Uranyl Carboxylates Research Articles

Woven, Polycatenated, or Cage Structures: Effect of Modulation of Ligand Curvature in Heteroleptic Uranyl Ion Complexes

Combining the flexible zwitterionic dicarboxylate 4,4'-bis(2-carboxylatoethyl)-4,4'-bipyridinium (L) and the anionic dicarboxylate ligands isophthalate (ipht2-) and 1,2-, 1,3-, or 1,4-phenylenediacetate (1,2-, 1,3-, and 1,4-pda2-), of varying shape and curvature, has allowed isolation of five uranyl ion complexes by synthesis under solvo-hydrothermal conditions. [(UO2)2(L)(ipht)2] (1) and [(UO2)2(L)(1,2-pda)2]·2H2O (2) have the same stoichiometry, and both crystallize as monoperiodic coordination polymers containing two uranyl-(anionic carboxylate) strands united by L linkers into a wide ribbon, all ligands being in the divergent conformation. Complex 3, [(UO2)2(L)(1,3-pda)2]·0.5CH3CN, with the same stoichiometry but ligands in a convergent conformation, is a discrete, binuclear species which is the first example of a heteroleptic uranyl carboxylate coordination cage. With all ligands in a divergent conformation, [(UO2)2(L)(1,4-pda)(1,4-pdaH)2] (4) crystallizes as a sinuous and thread-like monoperiodic polymer; two families of chains run along different directions and are woven into diperiodic layers. Modification of the synthetic conditions leads to [(UO2)4(LH)2(1,4-pda)5]·H2O·2CH3CN (5), a monoperiodic polymer based on tetranuclear (UO2)4(1,4-pda)4 rings; intrachain hydrogen bonding of the terminal LH+ ligands results in diperiodic network formation through parallel polycatenation involving the tetranuclear rings and the LH+ rods. Complexes 1-3 and 5 are emissive, with complex 2 having the highest photoluminescence quantum yield (19%), and their spectra show the maxima positions usual for tris-κ2O,O'-chelated uranyl cations.

Uranyl-organic hybrid material with semi-rigid polycarboxylate: Synthesis, structure, luminescence and photocatalytic properties

One novel uranyl carboxylate complex [(UO2) (L)·2H2O] (1) (H2L=4-carboxymethoxy-benzoic acid) was synthesized under hydrothermal conditions by the reaction of UO2(NO3)2·6H2O and H2L in aqueous system. X-ray analysis of single crystal structure determined that complex 1 exhibits one-dimensional ribbon structure, in which, the UO22+ ions are bridged by L2−. The one-dimensional chains are further connected into 3D supramolecular network by OH…O hydrogen bond interactions between the coordinated water molecule and the coordinated carboxylic oxygen. The π…π interactions between adjacent benzene rings also contribute the structure formation. Complex 1 exhibits emblematic charge-transfer green emission of uranyl-bearing complexes. The UV–vis absorption spectra reveal distinct light absorption both on the ultraviolet and visible regions for 1. Photocatalytic degradation of rhodamine B (RhB) in aqueous solutions using Complex 1 was studied, 78.3% of the RhB was degraded in only 80 min. Our obtained uranyl coordination polymer represents effective RhB photocatalytic degradation materials in aqueous solution. Infrared spectroscopy, TGA and PXRD were also discussed.

3D Uranyl Organic Frameworks Supported by Rigid Octadentate Carboxylate Ligand: Synthesis, Structure Diversity, and Luminescence Properties.

