Advancing understanding of actinide(iii) (Ac, Am, Cm) aqueous complexation chemistry.

The positive impact of having access to well-defined starting materials for applied actinide technologies - and for technologies based on other elements - cannot be overstated. Of numerous relevant 5f-element starting materials, those in complexing aqueous media find widespread use. Consider acetic acid/acetate buffered solutions as an example. These solutions provide entry into diverse technologies, from small-scale production of actinide metal to preparing radiolabeled chelates for medical applications. However, like so many aqueous solutions that contain actinides and complexing agents, 5f-element speciation in acetic acid/acetate cocktails is poorly defined. Herein, we address this problem and characterize Ac and Cm speciation as a function of increasing acetic acid/acetate concentrations (0.1 to 15 M, pH = 5.5). Results obtained X-ray absorption and optical spectroscopy show the aquo ion dominated in dilute acetic acid/acetate solutions (0.1 M). Increasing acetic acid/acetate concentrations to 15 M increased complexation and revealed divergent reactivity between early and late actinides. A neutral Ac(HO) (OCMe) compound was the major species in solution for the large Ac. In contrast, smaller Cm preferred forming an anion. There were approximately four bound OCMe ligands and one to two inner sphere HO ligands. The conclusion that increasing acetic acid/acetate concentrations increased acetate complexation was corroborated by characterizing (NH)M(OCMe) (M = Eu, Am and Cm) using single crystal X-ray diffraction and optical spectroscopy (absorption, emission, excitation, and excited state lifetime measurements).