H₄HBEDpa: Octadentate Chelate after A. E. Martell

Metal ion chelation is a well-established field in coordination chemistry and nuclear medicine. While many chelators have been developed for specific metal ions, few have demonstrated broad utility across multiple trivalent ions. Prior research has shown that octadentate ligands can offer enhanced stability and selectivity in metal complexation. However, the design of such ligands remains challenging due to the need for preorganization and compatibility with diverse metal geometries. The 1960s work of Arthur E. Martell on HBED ligands laid a foundation for modern chelator design. Yet, no single ligand has yet been shown to bind multiple trivalent ions with high affinity and structural consistency. This gap motivated the development of H₄HBEDpa, a new octadentate ligand inspired by Martell’s HBED. The research community has long sought ligands that can bind multiple diagnostic and therapeutic metal ions effectively. This paper's contribution is the synthesis and evaluation of H₄HBEDpa for its binding properties across a range of trivalent metals.

This study aimed to evaluate the binding properties of H₄HBEDpa with a range of trivalent metal ions relevant to diagnostic and therapeutic applications. The researchers focused on determining the stability, coordination geometry, and pH-dependent behavior of H₄HBEDpa-metal complexes. They also sought to compare the structural and energetic characteristics of complexes formed with different metal ions. The motivation stemmed from the need for a versatile chelator that could bind multiple trivalent ions with high affinity and structural consistency. By analyzing the ligand’s performance across Sc³⁺, Fe³⁺, Ga³⁺, In³⁺, and Lu³⁺, the study aimed to identify a ligand with broad applicability. The researchers also aimed to validate their findings using a combination of experimental and computational methods. The ultimate goal was to assess whether H₄HBEDpa could serve as a platform for developing multifunctional metal complexes.

The researchers used a combination of potentiometric and UV-vis spectrophotometric titrations to study complex formation equilibria. These methods allowed them to determine stability constants and coordination modes of H₄HBEDpa with various metal ions. X-ray diffraction was applied to analyze single crystals of the ligand-metal complexes, providing structural insights into the coordination geometry. Density functional theory (DFT) calculations were used to model the geometries and energies of all possible conformers of the complexes. The DFT approach enabled a detailed comparison of the binding patterns across different metal ions. The study also examined how pH affects the stability and structure of the complexes. By combining experimental and computational approaches, the researchers aimed to validate the ligand’s performance across a range of conditions. The methods were designed to ensure a comprehensive understanding of H₄HBEDpa’s chelation behavior.

The study found that H₄HBEDpa forms stable complexes with multiple trivalent metal ions. The strongest binding was observed with Fe³⁺, with a stability constant of log KFeL = 36.62. X-ray diffraction revealed that the ligand is preorganized and forms hexa-coordinated complexes with Fe³⁺ and Ga³⁺ at acidic pH. DFT calculations confirmed the experimental findings, showing that [Fe(HBEDpa)]⁻ is bound tightly in an asymmetric pattern. In contrast, [Ga(HBEDpa)]⁻ is symmetrically bound and more open, making it prone to hydrolysis at higher pH. The calculations also showed that Lu³⁺ fully coordinates with HBEDpa⁴⁻, forming a binary octadentate complex in its lowest-energy form. Smaller metal ions form six or seven coordinate complexes with HBEDpa⁴⁻. These results suggest that H₄HBEDpa can accommodate a range of metal ions with varying coordination geometries.

The authors concluded that H₄HBEDpa is a promising octadentate chelator for multiple trivalent metal ions. The ligand demonstrated high stability and metal-sequestering capacity, particularly for Fe³⁺. The X-ray and DFT findings confirmed that the ligand is preorganized and forms stable complexes with Fe³⁺ and Ga³⁺ at acidic pH. The asymmetric binding pattern of [Fe(HBEDpa)]⁻ contrasts with the symmetrically bound [Ga(HBEDpa)]⁻, which is more prone to hydrolysis. The study also showed that Lu³⁺ forms a fully coordinated octadentate complex with HBEDpa⁴⁻ in its lowest-energy form. Smaller metal ions form six or seven coordinate complexes with HBEDpa⁴⁻. These findings suggest that H₄HBEDpa can be tailored for different metal ions depending on the application. The researchers propose that the ligand’s versatility makes it suitable for use in diagnostic and therapeutic applications.