Seven three dimensional (3D) uranyl organic frameworks (UOFs), formulated as [NH4 ][(UO2 )3 (HTTDS)(H2 O)] (1), [(UO2 )4 (HTTDS)2 ](HIM)6 (2, IM=imidazole), [(UO2 )4 (TTDS)(H2 O)2 (Phen)2 ] (3, Phen=1,10-phenanthroline), [Zn(H2 O)4 ]0.5 [(UO2 )3 (HTTDS)(H2 O)4 ] (4), and {(UO2 )2 [Zn(H2 O)3 ]2 (TTDS)} (5), {Zn(UO2 )2 (H2 O)(Dib)0.5 (HDib)(HTTDS)} (6, Dib=1,4-di(1H-imidazol-1-yl)benzene) and [Na]{(UO2 )4 [Cu3 (u3 -OH)(H2 O)7 ](TTDS)2 } (7) have been hydrothermally prepared using a rigid octadentate carboxylate ligand, tetrakis(3,5-dicarboxyphenyl)silicon(H8 TTDS). These UOFs have different 3D self-assembled structures as a function of co-ligands, structure-directing agents and transition metals. The structure of 1 has an infinite ribbon formed by the UO7 pentagonal bipyramid bridged by carboxylate groups. With further introduction of auxiliary N-donor ligands, different structure of 2 and 3 are formed, in 2 the imidazole serves as space filler, while in 3 the Phen are bound to [UO2 ]2+ units as co-ligands. The second metal centers were introduced in the syntheses of 4-7, and in all cases, they are part of the final structures, either as a counterion (4) or as a component of framework (5-7). Interesting, in 7, a rare polyoxometalate [Cu3 (μ3 -OH)O7 (O2 CR)4 ] cluster was found in the structure. It acts as an inorganic building unit together with the dimer [(UO2 )2 (O2 CR)4 ] unit. Those uranyl carboxylates were sufficiently determined by single crystal X-ray diffraction, and their topological structures and luminescence properties were analyzed in detail.

Mass spectrometric and theoretical study on the formation of uranyl hydride from uranyl carboxylate.

Uranyl hydride in the form of HUO2Cl2- was prepared upon collision-induced dissociation of (RCO2)UO2Cl2- (R = H, CH3CH2, CH3CH2CH2, CH3CHCH, (CH3)2CH, C5H9, C6H11 and C6H5CH2CH2) in the gas phase. It was found that uranyl hydrides result from alkene and alkyne elimination with concomitant β-hydride transfer of uranyl alkylides RUO2Cl2- following decarboxylation of the carboxylates with the exception of (HCO2)UO2Cl2-, and formation of HUVIO2Cl2- through alkene/alkyne loss is in competition with neutral ligand loss to give UVO2Cl2-. According to the calculations at the B3LYP level, loss of a neutral ligand is slightly less favorable in the cases of (CH3CH2)UO2Cl2- and (CH3CH2CH2)UO2Cl2-, and the situations of (CH3CHCH)UO2Cl2-, ((CH3)2CH)UO2Cl2-, (C5H9)UO2Cl2-, (C6H11)UO2Cl2- and (C6H5CH2CH2)UO2Cl2- with β-hydrogen atoms should be similar despite the fact that the yield of uranyl hydride depends on the nature of the ligand. Although no uranyl hydride was observed when β-hydrogen is not available in the carboxylate precursor, there is no HUO2Cl2- generated from (C6H5CO2)UO2Cl2-, (2-C6H4FCO2)UO2Cl2- and (CH2CHCH2CO2)UO2Cl2- with β-hydrogen either. This is attributed to the much more favorable formation of UO2Cl2- over HUO2Cl2- as revealed by the B3LYP calculations, which is similar to the absence of HUO2Cl2- in the (CH3CO2)UO2Cl2- case where highly reactive CH2 would be formed.

Optimizing Photoluminescence Quantum Yields in Uranyl Dicarboxylate Complexes: Further Investigations of 2,5‐, 2,6‐ and 3,5‐Pyridinedicarboxylates and 2,3‐Pyrazinedicarboxylate

The 2,5‐, 2,6‐, and 3,5‐pyridinedicarboxylic (2,5‐, 2,6‐ and 3,5‐pydcH2), and 2,3‐pyrazinedicarboxylic (2,3‐pyzdcH2) acids have been used to synthesize six uranyl ion complexes including various counterions under solvo‐hydrothermal conditions. While [NH4]2[UO2(2,6‐pydc)2]·3H2O (1) is a discrete, mononuclear species, [UO2(2,6‐pydc)2Cu(R,S‐Me6cyclam)] (2) crystallizes as a monoperiodic coordination polymer through axial bonding of copper(II) to carboxylate donors. [PPh3Me][UO2(OH)(2,5‐pydc)]·H2O (3) and [Ni(R,S‐Me6cyclam)][UO2(OH)(2,5‐pydc)]2·2H2O (4) contain di‐hydroxo‐bridged dinuclear uranyl subunits assembled into homometallic, monoperiodic polymers. [(UO2)2(3,5‐pydc)2(HCOO)2Ni(R,S‐Me6cyclam)] (5) crystallizes as a heterometallic diperiodic network with the V2O5 topology, and [PPh4][UO2(OH)(2,3‐pyzdc)] (6) is a diperiodic species with sql topology. All complexes have well‐resolved uranyl emission spectra in the solid state, and three of them have photoluminescence quantum yields among the highest reported for uranyl carboxylate complexes, 44 % for 1, 71 % for 3, and 36 % for 6.

