Distorted equatorial coordination environments and weakening of U=O bonds in uranyl complexes containing NCN and NPN ligands

Treatment of [UO(2)Cl(2)(thf)(3)] in thf with 2 equiv of Na[PhC(NSiMe(3))(2)] (Na[NCN]) or Na[Ph(2)P(NSiMe(3))(2)] (Na[NPN]) gives uranyl complex [UO(2)(NCN)(2)(thf)] (1) or [UO(2)(NPN)(2)] (3), respectively. Each complex is a rare example of out-of-plane equatorial nitrogen ligand coordination; the latter contains a significantly bent O=U=O unit and represents the first example of a uranyl ion within a quadrilateral-faced monocapped trigonal prismatic geometry. Removal of the thf in 1 gives [UO(2)(NCN)(2)] (2) with in-plane N donor ligands. Addition of 3 equiv of Na[NCN] gives the tris complex [Na(thf)(2)PhCN][[UO(2)(NCN)(3)] (4.PhCN) with elongation and weakening of one U=O bond through coordination to Na(+). Hydrolysis of 4 provides the oxo-bridged dimer [Na(thf)UO(2)(NCN)(2)](2)(micro(2)-O) (6), a complex with the lowest reported O=U=O symmetrical stretching frequency (nu(1) = 757 cm(-)(1)) for a dinuclear uranyl complex. The anion in complex 4 is unstable in solution but can be stabilized by the introduction of 18-crown-6 to give [Na(18-crown-6)][UO(2)(NCN)(3)] (5). The structures of 1-4 and 6 have been determined by crystallography, and all except 2 show significant deviations of the N ligand atoms from the equatorial plane, driven by the steric bulk of the NCN and NPN ligands. Despite the unusual geometries, these distortions in structure do not appear to have any direct effect on the bonding and electronic structure of the uranyl ion. The main influences toward lowering the U=O bond stretching frequency (nu(1)) are the donating ability of the equatorial ligands, overall charge of the complex, and U=O.Na-type interactions. The intense orange/red colors of these compounds are because of low-energy ligand-to-metal charge-transfer electronic transitions.     

A trigonal bipyramidal uranyl amido complex: synthesis and structural characterization of [Na(THF)2][UO2(N(SiMe3)2)3

The synthesis and structural characterization of a rare example of a uranyl complex possessing three equatorial ligands, [M(THF)2][UO2(N(SiMe3)2)3] (3a, M = Na; 3b, M = K), are described. The sodium salt 3a is prepared by protonolysis of [Na(THF)2]2[UO2(N(SiMe3)2)4], whereas the potassium salt 3b is obtained via a metathesis reaction of uranyl chloride UO2Cl2(THF)2 (4) with 3 equiv of K[N(SiMe3)2]. A single-crystal X-ray diffraction study of 3a revealed a trigonal-bipyramidal geometry about uranium, formed by two axial oxo and three equatorial amido ligands, with average U=O and U-N bond distances of 1.796(5) and 2.310(4) A, respectively. One of the oxo ligands is also coordinated to the sodium counterion. 1H NMR spectroscopic studies indicate that THF adds reversibly as a ligand to 3 to expand the trigonal bipyramidal geometry. The degree to which the coordination sphere in 3 is electronically satisfied with only three amido donors is suggested by (1) the reversible THF coordination, (2) a modest elongation in the bond distances for a five-coordinate U(VI) complex, and (3) the basicity of the oxo ligands as evidenced in the contact to Na. The vibrational spectra of the series of uranyl amido complexes [UO2(N(SiMe3)2)n]2-n (n = 2-4) are compared, to evaluate the effects on the axial U=O bonding as a function of increased electron density donated from the equatorial region. Raman spectroscopic measurements of the nu 1 symmetric O=U=O stretch show progressive axial bond weakening as the number of amido donors is increased. Crystal data for [Na(THF)2][UO2(N(SiMe3)2)3]: orthorhombic space group Pna2(1), a = 22.945(1) A, b = 15.2830(7) A, c = 12.6787(6) A, z = 4, R1 = 0.0309, wR2 = 0.0524.     

