Oligomers Intermediates in Between Two New Distinct Homonuclear Uranium(IV) DOTP Complexes*

Two new aqueous U^IV complexes were synthesized by the interaction between the tetravalent uranium cation and the (1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid) (DOTP) macrocyclic ligand. Two distinct homonuclear complexes were identified; the first was characterized by X-ray crystallography as a unique "out-of-cage", [U(DOTPH₆ )₂ ] complex, in which the U^IV cation is octa-coordinated to 4 phosphonic arms from each ligand in a square anti-prism geometry, with a C₄ symmetry. The second is the "in-cage" [U(DOTPH₄ )] complex, in which the tetravalent cation is located between the macrocycle O₄ and N₄ planes. With the help of UV-Vis absorption, ¹ H/³¹ P NMR, ATR-IR, and MALDI-TOFMS analytical techniques, the chemical interchange between both species is presented. It is shown that the one-way transition is governed by the formation of a multiple number of soluble oligomeric species consisting of varied stoichiometric ratios of both characterized homonuclear complexes.

On the Aqueous Chemistry of the UIV -DOTA Complex

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complex of U^IV with H₂ O, OH^- , and F^- as axial ligands was studied by using UV/Vis spectrophotometry, ESI-MS, NMR spectroscopy, X-ray crystallography, and electrochemistry. The U^IV -DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The U^IV /U^III redox couple was found to be quasi-reversible for the complex with both axially bonded H₂ O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one-electron U^V /U^IV oxidation of the [U^IV (DOTA)(H₂ O)] complex quasi-reversible, which suggests the formation of the short-lived pentavalent form of the complex, an aqueous non-uranyl chelated U^V cation.

Investigating subtle 4f vs. 5f coordination differences using kinetically inert Eu(iii), Tb(iii), and Cm(iii) complexes of a coumarin-appended 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (DO3A) ligand

Coumarin appended DO3A complexes of Cm(iii), Eu(iii), and Tb(iii) show that more than ionic radius determines the coordination chemistry of the f-elements.