Complexometric Investigation to Differentiate Pd(II) and Pd(IV) ions in Solution

In an attempt to find a simple, fast, and inexpensive method to differntiate Pd(II) from Pd(IV) ions in aqueous solutions, the organic complexing agent N',N "-bis[(1E)-1-(2-Pyridyl)-ethylidene] ethanedihydrazide (H2L) was synthesized and used to study complexation with Pd(II) and Pd(IV) in two ways: (i) Formation of soluble colored complexes employing low concentration levels of both ligand and metal cations. The reaction of a solution of the ligand with Pd(II) or Pd(IV) ions at pH = 5-5.5 in a 1:1 molar ratio proceeded with the formation of two different soluble palladium complexes, Pd(IV)-H2L and Pd(II)-H2L. Each of the complexes was accompanied by a characteristic emission and absorption spectra. The ligand imposes a distinct emission and absorption peaks for each ion that could be used to characterize and identify it. The Pd(IV)-H2L is characterized by an emission peak at 500 nm and an absorption peak at 440 nm while the Pd(II)-H2L has two absorption shoulders at 460 and 480 nm and two emission shoulders at 440 and 470 nm. (ii) Continuous stirring for 45 min employing high concentrations of a 1:1 mixture of the ligand H2L with either of the metal cations Pd(II) or Pd(IV) in 2-propanol/water (5:1) produced solid powders corresponding to the formation of the complexes Pd(II)-H2L (yellow) and Pd(IV)-H2L (pale yellow). Differences between the uncomplexed ligand and the two solid complexes gave an indication that the Pd(II) or Pd(IV) metal ions are really coordinated to the N-atoms of the pyridine ring and hydrazide group. Complexation to Pd(II) or Pd(IV) was proved by (i) FTIR peak shifts in the stretching frequency values of the main peaks for the H2L ligand. (ii) The H-1-NMR signals due to the enolic form (-OH) were shifted to down field upon complexation and appeared at 9.28-9.33 ppm. Moreover, the H-1-NMR spectra of the complexes showed a higher number of peaks corresponding to the pyridine protons. Finally, elemental analysis was performed to prove further the complexation process.