A mononuclear cobalt(II) salophen-type complex: Synthesis, theoretical and experimental electronic absorption and...

A tetradentate Schiff base ligand, (4-nitro-N,N'-o-phenylene-bis(5-methoxysalicylideneimine)) (H₂L), and its cobalt(II) complex (CoL•DMF) were synthesized. Elemental analysis, and FT-IR and ¹H NMR spectra were used to confirm the molecular structure of H₂L ligand. In addition, elemental analysis, molar conductance measurement, infrared and electronic absorption spectroscopies as well as single crystal X-ray diffraction were employed to distinguish the molecular structure of the complex that has a slightly distorted square-planar geometry around the cobalt(II) ion. The geometries of H₂L and CoL were optimized by means of the DFT/(U)PBE0/Def2-TZVP level of theory. Good agreement of the optimized geometric parameters of the complex with the corresponding experimental ones and successfully reproduction of the FT-IR spectra of the ligand and complex validated the applied method. Based on the optimized structure of CoL, its electronic absorption spectrum was reproduced by using time-dependent density functional theory (TDDFT) at the same level. Its analysis was carried out to assign the observed electronic transitions and determine their characters. The computed nonlinear optical (NLO) parameters (β, γ and < γ > values) of H₂L and CoL by using TDDFT at the same level combined with the sum-over-states (SOS) method are considerable larger values than reference compound i.e. urea. The ligand and the complex in the absence of nitro and methoxy moieties referred to as H₂L' and CoL' respectively were optimized to elucidate the effect of these moieties on hyperpolarizabilities values.