Variability of Uranyl Carboxylates from Rigid Terophenyldicarboxylic Acid Ligands.

Three uranyl carboxylates, namely, (UO2)(L1)(H2O)0.5 (1), [(UO2)(L2)(H2O)]·2H2O (2), and [(UO2)(L2)(H2O)]·(CH3CN) (3), were synthesized hydrothermally from 2',3',5',6'-tetramethyl-(1,1':4',1″-terphenyl)-4,4″-dicarboxylic acid (H2L1) and 2',5'-dimethyl-(1,1':4',1″-terphenyl)-3,3″-dicarboxylic acid (H2L2), which are all steric carboxylic acid ligands but vary with the carboxylic acid group position and methyl group number. It is found that compound 1 displays a three-dimensional 8-fold-interpenetrated net with channels running along the c direction. Compounds 2 and 3 are isostructural, and all display two-dimensional-layered crystal structures but contain different guest molecules. The photophysical measurements reveal that compounds 1 and 2, which contain disordered water molecules, are luminescence-quenched, whereas compound 3 containing acetonitrile molecules is luminescent.

Insights into mechanism on organic acids assisted translocation of uranium in Brassica juncea var. foliosa by EXAFS

Citric acid (CA) and Lactic acid (LA) were used as additives to study the mechanism of organic acid promoting the root-to-shoot translocation of uranium (U) in Brassica juncea var. foliosa from molecular and tissue levels. Firstly, the distribution of U in plants under the condition of different organic acids concentrations were studied. The accumulation of U in leafs of 1 mM CA group and 5 mM LA group reached 2225 and 1848 mg/kg respectively, which was about 5 times that of the control group. Secondly, the speciation and distribution of U in plant roots after exposure to different culture solutions were studied by EXAFS and SEM. The result of EXAFS found that the complex of U with organic acids resulted in the U accumulated in the roots was the uranyl carboxylate speciation, while the control group only was the uranyl phosphate speciation. SEM results showed that the lactic acids could enhanced the translocation of U from the cortex to the stele. Thirdly, we further studied the apoplastic pathway and the symplastic pathway of U translocation using transpiration inhibitor and metabolism inhibitor. Compared with the control group, it was likely that the complex of U with organic acids were translocated into the shoot of plants through the apoplastic pathway.

Occurrence of polyoxouranium motifs in uranyl organic networks constructed by using silicon-centered carboxylate linkers: structures, spectroscopy and computation

Four novel polyoxouranium-based uranyl carboxylates have been constructed by using a combination of three silicon-centered carboxylate ligands and polyoxouranium building blocks. Interesting oligomerization of the uranyl groups from three types of tetrameric units in 1 and 2 to octameric [(UO2)8(μ3-OH)6(μ2-OH)2(H2O)4]8+ in 3 and ultimately infinite polyoxouranium chains in 4 was observed. The 3D structure of 2 contains two different linear tetramers, whose structures are totally different from seven types of tetranuclear motifs previously observed in uranyl carboxylates. Compound 3 displays a rare octanuclear polyoxouranium brick; it is fabricated by the association of eight uranyl units in pentagonal-bipyramidal geometry that involves the edge-sharing polyhedral connection mode, which are further linked by four isolated UO7 pentagonal bipyramids to create a complicated 3D framework. These newly synthesized hybrid materials were extensively characterized by single crystal analysis, thermogravimetric analysis (TG) and different spectroscopic techniques (IR, UV-vis, and luminescence spectroscopy), which show well-resolved characteristic "five-finger" emission of the uranyl ions under excitation at 420 nm. Relativistic density functional theory (DFT) was used to explore five model compounds that theoretically simulate experimental real compounds. Experimental spectroscopy was rationalized by electronic-structure analysis.

Structure-Directing Effects of Counterions in Uranyl Ion Complexes with Long-Chain Aliphatic α,ω-Dicarboxylates: 1D to Polycatenated 3D Species.

