X-ray crystallographic and molecular sensing properties of [ZnCl2(κ2-N,N′-dpknph)] (dpknph = di-2-pyridyl ketone-p-nitrophenyl hydrazone)

When ZnCl2 was allowed to react with di-2-pyridyl ketone p-nitrophenylhydrazone (dpknph) in acetonitrile under reflux, [ZnCl2(κ2-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 [ZnCl2(κ2-N,N′-dpknph)].dmf isolated from a dmf solution of [ZnCl2(κ2-N,N′-dpknph)] shows the pyridine N,N′-bidentate binding of dpknph and revealed a serpentine polymeric network of [ZnCl2(κ2-N,N′-dpknph)].dmf units locked via a web of hydrogen bonds. 1H NMR spectra of [ZnCl2(κ2-N,N′-dpknph)] measured in d6-dmso, d7-dmf and d6-acetone confirmed the coordination of dpknph and showed partial exchange of the amide proton with solvent protons in d6-acetone and that [ZnCl2(κ2-N,N′-dpknph)] is stable in dmso and dmf. 1H NMR variable temperature studies confirmed the stability of [ZnCl2(κ2-N,N′-dpknph)] in d6-dmso and d7-dmf and showed the presence of temperature-dependent shielded protons due to hydrogen bonding. The electronic absorption spectra of [ZnCl2(κ2-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 [ZnCl2(κ2-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 [ZnCl2(κ2-N,N′-dpknph)] and a base showed the disappearance of the peak at 580 ± 2 nm and reappearance of the peak due to [ZnCl2(κ2-N,N′-dpknph)]. These results established reversible inter-conversion between [ZnCl2(κ2-N,N′-dpknph)] and its conjugate base, and allow for the use of [ZnCl2(κ2-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 [ZnCl2(κ2-N,N′-dpknph)] and NaBH4 in dmso confirmed the reversible inter-conversion between [ZnCl2(κ2-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.