Structural and thermodynamic investigation of An(IV)LI(O)HOPO

For the first time, capillary electrophoresis coupled with inductively coupled plasma mass spectrometry has been used to determine the stability constants of Pu(IV) with the multidentate hydroxy-pyridinonate chelating agents 3,4,3-LI(1,2-HOPO) and 5-LIO(Me-3,2-HOPO) in 0.1 M NaNO₃ solution, p_cH = 1.395 at 25 deg. C through competition with the NTA ligand. The limiting electrophoretic mobility was found to be zero for An(IV)[5-LIO(Me-3,2-HOPO)] and slightly positive for An(IV)[3,4,3-LI(1,2-HOPO)] (An(IV) = Th, Pu). They were respectively assigned to the formation of the 1: 2 neutral species An[5-LIO(Me-3,2-HOPO)](2) and a mixture of the neutral species An(IV)[3,4,3-LI(1,2-HOPO)] and its protonated form An(IV)H[3,4,3-LI(1,2-HOPO)]⁺. The corresponding stability constants of Th(IV) with both chelators were evaluated through the same experiments for the sake of comparison. The stability of both Pu(IV)-HOPOs was about ten orders of magnitude better than that of the equivalent Th(IV) complexes. To complement these thermodynamic data, structural parameters of Pu[3,4,3-LI(1,2-HOPO)] and Th[3,4,3-LI(1,2-HOPO)] complexes in solution have been derived from EXAFS experiments and compared to previously reported crystallographic structures. (authors)