Uranyl Moiety Research Articles

DFT Study of Oxo-Functionalized Pentavalent Dioxouranium Complexes: Structure, Bonding, Ligand Exchange, Dimerization, and U(V)/U(IV) Reduction of OUOH and OUOSiH3 Complexes

The structural and electronic properties of model oxo-functionalized pentavalent dioxouranium complexes have been studied using scalar relativistic density functional theory (DFT) calculations. The electronic structures of these complexes are compared to those of their hexavalent and pentavalent counterparts with free axial oxo groups while paying particular emphasis on the effect of oxo-functionalization on the formation of binuclear complexes, the U(V)/U(IV) redox potentials, as well as ligand exchange between the axial and equatorial regions of the dioxouranium moiety. The stabilization of the σ(d) orbitals of the UO(2) moiety is one of the major effects of oxo-functionalization. The origin of this effect is the mixing of the σ(d) orbital of the uranyl group with the σ(OH)/σ(OSiH(3)) orbitals of the axial OH/OSiH(3) group. The 6p atomic orbitals of the uranium center are mixed to a greater extent with the σ(d) orbital after stabilization caused by oxo-functionalization. The asymmetric nature of the oxo-functionalization has dramatic effects not only on the U-O bond lengths (elongation by up to 0.24 Å) and U-O bond orders (loss of a full bond order) but also on the formation and type of U(2)O(4) core found in binuclear complexes. The loss of a full bond order upon oxo-functionalization means the axial U-OH/U-OSiH(3) bonds are only slightly stronger than they would be if they were found in the equatorial region of the uranyl moiety. This raises the possibility of ligand exchange between the axial and equatorial regions as well as increasing the stability of the binuclear complexes with butterfly-shaped U(2)O(4) cores relative to those with diamond U(2)O(4) cores. Reductive oxo-functionalization results in complexes with lower electron density at their U(V) centers in comparison to UO(2)(+) complexes. This has dramatic effects on the calculated U(V)/U(IV) redox potentials.

Uranium–Ligand Multiple Bonding in Uranyl Analogues, [L═U═L]n+, and the Inverse Trans Influence

The societal importance of uranium complexes containing the uranyl moiety [O═U═O](2+) continues to grow with the ongoing international nuclear enterprise and associated accumulating legacy waste. Further studies of the electronic structure of uranyl and its analogues are imperative for the development of crucial technologies, including lanthanide/actinide extractants and chemical and environmental remediation methodologies. Actinide oxo halides are a subset of the growing class of actinyl (uranyl) analogues. The understanding of their electronic structures links the detailed spectroscopic studies of uranyl, indicating the role of the pseudocore 6p orbitals in U-O bonding, to hypotheses about the 6p orbitals' role in the chemical bonding of uranyl analogues. These actinide oxo halides are a very small class of actinide compounds that present the inverse trans influence (ITI). This class of complexes was, until recently, limited to two crystallographically characterized compounds, namely, [UCl(5)O][PPh(4)] and [PaCl(5)O][NEt(4)](2). These complexes are important because they give a readily and clearly defined experimental observable: the difference between the M-X(trans) and M-X(cis) (here X = Cl) bond lengths in the solid state. This bond metric is a sensitive probe for the role of 6p, 6d, and 5f orbitals, as well as electrostatic interactions, in determining their electronic structure. This Viewpoint Article reviews the theoretical, experimental, and synthetic work on the ITI in actinide complexes and contextualizes it within broader studies on the electronic structure of uranyl and its analogues. Furthermore, our recent work on the ITI in high-valent uranium(V/VI) oxo and imido complexes is described as a whole. This work builds on the extant synthetic literature on the ITI and provides design parameters for the synthesis and characterization of high-valent uranium-ligand multiple bonds.

