Molecular sensing behavior of di-2-pyridyl ketone p-aminophenylhydrazone hydrate (dpkabh·H 2O) in non-aqueous media

Abstract

1H NMR studies on di-2-pyridyl ketone p-aminobenzoylhydrazone hydrate (dpkabh·H 2O) in non-aqueous solvents show high sensitivity to its surrounding. In protophilic solvents ( d 6-dmso or d 7-dmf), the amine protons are equivalent, while in CDCl 3 they are not. Variable temperature analysis in CDCL 3 show the NH proton to exhibit high temperature dependence due to strong intra-molecular hydrogen bonding of the type N–H⋯N between the amide (NH) and N atom of a pyridine ring. The temperature dependence for the same proton in d 6-dmso and d 7-dmf is due to hydrogen bonding of the type N–H⋯O between the amide proton and oxygen atom of the solvent. Optical measurements on dpkabh·H 2O show one intra-ligand charge transfer (ILCT) transition in CH 2Cl 2 and in protophilic solvents, two ILCT of the donor–acceptor type due to dpkabh·H 2O and its conjugate base appeared. Variable temperature studies on protophilic solution of dpkabh·H 2O confirm the sensitivity of dpkabh·H 2O to its surroundings and show facile reversible inter-conversion between dpkabh·H 2O and its conjugate base. Changes in enthalpy (Δ H ϕ) of −5.2 ± 0.4 and −24.2 ± 1.20 kJ mol −1, entropy (Δ S ϕ) of +9.6 ± 0.5 and −63.0 ± 2.0 JK −1 mol −1, and free energy (Δ G ϕ) of +2.3 ± 0.2 and +5.4 ± 0.2 kJ mol −1 were calculated for dpkabh·H 2O at 298 K in dmso and dmf, respectively. When stoichiometric amounts of NaBH 4 or MCl 2 (M = Zn, Cd or Hg) were added to protophilic solution of dpkanh·H 2O conversion from the high to low energy electronic transition was observed and show that substrates in low concentrations can be detected and determined using protophilic solution of dpkabh·H 2O.