Role of the uranyl oxo group as a hydrogen bond acceptor

Density functional theory calculations have been used to evaluate the geometries and energetics of interactions between a number of uranyl complexes and hydrogen bond donor groups. The results reveal that although traditional hydrogen bond donors are repelled by the oxo group in the [UO(2)(OH(2))(5)](2+) species, they are attracted to the oxo groups in [UO(2)(OH(2))(2)(NO(3))(2)](0), [UO(2)(NO(3))(3)](-), and [UO(2)Cl(4)](2-) species. Hydrogen bond strength depends on the equatorial ligation and can exceed 15 kcal mol(-1). The results also reveal the existence of directionality at the uranyl oxo acceptor, with a weak preference for linear U═O---H angles.     

Structural, spectroscopic and redox properties of uranyl complexes with a maleonitrile containing ligand

The reaction of uranyl nitrate hexahydrate with the maleonitrile containing Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (salmnt((Et(2)N)(2))H(2)) in methanol produces [UO(2)(salmnt((Et2N)2))(H(2)O)] (1) where the uranyl equatorial coordination plane is completed by the N(2)O(2) tetradentate cavity of the (salmnt((Et(2)N)(2)))(2-) ligand and a water molecule. The coordinated water molecule readily undergoes exchange with pyridine (py), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and triphenylphosphine oxide (TPPO) to give a series of [UO(2)(salmnt((Et(2)N)(2)))(L)] complexes (L = py, DMSO, DMF, TPPO; 2-5, respectively). X-Ray crystallography of 1-5 show that the (salmnt((Et(2)N)(2)))(2-) ligand is distorted when coordinated to the uranyl moiety, in contrast to the planar structure observed for the free protonated ligand (salmnt((Et(2)N)(2))H(2)). The Raman spectra of 1-5 only display extremely weak bands (819-828 cm(-1)) that can be assigned to the typically symmetric O=U=O stretch. This stretching mode is also observed in the infrared spectra for all complexes 1-5 (818-826 cm(-1)) predominantly caused by the distortion of the tetradentate (salmnt((Et(2)N)(2)))(2-) ligand about the uranyl equatorial plane resulting in a change in dipole for this bond stretch. The solution behaviour of 2-5 was studied using NMR, electronic absorption and emission spectroscopy, and cyclic voltammetry. Complexes 2-5 exhibit intense absorptions in the visible region of the spectrum due to intramolecular charge transfer (ICT) transitions and the luminescence lifetimes (< 5 ns) indicate the emission arises from ligand-centred excited states. Reversible redox processes assigned to the {UO(2)}(2+)/{UO(2)}(+) couple are observed for complexes 2-5 (2: E(1/2) = -1.80 V; 3,5: E(1/2) = -1.78 V; 4: E(1/2) = -1.81 V : vs. ferrocenium/ferrocene {Fc(+)/Fc}, 0.1 M Bu(4)NPF(6)) in dichloromethane (DCM). These are some of the most negative half potentials for the {UO(2)}(2+)/{UO(2)}(+) couple observed to date and indicate the strong electron donating nature of the (salmnt((Et(2)N)(2)))(2-) ligand. Multiple uranyl redox processes are clearly seen for [UO(2)(salmnt((Et(2)N)(2)))(L)] in L (L = py, DMSO, DMF; 2-4: 0.1 M Bu(4)NPF(6)) indicating the relative instability of these complexes when competing ligands are present, but the reversible {UO(2)}(2+)/{UO(2)}(+) couple for the intact complexes can still be assigned and shows the position of this couple can be modulated by the solvation environment. Several redox processes were also observed between +0.2 and +1.2 V (vs. Fc(+)/Fc) that prove the redox active nature of the maleonitrile-containing ligand.     