New Reactivity of the Uranyl(VI) Ion

The chemistry of the uranyl ion ([UO(2)](2+)) has evolved remarkably over the past few years, with unexpected reactivity observed that challenge our understanding of this ion, and of actinides in general. This review highlights some recent advances in the field, focussing on the organometallic chemistry of the uranyl moiety, which is not well developed in comparison to lower oxidation states of uranium. The use of uranyl as a catalyst is highlighted and the newly developed supramolecular chemistry is described. The uranyl oxygen atoms have been considered as inert, but recent work has shown that is not necessarily the case and is discussed herein. Finally, reduction to the [UO(2)](+) ion will be discussed.

Aqueous Uranium(VI) Complexes with Acetic and Succinic Acid: Speciation and Structure Revisited

We employed density functional theory (DFT) calculations, and ultraviolet-visible (UV-vis), extended X-ray absorption fine-structure (EXAFS), and attenuated total reflection Fourier-transform infrared (IR) spectroscopy analyzed with iterative transformation factor analysis (ITFA) to determine the structures and the pH-speciation of aqueous acetate (ac) and succinate (suc) U(VI) complexes. In the acetate system, all spectroscopies confirm the thermodynamically predicted pH-speciation by Ahrland (1951), with the hydrated uranyl ion and a 1:1, a 1:2 and a 1:3 U(VI)-ac complex. In the succinate system, we identified a new 1:3 U(VI)-suc complex, in addition to the previously known 1:1 and 1:2 U(VI)-suc complexes, and determined the pH-speciation for all complexes. The IR spectra show absorption bands of the antisymmetric stretching mode of the uranyl mojety (υ3(UO2)) at 949, 939, 924 cm(-1) and at 950, 938, 925 cm(-1) for the 1:1, 1:2 and 1:3 U(VI)-ac and U(VI)-suc complexes, respectively. IR absorption bands at 1535 and 1534 cm(-1) and at 1465 and 1462 cm(-1) are assigned to the antisymmetric υ3,as(COO) and symmetric υ3,s(COO) stretching mode of bidentately coordinated carboxylic groups in the U(VI)-ac and U(VI)-suc complexes. The assignment of the three IR bands (υ3(UO2), υ3,as(COO), υ3,s(COO)) and the stoichiometry of the complexes is supported by DFT calculations. The UV-vis spectra of the equivalent U(VI)-ac and U(VI)-suc complexes are similar suggesting common structural features. Consistent with IR spectroscopy and DFT calculations, EXAFS showed a bidentate coordination of the carboxylic groups to the equatorial plane of the uranyl moiety for all uranyl ligand complexes except for the newly detected 1:3 U(VI)-suc complex, where two carboxylic groups coordinate bidentately and one carboxylic group coordinates monodentately. All 1:1 and 1:2 complexes have a U-Owater distance of ∼2.36 Å, which is shorter than the U-Owater distance of ∼2.40 Å of the hydrated uranyl ion. For all complexes the U-Ocarboxyl distance of the bidentately coordinated carboxylic group is ∼2.47 Å, while the monodentately coordinated carboxylic group of the 1:3 U(VI)-suc complex has a U-Ocarboxyl distance of ∼2.36 Å, that is, similar to the short U-Owater distance in the 1:1 and 1:2 complexes.

Dianion N1,N4-bis(salicylidene)-S-allyl-thiosemicarbazide complexes: Synthesis, structure, spectroscopy and thermal behavior

The products with the general formulas VO(Salits) 1, Mn(Salits)Br 2, Fe(Salits)Cl 3 and UO2(Salits)(MeOH) 4 (Salits=N1,N4-bis(salicylidene)-S-allyl-thiosemicarbazide) are prepared by the reaction of salicylaldehyde-S-allyl-thiosemicarbazone (H2L) and salicylaldehyde with VOSO4·3H2O, Mn(CH3COO)2·4H2O, FeCl3·6H2O and UO2(CH3COO)2·2H2O, respectively. Complexes 1–4 were characterized by elemental analysis, spectroscopic (FT-IR and UV–Vis) and TGA techniques. Crystal and molecular structures of the complexes were determined by single crystal X-ray diffraction. The results revealed that the complexes 1, 2 and 3 contain a dideprotonated tetradentate form of Salits coordinated via N2O2 donors along with oxido, bromido or chlorido ligands, respectively, and they have distorted square pyramidal geometries. However, the complex 4 adopts a severely distorted pentagonal bipyramidal geometry consisting of N2O2 coordination of Salits, a methanol ligand and two oxygens of the uranyl moiety. TG analysis showed that complex 3 is more stable than the others.