Nine uranyl ion complexes were synthesized under (solvo-)hydrothermal conditions using α,ω-dicarboxylic acids HOOC-(CH2) n-2-COOH (H2C n, n = 6-9) and diverse counterions. Complexes [PPh4][UO2(C6)(NO3)] (1) and [PPh4][UO2(C8)(NO3)] (2) contain zigzag one-dimensional (1D) chains, with further polymerization being prevented by the terminal nitrate ligands. [PPh3Me][UO2(C7)(HC7)] (3) crystallizes as a 1D polymer with a curved section, with hydrogen bonding of the uncomplexed carboxylic groups giving rise to formation of 3-fold interpenetrated two-dimensional (2D) networks. [PPh4][H2NMe2][(UO2)2(C7)3] (4) and [PPh3Me]2[(UO2)2(C8)3] (5) contain 1D chains, either ladder-like or containing doubly bridged dimers, while [PPh3Me]2[(UO2)2(C9)3]·2H2O (6) displays interdigitated, strongly corrugated honeycomb 2D nets. Ladder-like 1D polymers in [Cu( R,S-Me6cyclam)][(UO2)2(C7)2(C2O4)]·4H2O (7) are associated into layers by the hydrogen bonded counterions, whereas the [Ni(cyclam)]2+ moieties are part of the 2D polymeric arrangement in [(UO2)2(C7)2(HC7)2Ni(cyclam)]·2H2O (8) because of axial coordination of the nickel(II) center, with hydrogen bonding mediated by water molecules generating a three-dimensional (3D) net. [(UO2)2K2(C7)3(H2O)]·0.5H2O (9) contains convoluted uranyl dicarboxylate 2D subunits, which generate a 3D framework through 2D → 3D parallel polycatenation similar to that previously found in [NH4]2[(UO2)2(C7)3]·2H2O; further linking of these subunits is provided by bonding of the potassium cations to carboxylate and uranyl oxido groups. The solid-state emission spectra of complexes 1-6 and 9 display maxima positions typical of hexacoordinated uranyl carboxylate complexes, but uranyl luminescence is quenched in 7. A solid-state photoluminescence quantum yield of 11.5% has been measured for complex 1, while those for compounds 3-6 and 9 are in the range of 2.0-3.5%.

AgI and PbII as Additional Assembling Cations in Uranyl Coordination Polymers and Frameworks

Five mono- or polycarboxylic acids have been used to generate a series of eight heterometallic uranyl complexes involving silver(I) or lead(II) cations, all synthesized under (solvo)-hydrothermal conditions. Pimelic acid (H2pim) gave complexes [Ag(bipy)2]2[UO2(pim)(NO3)]2 (1) and [UO2Pb(pim)2(bipy)(H2O)]·0.5bipy·H2O (2) (bipy = 2,2′-bipyridine), which both crystallize as one-dimensional (1D) polymers, but differ in that the silver(I) cations are separate counterions, while carboxylate-bound lead(II) cations are an essential component of the polymer. Only silver(I)-containing species were obtained with all-cis-1,3,5-cyclohexanetricarboxylic acid (H3chtc), [UO2Ag(chtc)(H2O)2] (3) and [Ag(bipy) (CH3CN)]2[UO2(chtc)]2 (4); both contain two-dimensional (2D) uranyl carboxylate subunits with honeycomb {63} topology, these being united into a three-dimensional (3D) framework with the lonsdaleite {66} topology by bridging, oxo-bound silver(I) cations in 3. Both silver- and lead-containing complexes were obtained with 3,3′,4,4′-biphenyltetracarboxylic acid (H4bptc), [UO2Ag(bptc)(4,4′-bipyH)] (5) and [UO2Pb(bptc)(bipy)2] (6) (4,4′-bipy = 4,4′-bipyridine), and they both display a 2D uranyl carboxylate network with the {44.62} topology, the additional cations and N-donors being decorating species. In this case, a higher dimensionality was obtained not with an additional cation, but with a coordinated N-methyl-2-pyrrolidone (NMP) molecule, since [(UO2)2(bptc)(NMP)1.5(H2O)1.5]·1.5H2O (7) crystallizes as a three-dimensional (3D) framework. In the presence of silver(I), 3-pyrimidin-2-yl-benzoic acid (Hpyb) gave the complex [UO2Ag(pyb)3(H2O)2]·4H2O (8), in which the two coordination sites are occupied in accord with Hard/Soft Acid/Base (HSAB) principles, uranyl being chelated by three carboxylate groups and silver(I) being bound to nitrogen atoms; the 1D polymer formed bridges to another through silver–uranyl oxo bonding. In contrast, the homometallic, molecular complex [UO2(pyb)2(bipy)] (9) was obtained in the presence of lead(II) cations. The lead-containing complex with 2,6-pyridinedicarboxylic acid (H2pydc), [UO2Pb2(pydc)2(phen)2(HCOO)1.5(NO3)0.5]·0.5H2O (10) (phen = 1,10-phenanthroline), crystallizes as a 1D polymer in which uranyl is bound to two O,N,O-donors, as usual with this ligand, polymerization being due to lateral double lead(II) bridges. Variations in uranyl emission maxima positions appear to be essentially related to the uranium coordination number within the present series.

