Complexation of Np(V) with three structurally related amine-functionalized diacetamide ligands, including 2,2'-azanediylbis( N, N'-dimethylacetamide) (ABDMA), 2,2'-(methylazanediyl)bis( N, N'-dimethylacetamide) (MABDMA), and 2,2'-(benzylazanediyl)bis( N, N'-dimethylacetamide) (BnABDMA), in aqueous solutions was investigated. The stability constants of two successive complexes, namely, NpO₂L⁺ and NpO₂L₂⁺, where L stands for the ligands, were determined by absorption spectrophotometry. The results suggest that the stability constants of corresponding Np(V) complexes follow the trend: MABDMA > ABDMA ≈ BnABDMA. The data are discussed in terms of the basicity of the ligands and compared with those for the complexation of Np(V) with an ether oxygen-linked diacetamide ligand. Extended X-ray absorption fine structure data indicate that, similar to the complexation with Nd³⁺ and UO₂²⁺, the ligands coordinate to NpO₂⁺ in a tridentate mode through the amine nitrogen and two oxygen atoms of the amide groups. Computational results, in conjunction with spectrophotometric data, verify that the 1:2 complexes (NpO₂(L)₂⁺) in aqueous solutions are highly symmetric with Np at the inversion center, so that the f-f transition of Np(V) is forbidden and NpO₂(L)₂⁺ does not display significant absorption in the near-IR region.

Thermodynamic properties of the complexation of 2,2'-(trifluoroazanediyl)bis(N,N'-dimethylacetamide) (CF₃ABDMA) with U(VI), Nd(III), and Np(V) have been studied in 1.0 M NaNO₃ at 25 °C. Equilibrium constants of the complexation were determined by potentiometry and spectrophotometry. In comparison with a series of structurally related amine-bridged diacetamide ligands, including 2,2'-(benzylazanediyl)bis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-(methylazanediyl)bis(N,N'-dimethylacetamide) (MABDMA), CF₃ABDMA forms weaker complexes with U(VI), Nd(III), and Np(V) due to the lower basicity of the center N atom in CF₃ABDMA resulting from the attachment of the strong electron-withdrawing CF₃- moiety. The complexation strength of CF₃ABDMA with the three metal ions follows the order: UO₂²⁺ > Nd³⁺ > NpO₂⁺, consistent with the order of the "effective" charges of the metal ions. Structural information on the U(VI)/CF₃ABDMA complexes in solution and in solid was obtained by theoretical computation, single crystal X-ray diffractometry, ¹⁹F NMR, and electrospray ionization mass spectrometry. The structural data indicate that, similar to the three previously studied amine-bridged diacetamide ligands (BnABDMA, ABDMA, and MABDMA), the CF₃ABDMA ligand coordinates to UO₂²⁺ in a tridentate mode, through the center nitrogen and the two amide oxygen atoms.