The complexation of uranium(VI) and atmospherically derived CO2 at the ferrihydrite–water interface probed by time-resolved vibrational spectroscopy

The sorption reactions of uranium(VI) at the ferrihydrite(Fh)–water interface were investigated in the absence and presence of atmospherically derived CO2 by time-resolved in situ vibrational spectroscopy. The spectra clearly show that a single uranyl surface species, most probably a mononuclear bidentate surface complex, is formed irrespective of the presence of atmospherically derived CO2. The character of the carbonate surface species correlates with the presence of the actinyl ions and changes from a monodentate to a bidentate binding upon sorption of U(VI). From the in situ sorption experiments under mildly acid conditions, the formation of a ternary surface complex is derived where the carbonate ligands coordinate bidentately to the uranyl moiety (UO2(O2CO)x). Furthermore, the release reaction of the carbonate ligands from the ternary surface complex is found to be considerably retarded compared to those from the pristine surface suggesting a tighter bonding of the carbonate ions in the ternary complex. Simultaneous sorption of U(VI) and atmospherically derived carbonate onto pristine Fh shows formation of binary monodentate carbonate surface complexes prior to the formation of the ternary complexes.

An integrated X-ray and molecular dynamics study of uranyl-salen structures and properties

Uranium complexes of bis(p-tert-butyl-salicylidene)-1,2-diphenylethylenediamine (1) and bis(salicylidene)-1,2-diphenylethylenediamine (2) have been synthesized and investigated by X-ray single crystal diffraction and MD calculations in Periodic Boundary Conditions. Both compounds form crystals which are densely packed and do not provide voids accessible to solvent molecules. The configurations adopted by 1 and 2 are determined by well defined T-shaped and π-stacking non covalent interactions between phenyl groups of adjacent molecules as well as by a network of hydrogen bonds. These interactions and the relative arrangements of the molecules, explain the packing in the crystal structures. Each uranyl moiety shows a penta-coordination in the equatorial plane perpendicular to the trans oxygens giving rise, in both compounds, to a bypiramidal geometry. As usual for this class of compounds, the 5th position is characterized by the presence of the coordinated solvent. The in silico simulations confirm this hypothesis in very fine details. Moreover, in 1, even the partial occupancy of the solvent molecule determined from the crystal structure refinement, was shown to be due to a constrained freedom of motion of the solvent molecule that can be reproduced by molecular dynamics. This suggests that the reported disorder is not due to a poor quality of the harvested crystals but to a structural feature. In further agreement with the above mentioned results, DFT calculations demonstrated that the molecular orbital configuration and energies suit the described properties of complexes 1 and 2 suggesting a potential enantioselective activity as already shown by molecules belonging to this class of compounds.

Uranyl Sensitization of Samarium(III) Luminescence in a Two-Dimensional Coordination Polymer

Heterometallic carboxyphosphonates UO(2)(2+)/Ln(3+) have been prepared from the hydrothermal reaction of uranyl nitrate, lanthanide nitrate (Ln = Sm, Tb, Er, Yb), and phosphonoacetic acid (H(3)PPA). Compound 1, (UO(2))(2)(PPA)(HPPA)(2)Sm(H(2)O)·2H(2)O (1) adopts a two-dimensional structure in which the UO(2)(2+) metal ions bind exclusively to the phosphonate moiety, whereas the Ln(3+) ions are coordinated by both phosphonate and carboxylate functionalities. Luminescence studies of 1 show very bright visible and near-IR samarium(III)-centered emission upon direct excitation of the uranyl moiety. The Sm(3+) emissive state exhibits a double-exponential decay with lifetimes of 67.2 ± 6.5 and 9.0 ± 1.3 μs as measured at 594 nm, after excitation at both 365 and 420 nm. No emission is observed in the region typical of the uranyl cation, indicating that all energy is either transferred to the Sm(3+) center or lost to nonradiative processes. Herein we report the synthesis, crystal structure, and luminescent behavior of 1, as well as those of the isostructural terbium, erbium, and ytterbium analogues.

