastereoselectively gives λ- or δ-Zn configuration, respectively, and forms enantiopure crystals. Single crystal structure determinations show two symmetry-independent molecules (A and B) in each asymmetric unit to give Z′ = 2 structures. Electronic circular dichroism (ECD) spectra show the expected mirror image relationship resulting from diastereomeric excess toward the λ-Zn for R-ligands and δ-Zn for S-ligands in solution. ECD spectra are well reproduced by TDDFT calculations, while the application of the exciton chirality method, in the common point-dipole approximation, predicts the wrong sign for the long-wavelength couplet. A dynamic diastereomeric equilibrium (λ vs δ) prevails for both R- and S-ligand-metal complexes in solution, respectively, evidenced by 1H NMR spectroscopy. Variable temperature 1H NMR spectra show a temperature-dependent shift of the diastereomeric equilibrium and confirm δ-Zn configuration (for S-ligand) to be the most stable one and favored at low temperature. DSC analyses provide quantitative diastereomeric excess in the solid state for Zn-2R and Zn-2S, which is comparable to the results of solution studies.