Structure and hydrolysis of the U(IV), U(V), and U(VI) aqua ions from ab initio molecular simulations

Ab initio molecular dynamics simulations at 300 K, based on density functional theory, are performed to study the hydration shell geometries, solvent dipole, and first hydrolysis reaction of the uranium(IV) (U(4+)) and uranyl(V) (UO(2)(+)) ions in aqueous solution. The solvent dipole and first hydrolysis reaction of aqueous uranyl(VI) (UO(2)(2+)) are also probed. The first shell of U(4+) is coordinated by 8-9 water ligands, with an average U-O distance of 2.42 ?. The average first shell coordination number and distance are in agreement with experimental estimates of 8-11 and 2.40-2.44 ?, respectively. The simulated EXAFS of U(4+) matches well with recent experimental data. The first shell of UO(2)(+) is coordinated by five water ligands in the equatorial plane, with the average U═O(ax) and U-O distances being 1.85 ? and 2.54 ?, respectively. Overall, the hydration shell structure of UO(2)(+) closely matches that of UO(2)(2+), except for small expansions in the average U═O(ax) and U-O distances. Each ion strongly polarizes their respective first-shell water ligands. The computed acidity constants (pK(a)) of U(4+) and UO(2)(2+) are 0.93 and 4.95, in good agreement with the experimental values of 0.54 and 5.24, respectively. The predicted pK(a) value of UO(2)(+) is 8.5.     

Probing the local coordination environment and nuclearity of uranyl(VI) complexes in non-aqueous media by emission spectroscopy

We describe the synthesis, solid state and solution properties of two families of uranyl(VI) complexes that are ligated by neutral monodentate and anionic bidentate P=O, P=NH and As=O ligands bearing pendent phenyl chromophores. The uranyl(VI) ions in these complexes possess long-lived photoluminescent LMCT (3)Π(u) excited states, which can be exploited as a sensitive probe of electronic structure, bonding and aggregation behaviour in non-aqueous media. For a family of well defined complexes of given symmetry in trans-[UO(2)Cl(2)(L(2))] (L = Ph(3)PO (1), Ph(3)AsO (2) and Ph(3)PNH (3)), the emission spectral profiles in CH(2)Cl(2) are indicative of the strength of the donor atoms bound in the equatorial plane and the uranyl bond strength; the uranyl LMCT emission maxima are shifted to lower energy as the donor strength of L increases. The luminescence lifetimes in fluid solution mirror these observations (0.87-3.46 μs) and are particularly sensitive to vibrational and bimolecular deactivation. In a family of structurally well defined complexes of the related anion, tetraphenylimidodiphosphinate (TPIP), monometallic complexes, [UO(2)(TPIP)(thf)] (4), [UO(2)(TPIP)(Cy(3)PO)] 5), a bimetallic complex [UO(2)(TPIP)(2)](2) (6) and a previously known trimetallic complex, [UO(2)(TPIP)(2)](3) (7) can be isolated by variation of the synthetic procedure. Complex 7 differs from 6 as the central uranyl ion in 7 is orthogonally connected to the two peripheral ones via uranyl → uranium dative bonds. Each of these oligomers exhibits a characteristic optical fingerprint, where the emission maxima, the spectral shape and temporal decay profiles are unique for each structural form. Notably, excited state intermetallic quenching in the trimetallic complex 7 considerably reduces the luminescence lifetime with respect to the monometallic counterpart 5 (from 2.00 μs to 1.04 μs). This study demonstrates that time resolved and multi-parametric luminescence can be of value in ascertaining solution and structural forms of discrete uranyl(VI) complexes in non-aqueous solution.     

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.     

Lanthanum(III) and uranyl(VI) diglycolamide complexes: synthetic precursors and structural studies involving nitrate complexation

