How Does the Replacement of Acetate Arms with Acetamide Ones in the Structure of Chelators Affect Complexation with Pb2+?

The growing interest in the application of Pb2+ complexes in fields such as radiopharmaceuticals encourages a more thorough study of the coordination chemistry of this metal ion and the search for a desirable chelating agent. In this work, a comparative study of the complexing properties of various acetate and acetamide chelators was carried out to determine which pendant arms are preferable for binding the Pb2+ ion. The study focused on chelators containing the same pyridine bisamide fragment in their structure, while the type of chelator, the size of the macrocyclic cavity, and the number of pendant arms were varied. To better understand the impact of specific factors on Pb2+ binding, the discussion was supplemented with literature data on the complexation of the well-known ligands DOTA and DTPA, along with their acetamide analogues DOTAM and DTPAM. Potentiometric titration was employed to determine the basicity of the ligands and the thermodynamic stability of their Pb2+ complexes. Their structures were characterized by single-crystal X-ray diffraction and NMR spectroscopy. The results revealed that macrocyclic chelators with flexible structures and acetate pendant arms form the most thermodynamically stable complexes with Pb2+, exhibiting shorter and more uniform coordination bond lengths.