Bis[N-2-(R-pyridyl)-2-oxo-1-naphthaldiminato-κ2N^O]copper(II) (R = H, 4/6-CH3): Combined studies on syntheses, spectroscopy, cyclic voltammetry, thermal analyses, crystal structure and DFT/TDDFT

•Bis[N-2-(R-pyridyl)-2-oxo-1-naphthaldiminato-κ2N,O]copper(II) with square-planar N2O2-coordination sphere.•Quasi-reversible two electron charge transfer process in acetonitrile.•Irreversible phase transformation from crystal to crystal-liquid, and to isotropic liquid.•Copper(II)-N^O-chelate complexes with one unpaired electron.•DFT/TDDFT simulations support the experimental results.

Reaction of pyridyl Schiff base ligands, N-2-(R-pyridyl)-2-hydroxy-1-naphthaldimine (HL) with copper(II) acetate provides the bis[N-2-(R-pyridyl)-2-oxo-1-naphthaldiminato-κ2N^O]copper(II) (R = H: 1, 4/6-CH3: 2/3). Electronic spectra show the bands at <350, 350–500, and 500–850 nm for intra-ligand, metal-ligand, metal-metal transitions, respectively in dichloromethane. Thermal analyses demonstrate an irreversible phase transformation from crystalline to crystal-liquid, and to isotropic liquid phase. Cyclic voltammogram exhibits a quasi-reversible two electrons transfer for [Cu(L)2]/[Cu(L)2]− and [Cu(L)2]−/[Cu(L)2]2− couples, respectively in acetonitrile. Magnetic moment values {μ = 1.35 (1), 1.40 (2), and 1.42 (3) μB at 23 °C} indicate the formation of copper(II)-N^O chelate complexes with one unpaired electron in dichloromethane. X-ray structure determination for 3 verifies its roughly square-planar coordination by two N^O-chelating ligands to the copper(II). Optimized geometry by DFT corresponds well to the experimental structures. Electronic spectra by TDDFT explore a combined metal-metal, metal-ligand, and ligand-ligand transitions band with the highest MO contributions of 98% at ca. 388 nm (HOMO−1 to LUMO+1 for 1) or 99% at ca. 413 nm (HOMO to LUMO for 3), very close to the experimental results.

Graphical abstractTwo N^O-chelates Schiff base ligands form a roughly square-planar N2O2-coordination sphere around the copper atom in bis[N-2-(R-pyridyl)-2-oxo-1-naphthaldiminato-κ2N^O]copper(II) as evidenced by X-ray structure determination.Figure optionsDownload full-size imageDownload as PowerPoint slide