The synthesis and structural characterization of lanthanum(III) and uranyl(VI) complexes coordinated by tridentate diglycolamide (DGA) ligands O(CH2C(O)NR2)2[R=i-Pr (L1), i-Bu (L2)] are described. Reaction of L with UO2Cl2(H2O) n forms the uranyl(VI) cis-dichloride adducts UO2Cl2L [L=L1 (1a), L2 (1b)], while reaction of excess L with the corresponding metal nitrate hydrate produces [LaL3][La(NO3)6] [L=L1 (2a), L2 (2b)] for lanthanum and UO2(NO3)2L [L=L1 (3a), L2 (3b)] for uranium. Compounds 2b and 3a have been structurally characterized. The solid-state structure of the cation of 2b shows a triple-stranded helical arrangement of three tridentate DGA ligands with approximate D3 point-group symmetry, while the counteranion consists of six bidentate nitrate ligands coordinated around a second La center. The solid-state structure of 3a shows a tridentate DGA ligand coordinated along the equatorial plane perpendicular to the OUO unit as well as two nitrate ligands, one bidentate and oriented in the equatorial plane and the other monodentate and oriented parallel to the uranyl unit with the oxygen donor atom situated above the mean equatorial plane. Ambient-temperature NMR spectra for 3a and 3b indicated an averaged chemical environment of high symmetry consistent with fluxional nitrate hapticity, while spectroscopic data obtained at -30 degrees C revealed lower symmetry consistent with the slow-exchange limit for this process.     

Aqueous reactions of U(VI) at high chloride concentrations: syntheses and structures of new uranyl chloride polymers

The reactions of UO(3) with acidic aqueous chloride solutions resulted in the formation of two new polymeric U(VI) compounds. Single crystals of Cs(2)[(UO(2))(3)Cl(2)(IO(3))(OH)O(2)].2H(2)O (1) were formed under hydrothermal conditions with HIO(3) and CsCl, and Li(H(2)O)(2)[(UO(2))(2)Cl(3)(O)(H(2)O)] (2) was obtained from acidic LiCl solutions under ambient temperature and pressure. Both compounds contain pentagonal bipyramidal coordination of the uranyl dication, UO(2)(2+). The structure of 1 consists of infinite [(UO(2))(3)Cl(2)(IO(3))(mu(3)-OH)(mu(3)-O)(2)](2-) ribbons that run down the b axis that are formed from edge-sharing pentagonal bipyramidal [UO(6)Cl] and [UO(5)Cl(2)] units. The Cs(+) cations separate the chains from one another and form long ionic contacts with terminal oxygen atoms from iodate ligands, uranyl oxygen atoms, water molecules, and chloride anions. In 2, edge-sharing [UO(3)Cl(4)] and [UO(5)Cl(2)] units build up tetranuclear [(UO(2))(4)(mu-Cl)(6)(mu(3)-O)(2)(H(2)O)(2)](2-) anions that are bridged by chloride to form one-dimensional chains. These chains are connected in a complex network of hydrogen bonds and interactions of uranyl oxygen atoms with Li(+) cations. Crystal data: 1, orthorhombic, space group Pnma, a = 8.2762(4) A, b = 12.4809(6) A, c = 17.1297(8) A, Z = 4; 2, triclinic, space group P1, a = 8.110(1) A, b = 8.621(1) A, c = 8.740(1) A, Z = 2.     

Cation-cation interactions between uranyl cations in a polar open-framework uranyl periodate

Novel open-framework alkali metal uranyl periodates, having the formula A[(UO2)3(HIO6)(OH)(O)(H2O)].1.5H2O (A = Li, Na, K, Rb, Cs), have been prepared through mild hydrothermal synthesis. These isostructural compounds contain distorted UO7 pentagonal bipyramids that are linked through a uranyl (UO22+) to uranyl cation-cation interaction. This interaction arises from a single axial uranyl oxygen coordinating at an equatorial site of an adjacent uranyl unit. These uranium oxide polyhedra are further bound by IO6 distorted octahedra creating an open-framework structure whose channels contain the alkali metal cations.     

Theoretical investigation of uranyl dihydroxide: oxo ligand exchange, water catalysis, and vibrational spectra

Density functional theory is employed to investigate uranyl dihydroxide, UO2(OH)2, isomerization reaction energy barriers, including those occurring via proton shuttles. The ground-state structure of a uranyl dihydroxide complex containing a uranyl moiety with a near 90 degrees O=U=O bond angle is reported for the first time. Furthermore, we predict the vibrational spectra of these compounds. Scalar-relativistic effects for uranium are treated by employing a relativistic effective core potential.     

