Insights into the Complexation of Actinides by Diethylenetriaminepentaacetic Acid from Characterization of the Americium(III) Complex.

Diethylenetriaminepentaacetic acid (DTPA) is a frequently used chelator in the nuclear and medical industries, especially for the complexation of trivalent actinides. However, structural data on these complexes in the solid-state have long remained elusive. Herein, a detailed structural analysis of the presented crystal structures of [C(NH)][Nd(DTPA)]·HO and [C(NH)][Am(DTPA)]·HO, where [C(NH)] is guanidinium, details the subtle differences in the Lewis acidity between a lanthanide/actinide pair of similar ionic sizes. Contractions in nitrogen-metal bond lengths between neodymium(III) and americium(III) were observed, while the metal-oxygen bonds remained relatively consistent, highlighting the marginal favorability for actinide complexation over the lanthanides with moderately soft N-donors. Spectroscopic analysis shows significant splitting of many transitions and relatively strong electronic interactions with traditionally low-intensity transitions in the americium complex, as is demonstrated in the F→F transitions. Pressure-induced spectroscopic analysis showed surprisingly little effect on the americium complex, with 5→5 transitions either not shifting or marginally shifting from 2 to 3 nm at 11.93 ± 0.06 GPa─atypical of a soft, N-donor americium complex under pressure. Large voids occupied by water molecules in between the complexes within the crystal structure may be responsible for the lack of pressure response in the 5→5 transitions.