Synthesis, spectral, electrochemical, crystal structures and nonlinear optical properties of unsymmetrical Ni(II) and Cu(II) Schiff base complexes

This paper explores the synthesis, spectroscopic and structural characterization, linear and nonlinear optical properties, as well as the electrochemical behavior of two series of robust neutral Ni(II)- and Cu(II)-centered organometallic macroacyclic salen-type Schiff base complexes, namely, the 5-nitrosubstituted derivatives M[Fc-C(O)CH = C(CH₃)N-o-C₆H₄-N = CH-(2-O,5-NO₂-C₆H₃)] (M = Ni (2), Cu (3)), and their 3,5-difluorosubstituted analogues M[Fc-C(O)CH = C(CH₃)N-o-C₆H₄-N = CH-(2-O-3,5-F₂-C₆H₂)] (M = Ni (4), Cu (5); Fc = ferrocenyl = (η⁵-C₅H₅)Fe(η⁵-C₅H₄))). Compounds 2-5 were synthesized in good yields (75-87%) in refluxing ethanol for 4 h, via template reaction by condensation of the tridentate half-unit metalloligand Fc-C(O)CH = C(CH₃)-N(H)-o-C₆H₄NH₂ (1) with 5-nitrosalicylaldehyde and 3,5-difluorosalicylaldehyde, respectively, in the presence of hydrated nickel(II) and copper(II) acetate salts, respectively. Their composition and identity were inferred from elemental analysis, FT-IR, UV-Vis, and multinuclear NMR spectroscopy. Single crystal X-ray diffraction analysis of the two 3,5-difluorosubstituted compounds 4 and 5 indicates a slightly bowed structure of the unsymmetrical Schiff base scaffold. Both Ni²⁺ and Cu²⁺ ions are tetracoordinated in a distorted square planar environment, with two nitrogen atoms and two oxygen atoms as donors. The electrochemical behavior of the four binuclear complexes was investigated by cyclic voltammetry, showing greater anodic shifts of the reversible redox process ascribed to the Fe^II/Fe^III couple of the ferrocenyl donor fragment of the 5-nitrosubstituted derivatives compared to their 3,5-difluorosubstituted counterparts. Harmonic Light Scattering measurements showed that compounds 2-5 exhibited rather high second-order nonlinear responses, found between 200 and 267 × 10^-30 esu, establishing a good correlation between redox potentials and the hyperpolarizability β_1.91 values.