Facile Reduction of a Uranyl(VI) β-Ketoiminate Complex to U(IV) Upon Oxo Silylation

Reaction of the uranyl β-ketoiminate complex UO(2)((tBu)acnac)(2) (1) ((tBu)acnac = (t)BuNC(Ph)CHC(Ph)O) with Me(3)SiI, in the presence of Ph(3)P, results in the reductive silylation of the uranyl moiety and formation of the U(V) bis-silyloxide complex [Ph(3)PI][U(OSiMe(3))(2)I(4)] (2). Subsequent reaction of 2 with Lewis bases, such as 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), and tetrahydrofuran (THF), results in a further reduction of the metal center and isolation of the U(IV) complexes U(OSiMe(3))(2)I(2)(bipy)(2) (3), U(OSiMe(3))(2)I(2)(phen)(2) (4), and [U(OSiMe(3))(2)I(THF)(4)][I(3)] (5), respectively.

Uranyl sequestration: synthesis and structural characterization of uranyl complexes with a tetradentate methylterephthalamide ligand

Uranyl complexes of a bis(methylterephthalamide) ligand (LH(4)) have been synthesized and characterized by X-ray crystallography. The structure is an unexpected [Me(4)N](8)[L(UO(2))](4) tetramer, formed via coordination of the two MeTAM units of L to two uranyl moieties. Addition of KOH to the tetramer gave the corresponding monomeric uranyl methoxide species [Me(4)N]K(2)[LUO(2)(OMe)].

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.

Effects of the self-interaction error in Kohn–Sham calculations: A DFT + U case study on penta-aqua uranyl(VI)

Using the LCGTO-DFT + U methodology, we analyze the potential effect of self-interaction artifacts in standard Kohn–Sham (KS) density functional calculations on geometries, vibrational frequencies, and bonding for the example of the uranyl (VI) dication, both without and with explicit aqua ligands. We attribute the bent structure of the uranyl moiety in the penta-aqua complex to the limited accuracy of the Coulomb interaction of the U 5 f orbitals afforded by common local or gradient-corrected exchange–correlation functionals. In particular, with scalar relativistic, all-electron Douglas–Kroll–Hess calculations we demonstrate that the distorted geometry of the uranyl penta-aqua complex is essentially rectified by the inclusion of a small positive on-site repulsion parameter, U eff, for the 5 f shell of the uranium atom.

Formation of Uranium(IV) Oxide Clusters from Uranocene [U(η8-C8H8)2] and Uranyl [UO2X2] Compounds

Uranocene [U(eta(8)-C(8)H(8))(2)] reacted in refluxing pyridine with 2 equiv of the uranyl(VI) compound [UO(2)(OTf)(2)] or [UO(2)I(2)(THF)(3)] (OTf = O(3)SCF(3); THF = tetrahydrofuran) or 1 equiv of the uranyl(V) complex [UO(2)(py)(2.3)K(OTf)(2)] to afford the hexanuclear uranium(IV) oxide cluster [U(6)O(8)(OTf)(8)(py)(8)] (1) or [U(6)O(8)I(8)(py)(10)] (3). Complexes 1 and 3 were easily isolated in good yield because they were deposited as microcrystalline powders with the release of free cyclooctatetraene as a unique byproduct. A similar reaction in THF gave [U(6)O(8)(OTf)(8)(THF)(8)] (2), whose isolation was impeded by polymerization of the solvent. With complexes [U(eta(8)-C(8)H(8))(2)] and [UO(2)(OTf)(2)] in a molar ratio of 4:3 or in the presence of excess uranocene, only crystals of the mono(eta(8)-C(8)H(8)) aggregate [U(6)O(8)(eta(8)-C(8)H(8))(OTf)(6)(py)(8)] (4) were obtained. The crystal structures of 1 x 2 py, 2 x 2 THF, 3 x py, and 4 x py were determined. Whereas uranocene and uranyl(VI) compounds are generally viewed as the most robust uranium compounds, they were found to fuse together into hexanuclear assemblages. These studies revealed the unusual four-electron reductive capacity of a uranium(IV) complex, in particular uranocene, which, through the loss of its two C(8)H(8) ligands, induces the activation and reduction of the strong U=O bonds of the uranyl moiety.