Steric control of substituted phenoxide ligands on product structures of uranyl aryloxide complexes

A series of uranyl aryloxide complexes has been prepared via metathesis reactions between [UO(2)Cl(2)(THF)(2)](2) and di-ortho-substituted phenoxides. Reaction of 4 equiv of KO-2,6-(t)()Bu(2)C(6)H(3) with [UO(2)Cl(2)(THF)(2)](2) in THF produces the dark red uranyl compound, UO(2)(O-2,6-(t)()Bu(2)C(6)H(3))(2)(THF)(2).THF, 1. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo groups, two cis-aryloxides, and two THF ligands. A similar product is prepared by reaction of KO-2,6-Ph(2)C(6)H(3) with [UO(2)Cl(2)(THF)(2)](2) in THF. Single-crystal X-ray diffraction analysis of this compound reveals it to be the trans-monomer UO(2)(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), 2. Dimeric structures result from the reactions of [UO(2)Cl(2)(THF)(2)](2) with less sterically imposing aryloxide salts, KO-2,6-Cl(2)C(6)H(3) or KO-2,6-Me(2)C(6)H(3). Single-crystal X-ray diffraction analyses of [UO(2)(O-2,6-Cl(2)C(6)H(3))(2)(THF)(2)](2), 3, and [UO(2)Cl(O-2,6-Me(2)C(6)H(3))(THF)(2)](2), 4, reveal similar structures in which each U atom is coordinated by seven ligands in a pseudopentagonal bipyramidal fashion. Coordinated to each uranium are two apical oxo groups and five equatorial ligands (3, one terminal phenoxide, two bridging phenoxides, and two nonadjacent terminal THF ligands; 4, one terminal chloride, two bridging phenoxides, and two nonadjacent terminal THF ligands). Apparently, the phenoxide ligand steric features exert a greater influence on the solid-state structures than the electronic properties of the substituents. Emission spectroscopy has been utilized to investigate the molecularity and electronic structure of these compounds. For example, luminescence spectra taken at liquid nitrogen temperature allow for a determination of the dependence of the molecular aggregation of 3 on the molecular concentration. Electronic and vibrational spectroscopic measurements have been analyzed to examine trends in emission energies and stretching frequencies. However, comparison of the data for compounds 1-4 reveals that the innate electron-donating capacity of phenoxide ligands is only subtly manifest in either the electronic or vibrational energy distributions within these molecules.     

Sterically congested uranyl complexes with seven-coordination of the UO2 unit: the peculiar ligation mode of nitrate in [UO2(NO3)2(Rbtp)] complexes

Addition of 1 or 2 molar equiv of Rbtp [Rbtp = 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine; R = Me, Pr ( n )] to UO 2(OTf) 2 in anhydrous acetonitrile gave the neutral compounds [UO 2(OTf) 2(Rbtp)] [R = Me ( 1), ( n )Pr ( 2)] and the cationic complexes [UO 2(Rbtp) 2][OTf] 2 [R = Me ( 3), Pr ( n ) ( 4)], respectively. No equilibrium between the mono and bis(Rbtp) complexes or between [UO 2(Rbtp) 2][OTf] 2 and free Rbtp in acetonitrile was detected by NMR spectroscopy. The crystal structures of 1 and 3 resemble those of their terpyridine analogues, and 3 is another example of a uranyl complex with the uranium atom in the unusual rhombohedral environment. In the presence of 1 molar equiv of Rbtp in acetonitrile, UO 2(NO 3) 2 was in equilibrium with [UO 2(NO 3) 2(Rbtp)] and the formation of the bis adduct was not observed, even with an excess of Rbtp. The X-ray crystal structures of [UO 2(NO 3) 2(Rbtp)] [R = Me ( 5), Pr ( n ) ( 6)] reveal a particular coordination geometry with seven coordinating atoms around the UO 2 fragment. The large steric crowding in the equatorial girdle forces the bidentate nitrate ligands to be almost perpendicular to the mean equatorial plane, inducing bending of the UO 2 fragment. The dinuclear oxo compound [U(CyMe 4btbp) 2(mu-O)UO 2(NO 3) 3][OTf] ( 7), which was obtained fortuitously from a 1:2:1 mixture of U(OTf) 4, CyMe 4btbp, and UO 2(NO 3) 2 [CyMe 4btbp = 6,6'-bis-(3,3,6,6-tetramethyl-cyclohexane-1,2,4-triazin-3-yl)-2,2'-bipyridine] is a very rare example of a mixed valence complex involving covalently bound U (IV) and U (VI) ions; its crystal structure also exhibits a seven coordinate uranyl moiety, with one bidentate nitrate group almost parallel to the UO 2 fragment. The distinct structural features of [UO 2(kappa (2)-NO 3) 2(Mebtp)], with its high coordination number and a noticeable bending of the UO 2 fragment, and of [UO 2(kappa (2)-NO 3)(kappa (1)-NO 3)(terpy)], which displays a classical geometry, were analyzed by Density Functional Theory, considering the bonding energy components and the molecular orbitals involved in the interaction between the uranyl, nitrate, and Mebtp or terpy moieties. The unusual geometry of the Mebtp derivative with the seven coordinating atoms around the UO 2 fragment was found very stable. In both the Mebtp and terpy complexes, the origin of the interaction appears to be primarily steric (Pauli repulsion and electrostatic); this term represents 62-63% of the total bonding energy while the orbital term contributes to about 37-38%.     

