Addition of H ₂O and O ₂ to acetone and dimethylsulfoxide ligated uranyl(V) dioxocations

Gas-phase complexes of the formula [UO ₂(lig)]+ (lig = acetone (aco) or dimethylsulfoxide (dmso)) were generated by electrospray ionization (ESI) and studied by tandem ion-trap mass spectrometry to determine the general effect of ligand charge donation on the reactivity of UO ₂ ⁺with respect to water and dioxygen. The original hypothesis that addition of O ₂ is enhanced by strong σ-donor ligands bound to UO ₂ ⁺ is supported by results from competitive collision-induced dissociation (CID) experiments, which show near exclusive loss of H ₂O from [UO ₂(dmso)(H ₂O)(O ₂)] ⁺, whereas both H ₂O and O ₂ are eliminated from the corresponding [UO ₂(aco)(H ₂O)(O ₂)] ⁺ species. Ligand-addition reaction rates were investigated by monitoring precursor and product ion intensities as a function of ion storage time in the ion-trap mass spectrometer: these experiments suggest that the association of dioxygen to the UO ₂ ⁺ complex is enhanced when the more basic dmso ligand was coordinated to the metal complex. Conversely, addition of H ₂O is favored for the analogous complex ion that contains an aco ligand. Experimental rate measurements are supported by density function theory calculations of relative energies, which show stronger bonds between UO ₂ ⁺ and O ₂ when dmso is the coordinating ligand, whereas bonds to H ₂O are stronger for the aco complex.