Schiff base ligands derived from 1,2-bis(2′-nitro-/amino-phenoxy)-3-R-benzene and 2-hydroxy-1-naphthaldehyde and their Cu/Zn(ii) complexes: synthesis, characterization, X-ray structures and computational studies

Abstract

1,2-Bis(2′-nitrophenoxy)-3-R-benzenes {R = H (1), CH₃ (2) and OCH₃ (3)} have been prepared from the S_NAr reaction between 1-fluoro-2-nitrobenzene and 3-R-catechol (aromatic diol), and are then reduced to the corresponding diamines 1,2-bis(2′-aminophenoxy)-3-R-benzene {R = H (4), CH₃ (5) and OCH₃ (6)}, respectively. Reaction of these diamines with 2-hydroxy-1-naphthaldehyde gives tetradentate Schiff base ligands {H₂L: R = H (H₂L¹), CH₃ (H₂L²) and OCH₃ (H₂L³)}, respectively. The Schiff base ligands coordinate to metal(II) ions to provide the complexes ML¹, ML² and ML³ (M = Cu and Zn), respectively. X-ray molecular structure determinations were performed to explore stereochemical rigidity as the origin of chirality induction in the dinitro (2), diamine (5) and Schiff base ligands (H₂L¹ and H₂L²) as well as in the complexes (ZnL², ZnL³ and CuL³), respectively. The structures feature an N₂O₂-chromophore from bis(2-oxo-1-naphthaldiminate) moieties with one additional weak Cu/Zn⋯O (ether) contact to give a 4 + 1 coordination with a distorted square-pyramidal geometry for ZnL², ZnL³ and CuL³, respectively. Without considering this additional contact, the structures are distorted tetrahedral or distorted square-planar for the Zn or Cu complexes, such that a chiral Λ- and Δ-configuration at the metal atom is induced. All three metal complexes crystallize as a crystalline racemate or racemic mixture of Λ/Δ-ML. The supramolecular packing in the structures is organized by inter-/intra-molecular π–π and C–H⋯π interactions, respectively. The optimized structures and excited state properties from DFT/TDDFT calculations support the experimental results.