Ions generated from uranyl nitrate solutions by electrospray ionization (ESI) and detected with fourier transform ion-cyclotron resonance (FT-ICR) mass spectrometry

Electrospray ionization (ESI) of uranyl nitrate solutions generates a wide variety of positively and negatively charged ions, including complex adducts of uranyl ions with methoxy, hydroxy, and nitrate ligands. In the positive ion mode, ions detected by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry are sensitive to instrumental tuning parameters such as quadrupole operating frequency and trapping time. Positive ions correspond to oligomeric uranyl nitrate species that can be characterized as having a general formula of [(UO(2))(n)(A)(m)(CH(3)OH)(s)](+) or [(UO(2))(n)(O)(A)(m)(CH(3)OH)(s)](+) with n = 1-4, m = 1-7, s = 0 or 1, and A = OH, NO(3), CH(3)O or a combination of these, although the formation of NO(3)-containing species is preferred. In the negative ion mode, complexes of the form [(UO(2))(NO(3))(m)](-) (m = 1-3) are detected, although the formation of the oxo-containing ions [(UO(2))(O)(n)(NO(3))(m)](-) (n = 1-2, m = 1-2) and the hydroxy-containing ions [(UO(2))(OH)(n)(NO(3))(m)](-) (n = 1-2, m = 0-1) are also observed. The extent of coordinative unsaturation of both positive and negative ions can be determined by ligand association/exchange and H/D exchange experiments using D(2)O and CD(3)OD as neutral reaction partners in the gas-phase. Positive ions are of varying stability and reactivity and may fragment extensively upon collision with D(2)O, CD(3)OD and N(2) in sustained off-resonance irradiation/collision-induced dissociation (SORI-CID) experiments. Electron-transfer reactions, presumably occurring during electrospray ionization but also in SORI-CID, can result in reduction of U(VI) to U(V) and perhaps even U(IV).     

Novel dinuclear uranyl complexes with asymmetric schiff base ligands: synthesis, structural characterization, reactivity, and extraction studies

