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

1,2-Bis(2'-nitrophenoxy)-3-R-benzenes {R = H (1), CH3 (2) and OCH3 (3)} have been prepared from the S-N-Ar 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 center dot center dot center dot 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 Lambda- and Delta-configuration at the metal atom is induced. All three metal complexes crystallize as a crystalline racemate or racemic mixture of Lambda/Delta-ML. The supramolecular packing in the structures is organized by inter-/intra-molecular pi-pi and C-H center dot center dot center dot pi interactions, respectively. The optimized structures and excited state properties from DFT/TDDFT calculations support the experimental results.