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

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 Cu[sbnd]O short contact interaction (2.63 Å). 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.