Binding of molecular O2 to di- and triligated [UO2]+

Gas-phase complexes containing dioxouranium(V) cations (UO(2)](+)) ligated with two or three sigma-donating acetone ligands reacted with dioxygen to form UO(2)(A)(2,3)(O(2))](+), where A is acetone. Collision-induced dissociation studies of UO(2)(A)(3)(O(2))](+) showed initial loss of acetone, followed by elimination of O(2), which suggested that O(2) was bound more strongly than the third acetone ligand, but less strongly than the second. Similar behavior was observed for complexes in which water was substituted for acetone. Binding of dioxygen to UO(2)](+) containing zero, one, or four ligands did not occur, nor did it occur for analogous ligated U(IV)O(2) or U(VI)O(2) ions. For example, only addition of acetone and/or H(2)O occurred for the U(VI) species UO(2)OH](+), with the ligand addition cascade terminating in formation of UO(2)OH(A)(3)](+). Similarly, the U(IV) species UOOH](+) added donor ligands, which produced the mixed-ligand complex UOOH(A)(3)(H(2)O)](+) as the preferred product at the longest reaction times accessible. Since dioxygen normally functions as an electron acceptor, an alternative mode for binding dioxygen to the cationic U(V)O(2) center is indicated that is dependent on the presence of an unpaired electron and donor ligands in the uranyl valence orbitals.