Synthesis, characterization and molecular sensing behavior of [ZnCl 2(η 3-N,N,O-dpkbh)] (dpkbh = di-2-pyridyl ketone benzoyl hydrazone)

Abstract

The reaction between ZnCl 2 and di-2-pyridyl ketone benzoyl hydrazone (dpkbh) in acetonitrile under ultrasonic or reflux conditions gave [ZnCl 2(η 3-N,N,O-dpkbh)] in good yield. The identity of the compound was established from the results of its elemental analysis and a number of spectroscopic measurements. Solid-state infrared spectra of [ZnCl 2(η 3-N,N,O-dpkbh)] reveal the coordination of dpkbh, the presence of the amide proton and binding of the oxygen atom of dpkbh. 1H NMR spectra of [ZnCl 2(η 3-N,N,O-dpkbh)] show sensitivity to solvent and temperature variations and confirm the coordination of dpkbh. The electronic absorption spectra of [ZnCl 2(η 3-N,N,O-dpkbh)] in non-aqueous media show high sensitivity to changes in their surroundings and divulge two interlocked intra-ligand-charge transfer (ILCT) transitions of the donor–acceptor type between 300 and 500 nm. Optical measurements show reversible inter-conversion between two forms of [ZnCl 2(η 3-N,N,O-dpkbh)] that may be due to complex–substrate interactions and thermodynamic analysis gave changes in enthalpy (Δ H θ ) of −23.3 and +14.1 kJ mol −1, entropy (Δ S θ ) of −51 and +31 J mol −1K −1, and free energy (Δ G θ ) of −8.16 and +4.54 kJ mol −1 at 298.15 K in DMF and DMSO, respectively. Substrates in concentrations as low as 10 −5 M can be detected and determined using [ZnCl 2(η 3-N,N,O-dpkbh)] in polar solvents. A provisional model for the binding of substrate to [ZnCl 2(η 3-N,N,O-dpkbh)] is developed and a binding constant in the range 6000–8000 M −1 is obtained in the case of ZnCl 2 in DMSO. Electrochemical measurements on [ZnCl 2(η 3-N,N,O-dpkbh)] in DMF show irreversible metal and ligand-based redox processes, and electrochemical reactions of dpkbh with ZnCl 2 show facile coordination of ZnCl 2 and formation of [ZnCl 2(η 3-N,N,O-dpkbh)]. X-ray structural analysis done on a crystal grown from a DMF solution of [ZnCl 2(η 3-N,N,O-dpkbh)] confirmed the identity of [ZnCl 2(η 3-N,N,O-dpkbh)] and shows an extensive network of non-covalent interactions that connects all molecules.