Coordination of 2,2'-(Trifluoroazanediyl)bis(N,N'-dimethylacetamide) with U(VI), Nd(III), and Np(V): A Thermodynamic and Structural Study.

Abstract

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.