X-ray crystallographic and molecular sensing properties of [ [formula omitted](κ 2- N, N′-dpknph)] (dpknph = di-2-pyridyl ketone- p-nitrophenyl hydrazone)

Abstract

When ZnCl 2 was allowed to react with di-2-pyridyl ketone p-nitrophenylhydrazone (dpknph) in acetonitrile under reflux, [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] was isolated in good yield. The formulation of the isolated product was established from the results of its elemental analysis and a number of spectroscopic measurements. Single crystal X-ray structural analysis done on [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] .dmf isolated from a dmf solution of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] shows the pyridine N, N′-bidentate binding of dpknph and revealed a serpentine polymeric network of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] .dmf units locked via a web of hydrogen bonds. 1H NMR spectra of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] measured in d 6 -dmso, d 7-dmf and d 6-acetone confirmed the coordination of dpknph and showed partial exchange of the amide proton with solvent protons in d 6-acetone and that [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] is stable in dmso and dmf. 1H NMR variable temperature studies confirmed the stability of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] in d 6-dmso and d 7-dmf and showed the presence of temperature-dependent shielded protons due to hydrogen bonding. The electronic absorption spectra of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] in non-aqueous media show a single solvent-dependent intra-ligand-charge transfer (ILCT) transition of the donor–acceptor type between 300 and 700 nm. Thermo-optical measurements in protophilic solvent (dmso or dmf) between 303.15 and 353.15 K show the stability of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] at those temperatures. In the presence of a base, a new ILCT peak appeared at 580 ± 2 nm in dmso, and the addition of an acid to a mixture of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] and a base showed the disappearance of the peak at 580 ± 2 nm and reappearance of the peak due to [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] . These results established reversible inter-conversion between [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] and its conjugate base, and allow for the use of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] in dmso to measure and detect substrates in concentrations as low as 1.00 × 10 −5 M. Thermo-optical measurements on a mixture of [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] and NaBH 4 in dmso confirmed the reversible inter-conversion between [ ZnCl 2 ( κ 2 - N , N ′ -dpknph ) ] and its conjugate base, and gave changes in enthalpy of 43.32 kJ/mol and entropy of 138.87 JK −1 mol −1 at 298.15 K.