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

Abstract

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 4h, 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 FeII/FeIII couple of the ferrocenyl donor fragment of the 5-nitrosubstituted derivatives compared to their 3,5-difluorosubstituted counterparts. Harmonic Light Scattering measurementsshowed 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 β1.91 values.