Zn(ii), Cd(ii) and Pb(ii) complexation with pyridinecarboxylate containing ligands.

Area of Science:

• Coordination Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Hexadentate ligands featuring pyridinecarboxylate groups are crucial in coordination chemistry.
• Understanding metal-ligand interactions is vital for developing selective metal extraction agents.
• The design of ligands with specific backbones (ethane-1,2-diamine vs. cyclohexane-1,2-diamine) influences coordination properties.

Purpose of the Study:

• To investigate the coordination behavior of two novel hexadentate ligands, bcpe and bcpc, with Zn(II), Cd(II), and Pb(II) metal ions.
• To elucidate the structural, conformational, and electronic properties of the resulting metal complexes.
• To evaluate the stability and selectivity of these ligands for potential applications in metal ion removal.

Main Methods:

• X-ray crystallography was used to determine the solid-state structures of metal complexes.
• Density Functional Theory (DFT) calculations (B3LYP) were employed to study ligand conformations.
• Potentiometric studies were conducted to determine protonation and stability constants.
• Natural Bond Orbital (NBO) analysis was performed to understand electronic properties.

Main Results:

• Hexadentate binding was observed for Zn(II) and Cd(II) complexes, forming distorted octahedral polyhedra.
• Pb(II) complexes exhibited tetrameric structures with carboxylate bridging.
• Ligand conformation varied: twist-wrap (tw) for Zn(II)/Cd(II) and twist-fold (tf) for Pb(II).
• DFT calculations confirmed the greater stability of the tf conformation for Pb(II) complexes.
• The cyclohexylene backbone significantly enhanced Pb(II) and Cd(II) complex stability compared to the ethylene backbone.
• The bcpc ligand demonstrated superior Pb/Ca and Cd/Ca selectivity (10^8.9 and 10^9.8) over EDTA.

Conclusions:

• The studied ligands effectively coordinate with Zn(II), Cd(II), and Pb(II) ions.
• Ligand conformation and metal-ion coordination geometry are influenced by the metal and backbone structure.
• The bcpc ligand exhibits promising selectivity for Pb(II) and Cd(II), suggesting potential for environmental remediation applications.
• The findings provide valuable insights into the design of selective metal-binding agents.