Spectroscopic, structure and DFT studies of copper(II) and palladium(II) complexes of pyridine-2-carboxaldehyde-2-pyridylhydrazone: A chromogenic agent for palladium(II)

A heterocyclic hydrazone ligand, pyridine-2-carboxaldehyde-2-pyridylhydrazone, HL, 1, was investigated as a new chromogenic agent for selective detection of Pd2+. The ligand HL, 1, undergoes 1:1 complexation with Pd2+ and Cu2+ to form complexes [Pd(L)Cl], 1a and [Cu(HL)Cl2], 1b respectively. The complex 1a gives a characteristic absorption peak at 536 nm with distinct reddish-pink coloration. The change in color can easily be distinguished from other metal complexes by the naked eye. No obvious interference was observed in the presence of other metal ions (Na+, K+, Mg2+, Ca2+, Al3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Sn2+, Hg2+, Pb2+). The association constants, Kass (UV–Vis), were found to be 5.52 ± 0.004 × 104 for 1a and 4.94 ± 0.006 × 104 for 1b at 298 K. On excitation at 295 nm, the ligand HL, 1 strongly emits at 372 nm due to an intraligand 1(π–π∗) transition. Upon complexation the emission peaks are blue shifted (λex 295 nm, λem 358 nm for 1a and λex 295 nm, λem 367 nm for 1b) along with a quenching (F/F0 0.32 for 1a and 0.88 for 1b) in the emission intensity. DFT and TDDFT calculations were highly consistent with the spectroscopic behavior of the ligand and complexes. The molecular structure of the complex 1b has been determined by single crystal X-ray diffraction studies.

Graphical abstractA heterocyclic hydrazone ligand, pyridine-2-carboxaldehyde-2-pyridylhydrazone, HL, 1, was investigated as a new chromogenic agent for selective detection of Pd2+. The change in color of the complex [Pd(L)Cl], 1a can easily be distinguished from other metal complexes by the naked eye. DFT and TDDFT calculations were highly consistent with the spectroscopic behavior of the ligand and complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide