Synthesis, Characterization, Catalytic Activity, and DFT Calculations of Zn(II) Hydrazone Complexes

Abstract

Two new Zn(II) complexes with tridentate hydrazone-based ligands (condensation products of 2-acetylthiazole) were synthesized and characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction methods. The complexes <b>1</b>, <b>2</b> and recently synthesized [Zn<b>L³</b>(NCS)₂] (<b>L³</b> = (<i>E</i>)-<i>N</i>,<i>N</i>,<i>N</i>-trimethyl-2-oxo-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)ethan-1-aminium) complex <b>3</b> were tested as potential catalysts for the ketone-amine-alkyne (KA²) coupling reaction. The gas-phase geometry optimization of newly synthesized and characterized Zn(II) complexes has been computed at the density functional theory (DFT)/B3LYP/6-31G level of theory, while the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO and LUMO) energies were calculated within the time-dependent density functional theory (TD-DFT) at B3LYP/6-31G and B3LYP/6-311G(d,p) levels of theory. From the energies of frontier molecular orbitals (HOMO-LUMO), the reactivity descriptors, such as chemical potential (<i>μ</i>), hardness (<i>η</i>), softness (<i>S</i>), electronegativity (<i>χ</i>) and electrophilicity index (<i>ω</i>) have been calculated. The energetic behavior of the investigated compounds (<b>1</b> and <b>2</b>) has been examined in gas phase and solvent media using the polarizable continuum model. For comparison reasons, the same calculations have been performed for recently synthesized [Zn<b>L³</b>(NCS)₂] complex <b>3</b>. DFT results show that compound <b>1</b> has the smaller frontier orbital gap so, it is more polarizable and is associated with a higher chemical reactivity, low kinetic stability and is termed as soft molecule.