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.