Phuong D. Dau

Probing the electronic structure and chemical bonding in tricoordinate uranyl complexes UO2X3- (X = F, Cl, Br, I): competition between Coulomb repulsion and U-X bonding.

While uranyl halide complexes [UO2(halogen)n](2-n) (n = 1, 2, 4) are ubiquitous, the tricoordinate species have been relatively unknown until very recently. Here photoelectron spectroscopy and relativistic quantum chemistry are used to investigate the bonding and stability of a series of gaseous tricoordinate uranyl complexes, UO2X3(-) (X = F, Cl, Br, I). Isolated UO2X3(-) ions are produced by electrospray ionization and observed to be highly stable with very large adiabatic electron detachment energies: 6.25, 6.64, 6.27, and 5.60 eV for X = F, Cl, Br, and I, respectively. Theoretical calculations reveal that the frontier molecular orbitals are mainly of uranyl U-O bonding character in UO2F3(-), but they are from the ligand valence np lone pairs in the heavier halogen complexes. Extensive bonding analyses are carried out for UO2X3(-) as well as for the doubly charged tetracoordinate complexes (UO2X4(2-)), showing that the U-X bonds are dominated by ionic interactions with weak covalency. The U-X bond strength decreases down the periodic table from F to I. Coulomb barriers and dissociation energies of UO2X4(2-) → UO2X3(-) + X(-) are calculated, revealing that all gaseous dianions are in fact metastable. The dielectric constant of the environment is shown to be the key in controlling the thermodynamic and kinetic stabilities of the tetracoordinate uranyl complexes via modulation of the ligand-ligand Coulomb repulsions.

Elucidating Protactinium Hydrolysis: The Relative Stabilities of PaO2(H2O)+ and PaO(OH)2+

It is demonstrated that the gas-phase oxo-exchange of PaO2(+) with water is substantially faster than that of UO2(+), indicating that the Pa-O bonds are more susceptible to activation and formation of the bis-hydroxide intermediate, PaO(OH)2(+). To elucidate the nature of the water adduct of PaO2(+), hydration of PaO2(+) and UO2(+), as well as collision induced dissociation (CID) and ligand-exchange of the water adducts of PaO2(+) and UO2(+), was studied. The results indicate that, in contrast to UO2(H2O)(+), the protactinium oxo bis-hydroxide isomer, PaO(OH)2(+), is produced as a gas-phase species close in energy to the hydrate isomer, PaO2(H2O)(+). CID behavior similar to that of Th(OH)3(+) supports the assignment as PaO(OH)2(+). The gas-phase results are consistent with the spontaneous hydrolysis of PaO2(+) in aqueous solution, this in contrast to later AnO2(+) (An = U, Np, Pu), which forms stable hydrates in both solution and gas phase. In view of the known propensity for Th(IV) to hydrolyze, and previous gas-phase studies of other AnO2(+), it is concluded that the stabilities of oxo-hydroxides relative to oxide hydrates decreases in the order: Th(IV) > Pa(V) > U(V) > Np(V) > Pu(V). This trend suggests increasing covalency and decreasing ionicity of An-O bonds upon proceeding across the actinide series.

Unusual complexation of nitrate with lanthanides in a wet ionic liquid: a new approach for aqueous separation of trivalent f-elements using an ionic liquid as solvent

The energetics of lanthanide nitrate complexation in a wet ionic liquid (IL; saturated with water as seen in the aqueous separation process) differs entirely from that in dry IL. A new approach to design more effective strategies for separation of actinides and lanthanides using an IL as a solvent has been revealed.

