Structural Variety, Luminescent Properties, and DFT Calculations of 2-Naphthaldehyde Thiosemicarbazone and Its Complexes with Zinc(II) and Cadmium(II)

A 2-naphthyl functionalized thiosemicarbazone has been synthesized in high yield by means of a Schiff base condensation reaction. The 2-(naphthalen-2-ylmethylene)hydrazinecarbothioamide (Hnatsc, 1) reacts with zinc(II) and cadmium(II) chlorides and acetates to obtain neutral complexes of formulae: [ZnCl2(Hnatsc)2]·H2O (2·H2O), [Zn(natsc)2]·3H2O (3·3H2O), [Zn(natsc)2(Hnatsc)2]·3H2O (4·3H2O), [Zn(OAc)(natsc)(Hnatsc)]·2H2O (5·2H2O), [CdCl2(Hnatsc)2] (6), [Cd(natsc)2] (7), and [Cd(OAc)(natsc)(MeOH)] (8), in which the thiosemicarbazone functions as a monodentate thione ligand or as a deprotonated bidentate chelating ligand coordinating to the metal ions via the imine nitrogen and thiolate sulfur atoms. The characterization of all compounds was through a combination of analytical techniques, including elemental analysis, nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and electrospray ionization (ESI) mass spectrometry. The molecular and crystal structures of Hnatsc and complexes 2 and 6 were determined by means of single-crystal X-ray diffraction. Furthermore, a fluorescence study was conducted on the thiosemicarbazone and the complexes. A density functional theory (DFT) study using MEP, QTAIM, and NCIplot analyzed the supramolecular architecture, revealing that metal coordination significantly strengthens hydrogen-bonded synthons. Furthermore, TD-DFT calculations provided insight into the photophysical properties, confirming the ligand-centered nature of the electronic transitions and identifying high internal conversion rates as the primary quenching mechanism in the zinc complex 2.