H₄octapa: synthesis, solution equilibria and complexes with useful radiopharmaceutical metal ions

H₄octapa is an extremely versatile acyclic chelator for a wide variety of medicinally relevant metal ions, forming complexes of both high thermodynamic and kinetic stability. This work reports a significantly simplified 3 step high yield straightforward synthesis of H₄octapa directly from EDDA. Crystals of the octa-protonated form of the ligand [H₈octapa]⁴⁺ as its tetrachloride salt, and of the mixed lanthanum-sodium salt of [La(octapa)]^- were isolated and characterized by X-ray diffraction. All eight protonation constants for the ligand were determined through combined potentiometric-spectrophotometric titrations and in batch experiments using UV spectrophotometry and ¹H NMR spectroscopy. Synthesis, characterisation and solution equilibria studies are presented for complexes [Ln(octapa)]^- (Ln = lanthanide element) with Sm(iii), Dy(iii), and Yb(iii) (each of which have radiopharmaceutical applications). Complex formation equilibria studies provided evidence of Ln(Hoctapa), [Ln(octapa)]^- and [Ln(octapa)(OH)]^2- species in solution, and their stability constants were evaluated by pH-potentiometric competition titrations using [ttha]^6- as a competing ligand, and by UV-vis spectrophotometric measurements. The high stability constants of the [Ln(octapa)]^- complexes with Sm(iii), Dy(iii), and Yb(iii) (log K_SmL = 20.10(2), log K_DyL = 20.14(3) and log K_YbL = 19.90(1)) are similar to the published values for other lanthanides and consequently these initial investigations confirm H₄octapa as a valuable ligand for Sm(iii), Dy(iii), Yb(iii) and other Ln for application in diagnostic and therapeutic nuclear medicine.