Unexpected cis-dioxido uranyl carboxylate compound: Synthesis, characterization and photocatalytic activity of uranyl-succinate complexes

Two new complexes formed by uranyl ions with succinic acid and 4,4′-dipyridine have been investigated. Interestingly, the uranium(VI) coordination polymers possess two types dioxido unit, a cis-dioxido bond angle with 115.421(2)°and a trans-dioxido bond angle with 178.421(2)°, respectively. The complexes were characterized by elemental analysis, IR and UV–vis spectroscopy and X-ray crystal diffraction. In addition, the properties of luminescence and photocatalytic analysis were also investigated. The result revealed that the two complexes show different photocatalytic activity in the decomposition of RhB under visible light and UV light irradiation.

Synthesis, Structure, and Nonlinear Optical Activity of K, Rb, and Cs Tris(crotonato)uranylates(VI)

AbstractThree uranyl crotonate complexes with composition RUO2(C3H5COO)3 [R = K (I), Rb (II), Cs (III)] were studied using FT‐IR spectroscopy and X‐ray diffraction analyses. The structures of I–III are constructed of typical anionic tris(carboxylato)uranylate complexes [UO2(C3H5COO)3]–. Cubic crystals of I and II have 3D framework structure, whereas triclinic crystals of III have layered structure. In the structure of III uranyl ions are nonlinear and non equal‐arm due to uranyl‐cation UO22+–Cs+ interactions. Nonlinear optical activity of noncentrosymmetric uranyl carboxylates was estimated by second harmonic generation measurements. Uranyl crotonate complexes show about twice higher SHG ability than uranyl butyrate and valerate complexes.

Spectroscopic study on uranyl carboxylate complexes formed at the surface layer of Sulfolobus acidocaldarius.

The complexation of U(vi) at the proteinaceous surface layer (S-layer) of the archaeal strain Sulfolobus acidocaldarius was investigated over a pH range from pH 1.5 to 6 at the molecular scale using time-resolved laser-induced fluorescence spectroscopy (TRLFS) and U L(III)-edge extended X-ray absorption fine structure (EXAFS). The S-layer, which represents the interface between the cell and its environment, is very stable against high temperatures, proteases, and detergents. This allowed the isolation and purification of S-layer ghosts (= empty cells) that maintain the size and shape of the cells. In contrast to many other microbial cell envelope compounds the studied S-layer is not phosphorylated, enabling the investigation of uranyl carboxylate complexes formed at microbial surfaces. The latter are usually masked by preferentially formed uranyl phosphate complexes. We demonstrated that at highly acidic conditions (pH 1.5 to 3) no uranium was bound by the S-layer. In contrast to that, at moderate acidic pH conditions (pH 4.5 and 6) a complexation of U(vi) at the S-layer via deprotonated carboxylic groups was stimulated. Titration studies revealed dissociation constants for the carboxylic groups of glutamic and aspartic acid residues of pK(a) = 4.78 and 6.31. The uranyl carboxylate complexes formed at the S-layer did not show luminescence properties at room temperature, but only under cryogenic conditions. The obtained luminescence maxima are similar to those of uranyl acetate. EXAFS spectroscopy demonstrated that U(vi) in these complexes is mainly coordinated to carboxylate groups in a bidentate binding mode. The elucidation of the molecular structure of these complexes was facilitated by the absence of phosphate groups in the studied S-layer protein.

