Spectroscopic and computational investigation of actinium coordination chemistry

Actinium-225 is a promising isotope for targeted-a therapy. Unfortunately, progress in developing chelators for medicinal applications has been hindered by a limited understanding of actinium chemistry. This knowledge gap is primarily associated with handling actinium, as it is highly radioactive and in short supply. Hence, Ac-III reactivity is often inferred from the lanthanides and minor actinides (that is, Am, Cm), with limited success. Here we overcome these challenges and characterize actinium in HCl solutions using X-ray absorption spectroscopy and molecular dynamics density functional theory. The Ac-Cl and Ac-OH2O distances are measured to be 2.95(3) and 2.59(3)angstrom, respectively. The X-ray absorption spectroscopy comparisons between Ac-III and Am-III in HCl solutions indicate Ac-III coordinates more inner-sphere Cl1-ligands (3.2 +/- 1.1) than Am-III (0.8 +/- 0.3). These results imply diverse reactivity for the +3 actinides and highlight the unexpected and unique Ac-III chemical behaviour.