The reaction of uranyl nitrate with asymmetric [3O, N] Schiff base ligands in the presence of base yields dinuclear uranyl complexes, [UO2(HL1)]2.DMF (1), [UO2(HL2)]2.2DMF.H2O (2), and [UO2(HL3)]2.2DMF (3) with 3-(2-hydroxybenzylideneamino)propane-1,2-diol (H3L1), 4-((2,3-dihydroxypropylimino)methyl)benzene-1,3-diol (H3L2), and 3-(3,5-di-tert-butyl-2-hydroxybenzylideneamino)propane-1,2-diol (H3L3), respectively. All complexes exhibit a symmetric U2O2 core featuring a distorted pentagonal bipyramidal geometry around each uranyl center. The hydroxyl groups on the ligands are attached to the uranyl ion in chelating, bridging, and coordinate covalent bonds. Distortion in the backbone is more pronounced in 1, where the phenyl groups are on the same side of the planar U2O2 core. The phenyl groups are present on the opposite side of U2O2 core in 2 and 3 due to electronic and steric effects. A similar hydrogen-bonding pattern is observed in the solid-state structures of 1 and 3 with terminal hydroxyl groups and DMF molecules, resulting in discrete molecules. Free aryl hydroxyl groups and water molecules in 2 give rise to a two-dimensional network with water molecules in the channels of an extended corrugated sheet structure. Compound 1 in the presence of excess Ag(NO3) yields {[(UO2)(NO3)(C6H4OCOO)](NH(CH2CH3)3)}2 (4), where the geometry around the uranyl center is hexagonal bipyrimidal. Two-phase extraction studies of uranium from aqueous media employing H3L3 indicate 99% reduction of uranyl ion at higher pH.     

Cation-cation complexes of pentavalent uranyl: from disproportionation intermediates to stable clusters

Three new cation-cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO(2)py(5))(KI(2)py(2))](n) (1) with the Schiff base ligands salen(2-), acacen(2-), and salophen(2-) (H(2)salen = N,N'-ethylene-bis(salicylideneimine), H(2)acacen = N,N'-ethylenebis(acetylacetoneimine), H(2)salophen = N,N'-phenylene-bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen(2-) in pyridine leads to immediate disproportionation, but in the presence of [18]crown-6 ([18]C-6) a stable complex forms. The solid-state structure of the four tetranuclear complexes, {[UO(2)(acacen)](4)[μ(8)-](2)[K([18]C-6)(py)](2)} (3) and {[UO(2)(acacen)](4)[μ(8)-]}?2?[K([222])(py)] (4), {[UO(2)(salophen)](4)[μ(8)-K](2)[μ(5)-KI](2)[(K([18]C-6)]}?2?[K([18]C-6)(thf)(2)]?2?I (5), and {[UO(2)(salen)(4)][μ(8)-Rb](2)[Rb([18]C-6)](2)} (9) ([222] = [222]cryptand, py = pyridine), presenting a T-shaped cation-cation interaction has been determined by X-ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U(V)O(2)(salen)(py)][Cp*(2)Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation-cation complexes. The nature of the cation plays a key role in the preparation of stable cation-cation complexes. Stable tetranuclear complexes form in the presence of K(+) and Rb(+), whereas Li(+) leads to disproportionation. A new uranyl-oxo cluster was isolated from this reaction. The reaction of [U(V)O(2)(salen)(py)][Cp*(2)Co] (Cp* = pentamethylcyclopentadienyl) with its U(VI) analogue yields the oxo-functionalized dimer [UO(2)(salen)(py)](2)[Cp*(2)Co] (8). The reaction of the {[UO(2)(salen)(4)][μ(8)-K](2)[K([18]C-6)](2)} tetramer with protons leads to disproportionation to U(IV) and U(VI) species and H(2)O confirming the crucial role of the proton in the U(V) disproportionation.     

Near- and mid-infrared spectroscopy of the uranyl selenite mineral haynesite (UO2)3(SeO3)2(OH)2.5H2O

Near- and mid-infrared spectra of uranyl selenite mineral haynesite (UO(2))(3)(SeO(3))(2)(OH)(2).5H(2)O, were studied and assigned. Observed bands were assigned to the stretching vibrations of uranyl and selenite units, stretching, bending and libration modes of water molecules and hydroxyl ions, and delta U-OH bending vibrations. U-O bond lengths in uranyl and hydrogen bond lengths O-H...O were inferred from the spectra.     