Unusual case of a polar copper(II) uranyl phosphonate that fluoresces

Abstract A polar Cu(II) uranyl diphosphonate, Cu(H2O)4(UO2)3(H2O)2[CH2(PO3)2]2·5H2O, has been prepared under mild hydrothermal conditions. This compound has direct linkages between the oxo atoms of the uranyl moieties and the Cu(II) centers. Despite the presence of Cu(II) in the structure, vibronically-coupled emission is still observed, most likely because there are two crystallographically unique uranyl moieties, only one of which bonds to Cu(II).

Synthesis, spectroscopic characterization and X-ray structure analyses of two uranyl complexes

Abstract The synthesis, spectroscopic characterization and crystal structures of two uranyl complexes, [UO2(SALEN)(DMF)] (1) and [UO2(BPHA)2(DMF)] (2) {SALEN = N,N′-ethylenebis(salicylimine), DMF = N,N-dimethylformamide, BPHA = N-phenyl benzohydroxamate} are described. The coordination geometry around the metal center in the mononuclear complexes can be best described as distorted pentagonal bipyramidal with an almost linear uranyl moiety. In addition to the van der Waal´s interactions, the crystal packing in the complexes is stabilized by weak intermolecular C–H…O hydrogen bonds involving the uranyl oxygen atoms, so generating two-dimensional arrays in both crystal structures.

Exploring the Effects of Reduction or Lewis Acid Coordination on the U═O Bond of the Uranyl Moiety

Reaction of Li(ArNC(Ph)CHC(Ph)O) ((Ar)acnac; Ar = 2,4,6-Me(3)C(6)H(2)) or Na(ArNC(Ph)CHC(Ph)O) (Ar = 3,5-(t)Bu(2)C(6)H(3)) with 0.5 equiv of UO(2)Cl(2)(THF)(3) results in the formation of UO(2)((Ar)acnac)(2) (Ar = 2,4,6-Me(3)C(6)H(2), 1; 3,5-(t)Bu(2)C(6)H(3), 2), which were isolated as orange crystalline solids in good yields. The structure of 2 has been confirmed by X-ray crystallography, while the solution redox properties of 1 and 2 have been measured by cyclic voltammetry. Complex 1 exhibits a reversible reduction feature at E(1/2) = -1.52 V (vs Fc/Fc(+)), while complex 2 exhibits a reduction feature at -1.35 V (vs Fc/Fc(+)). Complexes 1 and 2 react with Cp*(2)Co to generate [Cp*(2)Co][UO(2)((Ar)acnac)(2)] (Ar = 2,4,6-Me(3)C(6)H(2), 3; 3,5-(t)Bu(2)C(6)H(3), 4), in moderate to good yields. Both 3 and 4 have been fully characterized, while the structure of 4 has also been determined by X-ray crystallography. Reaction of 2 with 2 equiv of B(C(6)F(5))(3) in CH(2)Cl(2) leads to the isolation of UO(OB{C(6)F(5)}(3))((Ar)acnac)(2) (Ar = 3,5-(t)Bu(2)C(6)H(3)) (5). Complex 5, generated in situ, exhibits an irreversible reduction at -0.78 V (vs Fc/Fc(+), 100 mV/s scan rate) which is considerably lower than the reduction potential observed for 2, consistent with the removal of electron density from the uranyl moiety by coordination of B(C(6)F(5))(3).

