Nickel(II) and copper(II) complexes of new unsymmetrically-substituted tetradentate Schiff base ligands: Spectral, structural, electrochemical and computational studies

Abstract

The synthesis, spectroscopic and structural characterization, electrochemical properties and theoretical studies of a series of eight robust neutral Nickel(II) and Copper(II) complexes (4–11) supported by unsymmetrically-substituted N2O2-tetradentate Schiff base ligands are reported. The M(salophen)-type compounds are substituted by either a pair of donor (anisyl, ferrocenyl, methoxy) or acceptor (fluoro, nitro) groups, forming D-π-D and A-π-A systems, respectively. The compounds were prepared in good yields by condensation of the free amino group of the desired ONN-tridentate half-unit with the appropriate substituted salicylaldehyde in the presence of hydrated Nickel(II) or Copper(II) acetate salts. They were characterized by elemental analysis, FT-IR, UV–vis, and for diamagnetic species by multinuclear NMR spectroscopy, mass spectrometry and cyclic voltammetry. The crystal structures of one Ni(II) (4) and four Cu(II) complexes (5, 7, 9 and 11) revealed a four-coordinate square-planar environment for the nickel and copper metal ions, with two nitrogen and two oxygen atoms as donors. In 4, 5, 7 and 9, the crystallization solvent interacts through hydrogen bonding with the phenolato oxygen atoms of the Schiff base pocket, while 11 packs as centrosymmetric dimers with an apical CuO short contact interaction (2.63Å). the cyclovoltammograms of the nickel complexes present an irreversible mono-electronic Ni(II)/Ni(I) reduction wave while those of their copper counterparts exhibit a reversible or quasi-reversible one-electron Cu(II)/Cu(I) redox process. The electronic structures of the eight complexes were analyzed by DFT and TD-DFT calculations.