Hydrolysis of uranyl(VI) in acidic and basic aqueous solutions using a noncomplexing organic base: a multivariate spectroscopic and statistical study

In the field of actinide aqueous chemistry, this work aims to resolve some controversy about uranyl(VI) hydroxide species present in basic aqueous solutions. We revisit the Raman, IR, and UV-visible spectra with two new approaches. First, Raman, IR and UV data were recorded systematically from aqueous solutions with the noncomplexing electrolyte (C(2)H(5))(4)NNO(3) at 25 °C and 0.1 MPa ([U(total)] = 0.005-0.105 M) in H(2)O and D(2)O over a wide range of -log mH(D)(+) between 2.92 and 14.50. Second, vibrational spectra (IR and Raman) of basic solutions in H(2)O and D(2)O were analyzed using the Bayesian Positive Source Separation method to estimate pure spectra of individual species. In D(2)O solutions, the new spectroscopic data showed the occurrence of the same species as those in H(2)O. As observed for the wavenumber of the symmetric stretching mode, the wavenumber characteristic of the O═U═O antisymmetric stretching mode decreases as the number of OH(D)(-) ligands increases. These kinds of data, completed by (1) analysis of the signal widths, (2) persistence of the apparent exclusion rule between IR and Raman spectra of the uranyl species stretching modes, and (3) interpretation of the absorption UV-visible spectra, allow discussion of the chemistry, structures, and polynuclearity of uranyl(VI) species. In moderate basic solutions, the presence of two trimers is suggested. In highly basic solutions ([OH(-)] ≈ 3 M), the two monomers UO(2)(OH)(4)(2-) and UO(2)(OH)(5)(3-) are confirmed to be in good agreement with earlier EXAFS and NMR results. The occurrence of the UO(2)(OH)(6)(4-) monomer is also suggested from the more basic solutions investigated.     

Raman spectroscopic study of the molecular structure of the uranyl mineral zippeite from Jáchymov (Joachimsthal), Czech Republic

Raman spectra at 298 and 77K and infrared spectra of the uranyl sulfate mineral zippeite from Jáchymov (Joachimsthal), Czech Republic, K(0.6)(H(3)O)0.4[(UO(2))6(SO(4))3(OH)7].8H2O, were studied. Observed bands were tentatively attributed to the (UO(2))2+ and (SO(4))2- stretching and bending vibrations, the OH stretching vibrations of water molecules, hydroxyls and oxonium ions, and H(2)O, oxonium, and delta U-OH bending vibrations. Empirical relations were used for the calculation of U-O bond lengths in uranyl R (A)=f(nu(3) or nu(1)(UO(2))2+). Calculated U-O bond lengths are in agreement with U-O bond lengths from the single crystal structure analysis and those inferred for uranyl anion sheet topology of uranyl pentagonal dipyramidal coordination polyhedra. The number of observed bands supports the conclusion from single crystal structure analysis that at least two symmetrically distinct U6+ (in uranyls) and S6+ (in sulfates), water molecules and hydroxyls may be present in the crystal structure of the zippeite studied. Strong to very weak hydrogen bonds present in the crystal structure of zippeite studied were inferred from the IR spectra.     

On the "yl" bond weakening in uranyl(VI) coordination complexes

The U-O(yl) triple bonds in the UO(2)(2+) aquo ion are known to be weakened by replacing the first shell water with organic or inorganic ligands. Weakening of the U-O(yl) bond may enhance the reactivity of "yl" oxygens and uranyl(VI) cation-cation interactions. Density functional theory calculations as well as previously published vibrational spectroscopic data have been used to study the origin of the U-O(yl) bond weakening in uranyl(VI) coordination complexes. Natural population analyses (NPA) revealed that the electron localization on the O(yl) 2p orbital is a direct measure of the U-O(yl) bond weakening, indicating that the bond weakening is correlated to the weakening of the U-O(yl) covalent bond and not that of the ionic bond. The Mulliken analysis gives poor results for uranium to ligand electron partitioning and is thus unreliable. Further analyses of molecular orbitals near the highest occupied molecular orbital (HOMO) show that both the σ and π donating abilities of the ligands may account for the U-O(yl) bond weakening. The mechanism of the bond weakening varies with coordinating ligand so that each case needs to be examined independently.