Syntheses, Structures, Luminescence, and Photocatalytic Properties of a Series of Uranyl Coordination Polymers

A new series of uranium coordination polymers have been hydrothermally synthesized by using 4,4′-oxidiphthalic acid (H4L) in the presence of N-bearing species, namely, (H2dpa)UO2L·3H2O (1), (H2dib)UO2L·H2O (2), (H2bbi)(UO2)L·3H2O (3), (H2dib)(dib)0.5[UO2L(dib)]·3H2O (4), (H2bbi)(U2O5)L·2H2O (5), and (Hbpi)2(UO2F)2L (6) [dpa = di(pyridin-4-yl)amine, dib = 1,4-di(1H-imidazol-1-yl)benzene, bbi = 1,1′-(1,4-butanediyl)bis(imidazole), and bpi = 1-(biphenyl-4-yl)-1H-imidazole]. Compounds 1–3 exhibit the ribbon structures with dpa, dib, and bbi as the templates, respectively. Compound 4 represents a one-dimensional chain assembly of uranyl centers, L ligands, and coordinated dib molecules. Compound 5 contains the tetranuclear motifs of edge-sharing of two UO7 pentagonal and two UO8 hexagonal bipyramids, which are linked by L ligands forming a layered structure with bbi as the template. Compound 6 forms a two-dimensional arrangement, in which the chain structures of edge-sharing UO5F2 pentagonal bipyramids are pillared by L ligands. Luminescence studies reveal the characterized green light emission from uranyl centers for these uranyl carboxylates. Moreover, the excellent visible light photodegradation activities for RhB are displayed by these coordination polymers.

Room temperature crystallization of trichlorodioxouranate [UO2Cl3(L)] species in molecular assemblies involving aliphatic dicarboxylate linkers

Three uranyl carboxylates (UO2)2Cl6(succ)·4dmaH (1), (UO2)2Cl6(adip)·4dmaH (2) and (UO2)2Cl6(1,4-chdc)·4dmaH (3) have been synthesized from slow evaporation in ambient atmosphere of solutions of aliphatic carboxylic acids and UCl4 in N,N-dimethyformamide (DMF) after a solvothermal treatment (succ=succinate, adip=adipate, 1,4-chdc=1,4-cyclohexanedicarboxylate, dmaH=monoprotonated dimethylamine). These three compounds possess a similar crystal structure based on the molecular assembly of discrete dinuclear motifs (UO2)2Cl6(L) separated by dimethylammonium cations (coming from the in situ decomposition of DMF). Hexavalent uranium cation is found in seven-fold coordination (pentagonal bipyramid) with the typical double uranyl bond. Three chloride anions and two carboxyl oxygen atoms are located in the equatorial pentagonal plane, resulting in a trichlorodioxouranate ‘UO2Cl3(OC)2’ tecton. The uranyl centers are linked to each other through the ditopic ligand via the two carboxylate arms. The three compounds have been characterized by infrared and luminescence spectroscopies.

Crystal Chemistry of Uranyl Carboxylate Coordination Networks Obtained in the Presence of Organic Amine Molecules

AbstractThree uranyl isophthalates (1,3‐bdc) and two uranyl pyromellitates (btec) of coordination‐polymer type were hydrothermally synthesized (200 °C for 24 h) in the presence of different amine‐based molecules [1,3‐diaminopropane (dap) or dimethylamine (dma) originating from the in situ decomposition of N,N‐dimethylformamide]. (UO2)2(OH)2(H2O)(1,3‐bdc)·H2O (1) is composed of inorganic tetranuclear cores, which are linked to each other through the isophthalato ligand to generate infinite neutral ribbons, which are intercalated by free H2O molecules. The compounds (UO2)1.5(H2O)(1,3‐bdc)2·0.5H2dap·1.5H2O (2) and UO2(1,3‐bdc)1.5·0.5H2dap·2H2O (3) consist of discrete uranyl‐centered hexagonal bipyramids connected to each other by a ditopic linker to form a single‐layer network for 2 or a double‐layer network for 3. The protonated diamine molecules are located between the uranyl–organic sheets and balance the negative charge of the layered sub‐networks. The phase (UO2)2O(btec)·2Hdma·H2O (4) presents a 2D structure built up from tetranuclear units, which consist of two central sevenfold coordinated uranium centers and two peripheral eightfold coordinated uranium centers. The connection of the resulting tetramers through the pyromellitate molecules generates an anionic layerlike structure, in which the protonated dimethylammonium species are inserted. The compound UO2(btec)·2Hdma (5) is also a lamellar coordination polymer, which contains isolated eightfold coordinated uranium cations linked through pyromellitate molecules and intercalated by protonated dimethylammonium species. In both phases 4 and 5, the btec linker has non‐bonded carboxyl oxygen atoms, which preferentially interact with the protonated amine molecules through a hydrogen‐bond network. The different illustrations show the structural diversity of uranyl–organic coordination polymers with organic amine molecules as countercations.

