Design, synthesis, solid-state and solution structures, non linear optical and computational studies of copper(II) complexes supported by variously substituted enantiomerically pure push-pull tetradentate Schiff base ligands

The synthesis, spectroscopic and structural characterization, optical and electrochemical properties and theoretical studies of a family of four new copper(II) complexes supported by a ferrocene-containing N₂O₂-tetradentate chiral Schiff base ligand derived from enantiomerically pure (1R,2R)-(−)-1,2-diaminocyclohexane are reported. The push-pull Cu(II) complexes were prepared following a one-pot three-component reaction involving the chiral half-unit (1R,2R)-Fc-C(=O)CH[dbnd]C(CH₃)NH-c-C₆H_10–NH_2, copper acetate monohydrate and the appropriately 3-R,5-R’-substituted salicylaldehyde derivative in a 1:1:1 ratio (Fc = ferrocenyl; 3: R=H, R’ = Cl; 4: R=H, R’ = Br; 5: R=R’ = F; 6: R=R’ = NO₂), and isolated in 65–97% yields as brown microcrystalline products. The characterization of the synthesized compounds were investigated through CHN elemental analysis, UV–vis, FT-IR, high-resolution mass spectrometry, and X-ray diffraction analysis in the case of the difluoro substituted complex 5 that was obtained in the form of single crystals. It crystallizes in the orthorhombic non-centrosymmetric space group P2₁2₁2₁, with two (R,R)-(−)-chiral carbon atoms in the structure. The four-coordinate Cu(II) metal ion adopts a slightly distorted square planar geometry. Magnetic properties of powdered samples have been investigated (2-300 K) and found consistent with a single isolated copper(II) ion (s = 1/2). Cyclic voltammetry showed that the stronger the electron withdrawing effect of the 3,5-substituents, the more anodically shifted the oxidation potential (E_1/2) of the ferrocenyl moiety, following the order: 5-Cl < 5-Br < 3,5-F₂ < 3,5-(NO₂)₂. This trend is similar when considering the values of the second-order polarizabilities β of these compounds as measured using Harmonic Light Scattering (HLS) at 1.9 µm, confirming the increasing electron withdrawing effect evidenced by electrochemical studies as well as the conjugation between the electron donor ferrocenyl moiety and the different halogen or nitro substituents through the square planar copper Schiff base framework. Finally, DFT and TD-DFT calculations allow to rationalize the structure and properties of the complexes.