Binuclear Hexa- and Pentavalent Uranium Complexes with a Polypyrrolic Ligand: A Density Functional Study of Water- and Hydronium-Induced Reactions

Binuclear uranyl (VI) and (V) complexes of a Pacman-like polypyrrolic macrocycle (H4L) were investigated using relativistic density functional theory. The reactivity of the bis-uranyl(VI) complex (UO2)(2)(L) (2a) was explored computationally. Although 2a has not been obtained experimentally, its structural analogue (UO2)(OH)K(THF)(2)(H2LMe)] has been synthesized recently. The high reactive activity of 2a originates from its unique butterfly like cation cation structure, containing an active 0 center with easily broken U-O bonding and having unsaturated coordination sites of uranium(VI) along the equatorial plane. The present study indicates that 2a can react with water (Path 3) and hydronium (Path 4), which lead to the formation of a series of complexes with a triangle-like O=U=O=U=O skeleton. Path 3 results in an unusual complex containing a combined cis-uranyl/trans-uranyl cation cation structure, (cis-UO2)( trans-UO2)(H2O)(L) (4b), where the oxo atom of the trans-uranyl coordinates the uranium center of the cis-uranyl and the water bonds to the uranium of trans-uranyl in the equatorial plane. After a process of hydrogen transfer with an extremely low energy barrier (<1.5 kcal/mol), 4b is converted into a slightly more stable isomer (U2O3)(OH)(2)(L) (4a), where two hydroxyl groups link to two uranium atoms, respectively. In conjunction with previous studies, the free energies of reactions of 2a induced by isomerization (Path 1), proton (Path 2), water (Path 3), and hydronium (Path 4) were calculated in the gas phase and aqueous solution. Solvation stabilizes the free energy of the formation reactions of the neutral complexes but destabilizes that of the charged complexes. In these reactions, three pairs of isomers were obtained for binuclear uranium(VI) complexes, but only the most stable in each pair exists for the binuclear uranium(V) analogues.