Uranyl-organic complexes: structure symbols, classification of carboxylates, and uranyl polyhedral geometries

A system of structure symbols to classify and group uranyl-organic complexes is devised. The method is applied to describe 604 published structures of uranyl-organic complexes, then to consider the complex chemistry of uranyl carboxylates. A review of the uranyl-organic structural units in monocarboxylates, polycarboxylates and mixed carboxylates is provided. The uranyl (U–O) and equatorial bond lengths are considered for well-refined structures and their trends are briefly discussed.

Crystal Structure of Uranyl Carboxylates

AbstractUranyl-organic complexation in geologic fluids can have a profound impact upon uranium solubility and transport. Studies of uranyl organometallic crystal structures provide a basis for understanding complexation of the uranyl ion in solution.The crystal structures of several novel uranyl oxalates, synthesized under mild hydrothermal conditions, have been determined. These structures demonstrate new features little seen or not previously known in this chemical system, in particular polymerization into infinite sheets and direct linkage of uranyl polyhedra. Further work on the chemistry of this and other systems of hexavalent uranium and low molecular weight carboxylic acids, especially formic acid, is likely to turn up new insights.Although a hierarchical scheme exists for classifying inorganic uranyl compounds [1], no similar work has been done for organic compounds. Such a hierarchy would have practical benefits, in particular making structural information more accessible and understandable to workers studying related problems such as the environmental transport of hexavalent uranium as dissolved organic complexes. We offer a simple scheme that classifies uranyl oxalate structures by analyzing the long-range structural features and the coordination environments of uranyl ions, which leads to a structural symbol that can be used to easily identify uranyl oxalates with common structural features. This system is equally applicable to other carboxylate complexes with the uranyl ion, and could be extended to apply to any organic complex of the uranyl ion.

EXAFS Investigations of the Interaction of Humic Acids and Model Compounds with Uranyl Cations in Solid Complexes

Natural humic acids, HA`s, having varying amounts of their proton exchange capacities, PEC`s, loaded with uranyl ions and synthetic HA loaded with 14% PEC were prepared either from solution or from suspension. The interaction of uranium with the humates was studied using U L{sub III}-edge extended X-ray absorption fine structure, EXAFS, and infrared, IR, spectroscopy. IR results indicate a direct complexation of the uranyl ions onto the HA`s. The spectral positions of the asymmetric and symmetric IR stretching frequencies for COO{sup -} in the complex suggest monodentate coordination of HA carboxylate groups onto the uranyl cation. In all samples studied, the EXAFS analysis yielded axial uranium-oxygen distances of 1.77-1.78 A and five oxygen atoms in the plane equatorial to the uranyl unit at distances of 2.37-2.39 A. The bond distances determined from the EXAFS are the same, within the experimental error, for both synthetic and two different natural uranyl humates, for samples with large loadings and samples with relatively low uranyl loadings, as well as for dry and wet paste samples. Comparison of the EXAFS from the uranyl humates with that observed for two crystalline uranyl carboxylate complexes indicates that the HA carboxylate groups act predominantly as monodentate ligands when bound to the uranyl unit. Additional, neutral ligands must also be coordinated to the uranyl ion in order to satisfy the uranyl cation coordination number determined as 5 {+-} 0.7. (orig.)

High efficiency photoresist-free lithography of UO3 patterns from amorphous films of uranyl complexes

The solid state photochemistry of uranyl carboxylate complexes is presented with the purpose of developing methods for optical lithography of uranium oxide films. These complexes of the general formula, UO2(OOCR)2 (R = i-C3H7, C5H11, CH2C6H5, CH2OC2H5, C2H4OC2H5), were all photosensitive as thin amorphous films. The primary photochemical reaction for each of these complexes was the extrusion of a CO2 from the ligand and the production of radicals which initiated a chain reaction. The nature of this chain reaction was dependent upon the identity of the organic substituents, R. In some cases the chain reaction required a photochemical step while others were entirely thermal in nature. Of importance are the potentially high quantum yields which can be associated with thermal chain reactions. Some of the systems presented here exhibit quantum yields in excess of 1. This process was shown to be compatible with optical lithography by the patterning of the uranium oxide product on silicon surfaces.