Activation of carbon dioxide by a terminal uranium–nitrogen bond in the gas-phase: a demonstration of the principle of microscopic reversibility

Activation of CO2 is demonstrated by its spontaneous dissociative reaction with the gas-phase anion complex NUOCl2(-), which can be considered as NUO(+) coordinated by two chloride anion ligands. This reaction was previously predicted by density functional theory to occur exothermically, without barriers above the reactant energy. The present results demonstrate the validity of the prediction of microscopic reversibility, and provide a rare case of spontaneous dissociative addition of CO2 to a gas-phase complex. The activation of CO2 by NUOCl2(-) proceeds by conversion of a U[triple bond, length as m-dash]N bond to a U[double bond, length as m-dash]O bond and creation of an isocyanate ligand to yield the complex UO2(NCO)Cl2(-), in which uranyl, UO2(2+), is coordinated by one isocyanate and two chloride anion ligands. This activation of CO2 by a uranium(vi) nitride complex is distinctive from previous reports of oxidative insertion of CO2 into lower oxidation state U(iii) or U(iv) solid complexes, during which both C-O bonds remain intact. This unusual observation of spontaneous addition and activation of CO2 by NUOCl2(-) is a result of the high oxophilicity of uranium. If the computed Gibbs free energy of the reaction pathway, rather than the energy, is considered, there are barriers above the reactant asymptotes such that the observed reaction should not proceed under thermal conditions. This result provides a demonstration that energy rather than Gibbs free energy determines reactivity under low-pressure bimolecular conditions.

Halide abstraction from halogenated acetate ligands by actinyls: a competition between bond breaking and bond making.

Transfer of halogen atoms from halogenated acetate ligands, CX3CO2 (X = F, Cl, Br), to actinyls, AnO2(2+) (An = U, Np, Pu) is stimulated by collision-induced dissociation (CID) in a quadrupole ion trap. CID of [AnO2(CF3CO2)3](-) complexes results exclusively in F atom transfer, concomitant with elimination of CF2CO2, to produce [(CF3CO2)2AnO2F](-), [(CF3CO2)AnO2F2](-), and [AnO2F3](-). This contrasts with CID of transition metal fluoroacetates for which CO2-elimination to produce organometallics is an important pathway, a disparity that can be attributed to the differing bond dissociation energies (BDEs) of the created metal-carbon and metal-fluorine bonds. The dominant pathway for CID of [AnO2(CF3CO2)(CCl3CO2)(CBr3CO2)](-) is Br-atom transfer to produce [(CF3CO2)(CCl3CO2)AnO2Br](-). The preferential formation of bromides, despite that the BDEs of An-F bonds are substantially greater than those of An-Br bonds, is attributed to the offsetting effect of higher BDEs for C-F versus C-Br bonds. The results for the trihaloacetates are similar for uranyl, neptunyl and plutonyl, indicating that for all three the An-X bond dissociation energies are sufficiently high that X atom transfer is overwhelmingly dominant. CID of [UO2(CH2XCO2)2(CX3CO2)](-) (X = F, Cl, Br) resulted in F-transfer only from CH2XCO2, but Cl- and Br-transfer from both CH2XCO2 and CX3CO2, a manifestation of the characteristic increase in BDE[C-F] in CHx-nFn species as n increases; the overall thermochemistry determines the observed CID processes, providing clear distinctions between fluorides and chlorides/bromides. The results of this work reveal the propensity of the actinides to form strong bonds with halogens, and suggest that there is not a large variation in actinyl-halogen BDEs between uranyl, neptunyl, and plutonyl.

Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO4–

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, AnVO2(C2O4)-, where An = Pa or U, are essentially actinyl ions, AnVO2+, coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, AnVO(OH)2(C2O4)-. The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas PaVI is inaccessible, UVI is very stable. The UVO2(C2O4)- complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O2 to yield UO4- and two CO2 molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, UVIO22+, coordinated in the equatorial plane by two equivalent O atoms each having formal charges of -1.5 and U-O bond orders intermediate between single and double. The unreactive nature of PaVO2(C2O4)- toward O2 is a manifestation of the resistance toward oxidation of PaV, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO4-, reacts with water to yield UO5H2-. Infrared spectra obtained for UO5H2- confirm the computed lowest-energy structure, UO3(OH)2-.

