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

• DOI: 10.1021/acs.inorgchem.6b00403
• Publication Date: 2016-06-13T13:42:29Z
• Authors: Mohammed Enamullah; Gamall Makhloufi; Rifat Ahmed; Baitul Alif Joy; Mohammad Ariful I...; Daniele Padula; Howard Hunter; Gennaro Pescitelli; Christoph Janiak
• Keywords: Diastereomer; Enantiopure drug; Chirality

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