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Electrospray ionization was used to generate doubly charged complex ions composed of the uranyl ion and nitrile ligands. The complexes, with general formula [UO2(RCN)n]2+, n = 0−5 (where R=CH3-, CH3CH2-, or C6H5-), were isolated in an ion-trap mass spectrometer to probe intrinsic reactions with H2O. For these complexes, two general reaction pathways were observed:  (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-reduction reactions. For the latter, the reactions produced uranyl hydroxide, [UO2N)], complexes via collisions with gas-phase H2O molecules and the elimination of protonated nitrile ligands. [ABSTRACT FROM AUTHOR]

Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles by using 355 nm laser pulses that were synchronized with pulses of nitrogen gas admitted to the irradiation chamber. The gas pulse stabilized the nascent clusters that are largely anhydrous [CexOy] ions and neutrals. These initially formed species react with water, principally forming oxohydroxy species that are described by the general formula [CexOy(OH)z]−for which all the Ce atoms are in the IV oxidation state. In general, the extent of hydroxylation varies from a value of three OH per Ce atom when x= 1 to a value slightly greater than 1 for x≥ 8. The Ce3and Ce6species deviate significantly from this trend: the x= 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x= 2 and 4. Conversely, the x= 6 cluster is significantly less hydroxylated than its x= 5 and 7 neighbors. Density functional theory (DFT) modeling of the cluster structures shows that the hydrated clusters are hydrolyzed, and contain one-to-multiple hydroxide moieties, but not datively bound water. DFT also predicts an energetic preference for formation of highly symmetric structures as the size of the clusters increases. The calculated structures indicate that the ability of the Ce3oxyhydroxide to accommodate more extensive hydroxylation is due to a more open, hexagonal structure in which the Ce atoms can participate in multiple hydrolysis reactions. Conversely the Ce6oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the fully oxidized (Ce(IV)) oxyhydroxides, reduced oxyhydroxides (containing a Ce(III) center) are also formed. These become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to accommodate a reduced Ce(III) moiety. In addition, the spectra offer evidence for the formation of superoxide derivatives that may arise from reaction of the reduced oxyhydroxides with dioxygen. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases; however, this trend is interrupted at Ce13, which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure. [ABSTRACT FROM AUTHOR]

Gas-phase complexes of the formula [UO2(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 UO2+with respect to water and dioxygen. The original hypothesis that addition of O2is enhanced by strong σ-donor ligands bound to UO2+is supported by results from competitive collision-induced dissociation (CID) experiments, which show near exclusive loss of H2O from [UO2(dmso)(H2O)(O2)]+, whereas both H2O and O2are eliminated from the corresponding [UO2(aco)(H2O)(O2)]+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 UO2+complex is enhanced when the more basic dmso ligand was coordinated to the metal complex. Conversely, addition of H2O 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 UO2+and O2when dmso is the coordinating ligand, whereas bonds to H2O are stronger for the aco complex. [ABSTRACT FROM AUTHOR]

Theoretical calculations suggest a novel two-electron three-atom bonding scheme for complexation of O 2with U(V) compounds, leading to the stabilization of superoxo complexes in the side-on (η 2) configuration. This binding motif is likely to play an important role in the oxidative processes involving trans-uranium compounds having valence 5f φelectrons. [ABSTRACT FROM AUTHOR]

The Free-Electron Laser for Infrared Experiments (FELIX) was used to study the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl (UO22) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula UO2A(S)n, where A was hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n0−3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion (UO2A) were as low or lower than measurements for UO22ligated with as many as five strong neutral donor ligands and are comparable to solution-phase values. This result was surprising because initial DFT calculations predicted values that were 30−40 cm-1higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to UO2Aproduced UO2AS, which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for UO2ASwere in accord with trends in the data and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for UO2ASspecies having different solvent neutrals, values decreased with increasing neutral nucleophilicity. [ABSTRACT FROM AUTHOR]

Cationic metal species normally function as Lewis acids, accepting electron density from bound electron-donating ligands, but they can be induced to function as electron donors relative to dioxygen by careful control of the oxidation state and ligand field. In this study, cationic vanadium(IV) oxohydroxy complexes were induced to function as Lewis bases, as demonstrated by addition of O2to an undercoordinated metal center. Gas-phase complex ions containing the vanadyl (VO2), vanadyl hydroxide (VOOH), or vanadium(V) dioxo (VO2) cation and nitrile (acetonitrile, propionitrile, butyronitrile, or benzonitrile) ligands were generated by electrospray ionization (ESI) for study by multiple-stage tandem mass spectrometry. The principal species generated by ESI were complexes with the formula VO(L)n2, where L represents the respective nitrile ligands and n 4 and 5. Collision-induced dissociation (CID) of VO(L)52eliminated a single nitrile ligand to produce VO(L)42. Two distinct fragmentation pathways were observed for the subsequent dissociation of VO(L)42. The first involved the elimination of a second nitrile ligand to generate VO(L)32, which then added neutral H2O via an association reaction that occurred for all undercoordinated vanadium complexes. The second UO(L)42fragmentation pathway led instead to the formation of VOOH(L)2through collisions with gas-phase H2O and concomitant losses of L and L H. CID of VOOH(L)2caused the elimination of a single nitrile ligand to generate VOOH(L), which rapidly added O2(in addition to H2O) by a gas-phase association reaction. CID of VONO3(L)2, generated from spray solutions created by mixing VOSO4and Ba(NO3)2(and precipitation of BaSO4), caused elimination of NO2to produce VO2(L)2. CID of VO2(L)2produced elimination of a single nitrile ligand to form VO2(L), a V(V) analogue to the O2-reactive V(IV) species VOOH(L); however, this V(V) complex was unreactive with O2, which indicates the requirement for an unpaired electron in the metal valence shell for O2addition. In general, the VO2(L)2species required higher collisions energies to liberate the nitrile ligand, suggesting that they are more strongly bound than the VOOH(L)2counterparts. [ABSTRACT FROM AUTHOR]