Cleaving Off Uranyl Oxygens through Chelation: A Mechanistic Study in the Gas Phase

Recent efforts to activate the strong uranium-oxygen bonds in the dioxo uranyl cation have been limited to single oxo-group activation through either uranyl reduction and functionalization in solution, or by collision induced dissociation (CID) in the gas-phase, using mass spectrometry (MS). Here, we report and investigate the surprising double activation of uranyl by an organic ligand, 3,4,3-LI(CAM), leading to the formation of a formal U6+ chelate in the gas-phase. The cleavage of both uranyl oxo bonds was experimentally evidenced by CID, using deuterium and 18O isotopic substitutions, and by infrared multiple photon dissociation (IRMPD) spectroscopy. Density functional theory (DFT) computations predict that the overall reaction requires only 132 kJ/mol, with the first oxygen activation entailing about 107 kJ/mol. Combined with analysis of similar, but unreactive ligands, these results shed light on the chelation-driven mechanism of uranyl oxo bond cleavage, demonstrating its dependence on the presence of ligand hydroxyl protons available for direct interactions with the uranyl oxygens.

Oxidation of Actinyl(V) Complexes by the Addition of Nitrogen Dioxide Is Revealed via the Replacement of Acetate by Nitrite.

The gas-phase complexes AnO2(CH3CO2)2(-) are actinyl(V) cores, An(V)O2(+) (An = U, Np, Pu), coordinated by two acetate anion ligands. Whereas the addition of O2 to U(V)O2(CH3CO2)2(-) exothermically produces the superoxide complex U(VI)O2(O2)(CH3CO2)2(-), this oxidation does not occur for Np(V)O2(CH3CO2)2(-) or Pu(V)O2(CH3CO2)2(-) because of the higher reduction potentials for Np(V) and Pu(V). It is demonstrated that NO2 is a more effective electron-withdrawing oxidant than O2, with the result that all three An(V)O2(CH3CO2)2(-) exothermically react with NO2 to form nitrite complexes, An(VI)O2(CH3CO2)2(NO2)(-). The assignment of the NO2(-) anion ligand in these complexes, resulting in oxidation from An(V) to An(VI), is substantiated by the replacement of the acetate ligands in AnO2(CH3CO2)2(NO2)(-) and AnO2(CH3CO2)3(-) by nitrites, to produce the tris(nitrite) complexes AnO2(NO2)3(-). The key chemistry of oxidation of An(V) to An(VI) by the addition of neutral NO2 is established by the substitution of acetate by nitrite. The replacement of acetate ligands by NO2(-) is attributed to a metathesis reaction with nitrous acid to produce acetic acid and nitrite.

Heptavalent Actinide Tetroxides NpO4 - and PuO4 -: Oxidation of Pu(V) to Pu(VII) by Adding an Electron to PuO4

The highest known actinide oxidation states are Np(VII) and Pu(VII), both of which have been identified in solution and solid compounds. Recently a molecular Np(VII) complex, NpO3(NO3)2-, was prepared and characterized in the gas phase. In accord with the lower stability of heptavalent Pu, no Pu(VII) molecular species has been identified. Reported here are the gas-phase syntheses and characterizations of NpO4- and PuO4-. Reactivity studies and density functional theory computations indicate the heptavalent metal oxidation state in both. This is the first instance of Pu(VII) in the absence of stabilizing effects due to condensed phase solvation or crystal fields. The results indicate that addition of an electron to neutral PuO4, which has a computed electron affinity of 2.56 eV, counterintuitively results in oxidation of Pu(V) to Pu(VII), concomitant with superoxide reduction.

Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States

Pentavalent actinyl nitrate complexes AnVO2(NO3)2- were produced by elimination of two NO2 from AnIII(NO3)4- for An = Pu, Am, Cm, Bk, and Cf. Density functional theory (B3LYP) and relativistic multireference (CASPT2) calculations confirmed the AnO2(NO3)2- as AnVO2+ actinyl moieties coordinated by nitrates. Computations of alternative AnIIIO2(NO3)2- and AnIVO2(NO3)2- revealed significantly higher energies. Previous computations for bare AnO2+ indicated AnVO2+ for An = Pu, Am, Cf, and Bk, but CmIIIO2+: electron donation from nitrate ligands has here stabilized the first CmV complex, CmVO2(NO3)2-. Structural parameters and bonding analyses indicate increasing An-NO3 bond covalency from Pu to Cf, in accordance with principles for actinide separations. Atomic ionization energies effectively predict relative stabilities of oxidation states; more reliable energies are needed for the actinides.