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 (1)H NMR spectroscopy. Variable temperature (1)H 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.
Department of Chemistry, Jahangirnagar University , Dhaka-1342, Bangladesh.|Department of Chemistry, York University , 4700 Keele Street, Toronto, Canada.
Mohammed
M
Enamullah
Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf , Universitätsstr. 1, D-40225 Düsseldorf, Germany.
Gamall
G
Makhloufi
Department of Chemistry, Jahangirnagar University , Dhaka-1342, Bangladesh.
Rifat
R
Ahmed
Department of Chemistry, Jahangirnagar University , Dhaka-1342, Bangladesh.
Baitul Alif
BA
Joy
Department of Chemistry, Jahangirnagar University , Dhaka-1342, Bangladesh.
Mohammad Ariful
MA
Islam
Department of Chemistry, University of Pisa , via Moruzzi 13, 56124 Pisa, Italy.
Daniele
D
Padula
Department of Chemistry, York University , 4700 Keele Street, Toronto, Canada.
Howard
H
Hunter
Department of Chemistry, University of Pisa , via Moruzzi 13, 56124 Pisa, Italy.
Gennaro
G
Pescitelli
Institut für Anorganische Chemie und Strukturchemie, Universität Düsseldorf , Universitätsstr. 1, D-40225 Düsseldorf, Germany.
Christoph
C
Janiak