Synthesis, crystallographic characterization, and conformational prediction of a structurally unique molecular mixed-ligand U(VI) solid, Na6[UO2(O2)2(OH)2](OH)2·14H2O

Abstract The first mononuclear molecular mixed-ligand U(VI) solid containing hydroxide and peroxide ligands, Na6[UO2(O2)2(OH)2](OH)2·14H2O (I), was synthesized and structurally characterized using single crystal X-ray diffraction. The crystal structure of I consisted of [UO2(O2)2(OH)2]4− molecular units, with a uranyl(VI) moiety perpendicular to 6 equatorial O atoms belonging to two side-on trans peroxo groups and two terminal trans hydroxo groups. Density functional theory (DFT) calculations determined that the trans -conformer of the [UO2(O2)2(OH)2]4− molecular unit found in I was 24 kcal/mol lower in energy than the previously proposed cis -conformer. Crystal data: monoclinic, space group P21/n with a=13.357(4) Å, b=5.8521(15) Å, c=15.948(6) Å, β=112.292(4)°, and Z=2.

Parametric Investigation of Laser-Induced Fluorescence of Solid-State Uranyl Compounds

The combination of remote/standoff sensing and laser-induced fluorescence (LIF) spectroscopy shows potential for detection of uranyl (UO2(2+)) compounds. Uranyl compounds exhibit characteristic emission in the 450-600 nm (22,200 to 16,700 cm(-1)) spectral region when excited by wavelengths in the ultraviolet or in the short-wavelength portion of the visible spectrum. We report a parametric study of the effects of excitation wavelength [including 532 nm (18,797 cm(-1)), 355 nm (28,169 cm(-1)), and 266 nm (37,594 cm(-1))] and excitation laser power on solid-state uranium compounds. The uranium compounds investigated include uranyl nitrate, uranyl sulfate, uranyl oxalate, uranium dioxide, triuranium octaoxide, uranyl acetate, uranyl formate, zinc uranyl acetate, and uranyl phosphate. We observed the characteristic uranyl fluorescence spectrum from the uranium compounds except for uranium oxide compounds (which do not contain the uranyl moiety) and for uranyl formate, which has a low fluorescence quantum yield. Relative uranyl fluorescence intensity is greatest for 355 nm excitation, and the order of decreasing fluorescence intensity with excitation wavelength (relative intensity/laser output) is 355 nm > 266 nm > 532 nm. For 532 nm excitation, the emission spectrum is produced by two-photon excitation. Uranyl fluorescence intensity increases linearly with increasing laser power, but the rate of fluorescence intensity increase is different for different emission bands.

H2bipy]2[(UO2)6Zn2(PO3OH)4(PO4)4]·H2O: An Open-Framework Uranyl Zinc Phosphate Templated by Diprotonated 4,4′-Bipyridyl

Under mild hydrothermal conditions, a new organically templated uranyl zinc phosphate, [H 2bipy] 2[(UO 2) 6Zn 2(PO 3OH) 4(PO 4) 4].H 2O ( UZnP-2), has been synthesized. Structural analysis reveals that UZnP-2 is constructed from UO 7 pentagonal bipyramids that are linked into edge-sharing dimers that are in turn joined together by ZnO 4 and PO 4 tetrahedra to form a three-dimensional network. Intersecting channels occur along the a, b, and c axes. These channels house the diprotonated 4,4'-bipyridyl cations and water molecules. Ion-exchange experiments demonstrate that replacement of the 4,4'-bipyridyl cations by alkali and alkaline-earth metal cations results in a rearrangement of the framework. Further characterization of UZnP-2 is provided by Raman and fluorescence spectroscopy. The latter method reveals strong emission from the uranyl moieties with characteristic fine structure.

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%.