Complexation of 1,3-Diamino-2-hydroxypropane-N,N,N',N'-tetra acetic Acid (DHPTA) with Heavy Lanthanides (Tb³⁺, Ho³⁺, Lu³⁺) in Aqueous Solution

Thermodynamic equilibria of complexes of 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid (DHPTA) with heavy lanthanides (Tb3+, Ho3+ and Lu3+) in aqueous solution have been investigated with potentiometry, spectrophotometry, luminescence spectroscopy and nuclear magnetic resonance spectroscopy (NMR). The results identified three 1:1 Ln/DHPTA (Ln: Tb3+, Ho3+ and Lu3+) complexes with different degrees of deprotonation, LnL(-), Ln(H-1L)(2-), and Ln(OH)(H-1L)(3-), where H-1 represents the deprotonation of the hydroxyl group between two methyliminodiacetate groups in the DHPTA structure. The alkoxide form of the DHPTA hydroxyl group directly binds to the lanthanide atom, forming highly strong chelation. The complex of Ln(H-1L)(2-) could be present as a dimeric or polymeric complex in solution.