Synthesis, X-ray structure, and DFT modeling of a new polymeric zinc(II) complex of 2-mercaptonicotinic acid (MntH), {[Zn(Mnt–Mnt)(en)]·H₂O}_n

Abstract: Two zinc(II) complexes of 2-mercaptonicotinic acid (MntH), {[Zn(Mnt–Mnt)(en)]·H₂O}_n and [Zn(Mnt–Mnt)(H₂O)], were prepared by the reaction of ZnCl₂ and MntH in the presence of ethylenediamine (en). They were characterized by elemental analysis, and IR, ¹H, ¹³C NMR and UV–Vis spectroscopic studies. In the presence of en, the sulfur atoms of the MntH ligands were connected and formed a disulfide linkage in the Mnt–Mnt dimer. The single crystal X-ray diffraction analysis of {[Zn(Mnt–Mnt)(en)]·H₂O}_n complex revealed the polymer structure organization. Each zinc atom is five coordinated in distorted trigonal bipyramidal polyhedron by three oxygen atoms of two bridging Mnt–Mnt ligands and two nitrogen atoms of an en molecule. The vibrational modes, which characterized the carboxylic oxygen coordination of the Mnt–Mnt ligand to Zn(II), were determined by periodic DFT/PAW/PW91 calculations. Molecular structure modeling, vibrational spectra calculations, and natural bond orbital analysis of the monomer and dimer ligands as well as of the Zn(II) complexes were performed at the DFT/TDDFT/mPW1PW91 level to explain the available IR, NMR and UV–Vis spectroscopic data and to confirm the ligand coordination to the metal ion. The formation of disulfide Mnt–Mnt ligand in [Zn(Mnt–Mnt)(H₂O)] was confirmed by the absence of the absorption band at 377 nm in the UV–Vis spectrum. The molecular modeling fragment of [Zn(Mnt–MntH)₂(H₂O)] suggested that the most probable structure is that consisting of five coordinated Zn(II) with one aqueous oxygen and four carboxylic oxygens of two bidentate bound Mnt–Mnt ligands in a polymeric structure. Graphical abstract: [Figure not available: see fulltext.].