Divalent transition metal complexes of nitrogen, oxygen and sulfur containing ligand: design, structural, spectral, pH-metric, theoretical molecular modeling, analytical and mechanism studies.

In the present study, the 1-Nicotinoyl-4-phenyl thiosemicarbazide (H₂NPT) multidentate nitrogen, oxygen and sulfur containing ligand is being synthesized in a simple single step reaction by condensing 1:1 molar ratio of ethanolic solutions of both phenyl isothiocyanate and nicotinoyl hydrazine. The prepared H₂NPT and its complexes with Co²⁺, Ni²⁺ and Zn²⁺ and Cu²⁺ were characterized by pH-metric titrations, elemental analysis, FTIR, electronic spectra and thermogravimetric (TGA) analyses. The association constant of the ligand and the stability constants of its complexes were calculated by pH-metric measurements in 50% ethanol–water mixture. Also, DFT method was used to draw the geometry of all compounds and the parameters such as bond lengths, bond angles, dipole moment, Frontier orbitals (HOMO, LUMO), MEP and other energetic parameters (optical energy gap, softness, hardness, electronegativity) were evaluated. The prepared H₂NPT reacted with Co²⁺, Ni²⁺ and Zn²⁺ and Cu²⁺ metal ions to form colored precipitates that were readily floated to the surface solution using oleic acid (HOL) surfactant with vigorous shaking of the flotation cell. The heavy metals' concentrations in the dissolved precipitates are determined by flame atomic absorption spectrometry (FAAS). The different parameters affecting the flotation process were thoroughly investigated, viz. pH of sample solution, concentration of ligand, metal ions and HOL, temperature and interfering ions. Nano-gram quantities of Cu²⁺, Co²⁺, Ni²⁺ and Zn²⁺in 1500-ml samples are quantitatively determined with 1-Nicotinoyl-4-phenyl thiosemicarbazide (H₂NPT) at pH 3.0–6.5 for Cu²⁺ and at 7.0–7.5 for Co²⁺, Ni²⁺ and Zn²⁺. The concentrations of these heavy metals are increased 300-fold. Interferences, on the flotation process, from various foreign ions were avoided by adding excess H₂NPT. The mode of chelation between H₂NPT and the investigated metal ions is proposed to be through the S, N and/or O atoms of the ligand. The mechanism of flotation is proposed to be due to an electrostatic interaction between the HOL surfactant and the formed metal–ligand complexes through H-bond formation. The proposed multi-element flotation methodology has been utilized for the determination of the analytes in certified reference materials, alloys, synthetic mixtures and in water samples with a recovery % more than 90% and a relative standard deviation (RSD), as a precision < 2%. [ABSTRACT FROM AUTHOR]