Uranyl Ion Complexes with Chiral Malic and Citramalic, and Prochiral Citric and Tricarballylic Acids: Influence of Coligands and Additional Metal Cations

Nanotubule inclusion in the channels formed by a six-fold interpenetrated, triperiodic framework

When reacted together with uranyl ions under solvo-hydrothermal conditions, a bis(pyridiniumcarboxylate) zwitterion (L) and tricarballylic acid (H3tca) give the complex [NH4]2[UO2(L)2][UO2(tca)]4·2H2O (1). The two ligands are segregated into different units, an anionic nanotubule for tca3- and a six-fold interpenetrated cationic framework with lvt topology for L. The entangled framework defines large channels which contain the square-profile nanotubules. Complex 1 has a photoluminescence quantum yield of 19% and its emission spectrum shows the superposition of the signals due to the two independent species.

Ni(2,2':6',2″-Terpyridine-4'-carboxylate)2 Zwitterions and Carboxylate Polyanions in Mixed-Ligand Uranyl Ion Complexes with a Wide Range of Topologies

The zwitterionic complex formed by Ni^II and 2,2':6',2″-terpyridine-4'-carboxylate, Ni(tpyc)₂, has been used as a coligand with a diverse group of polycarboxylates in uranyl ion complexes synthesized under solvo-hydrothermal conditions, thus giving a series of 14 mixed ligand, heterometallic compounds. Both [(UO₂)₂(c-1,2-chdc)Ni(tpyc)₂(NO₃)₂]₂·4CH₃CN (1) and [(UO₂)₂(tdc)Ni(tpyc)₂(NO₃)₂]₂ (2), where c-1,2-chdc^2- is cis-1,2-cyclohexanedicarboxylate and tdc^2- is 2,5-thiophenedicarboxylate, display discrete U₄Ni₂ dinickelatetrauranacycles, a motif which is also found as part of a daisychain coordination polymer in [(UO₂)₄(bdc)₃Ni₂(tpyc)₄(NO₃)₂]·2CH₃CN·2H₂O (3), where bdc^2- is 1,4-benzenedicarboxylate. Similar U₄Ni₂ rings associate to form a nanotubular polymer in [(UO₂)₂(tca)Ni(tpyc)₂(NO₃)]·2CH₃CN·2H₂O (4), where tca^3- is tricarballylate. [(UO₂)₂(1,2-pda) (1,2-pdaH)Ni(tpyc)₂(NO₃)]·CH₃CN (5), where 1,2-pda^2- is 1,2-phenylenediacetate, crystallizes as a meander-like chain in which each bent section can be seen as an open, semi-U₄Ni₂ ring. Oxalate (ox^2-) gives [(UO₂)₂(ox)₂Ni(tpyc)₂] (6), a monoperiodic polymer containing smaller U₄Ni rings, while 1,2,3-benzenetricarboxylate (1,2,3-btc^3-) and citrate (citH^3-) give [Ni(tpycH)(H₂O)₃][UO₂(1,2,3-btc)]₂·2H₂O (7) and [UO₂Ni₂(tpyc)₄][UO₂(citH)]₂ (8), two complexes with charge separation, the latter displaying one-periodic + two-periodic semi-interpenetration. [(UO₂)₂(btcH)Ni(tpyc)₂(NO₃)] (9) and [(UO₂)₂(cbtcH)Ni(tpyc)₂(NO₃)] (10), where btc^4- and cbtc^4- are 1,2,3,4-butanetetracarboxylate and cis,trans,cis-1,2,3,4-cyclobutanetetracarboxylate, respectively, are diperiodic networks with hcb topology, and [(UO₂)₂(ndc)Ni(tpyc)₂(OH)(NO₃)] (11), where ndc^2- is 2,6-naphthalenedicarboxylate, is a sql network containing dinuclear nodes and involving 100-membered U₁₀Ni₄ metallacyclic units. U₄Ni₂ rings are found in the diperiodic polymer formed in [(UO₂)₄(t-R-1,2-chdc)₄Ni₂(tpyc)₄] (12), where t-R-1,2-chdc^2- is trans-R,R-1,2-cyclohexanedicarboxylate, the heavily puckered sheets being interlocked. 1,3-Phenylenediacetate (1,3-pda^2-) gives a very thick diperiodic polymer with KIa topology, [(UO₂)₄(1,3-pda)₄Ni₂(tpyc)₄]·CH₃CN·2H₂O (13). A triperiodic framework is formed with nitrilotriacetate (nta^3-) in [(UO₂)₂(nta)₂Ni₂(tpyc)₂] (14), where Ni^II is found in Ni(tpyc)₂ units as well as in Ni(nta)₂^4- moieties which both act as 4-coordinated nodes.

Pb-Oxo Interactions in Uranyl Hybrid Materials: A Combined Experimental and Computational Analysis of Bonding and Spectroscopic Properties

Reported are the syntheses and characterization of six new heterometallic UO₂²⁺/Pb²⁺ compounds. These materials feature rare instances of M-oxo interactions, which influence the bonding properties of the uranyl cation. The spectroscopic effects of these interactions were measured using luminescence and Raman spectroscopy. Computational density functional theory-based natural bonding orbital and quantum theory of atoms in molecules methods indicate interactions arise predominantly through charge transfer between cationic units via the electron-donating uranyl O sp^x lone pair orbitals and electron-accepting Pb²⁺ p orbitals. The interaction strength varies as a function of Pb-oxo interaction distance and angle with energy values ranging from 0.47 kcal/mol in the longer contacts to 21.94 kcal/mol in the shorter contacts. Uranyl units with stronger interactions at the oxo display an asymmetric bond weakening and a loss of covalent character in the U═O bonds interacting closely with the Pb²⁺ ion. Luminescence quenching is observed in cases in which strong Pb-oxo interactions are present and is accompanied by red-shifting of the uranyl symmetric Raman stretch. Changes to inner sphere uranyl bonding manifest as a weakening of the U═O bond as a result of interaction with the Pb²⁺ ion. Comprehensive evaluation of the effects of metal ions on uranyl spectra supports modeling efforts probing uranyl bonding and may inform applications such as forensic signatures.

Dimeric and Trimeric Uranyl(VI)-Citrate Complexes in Aqueous Solution

This research addresses a subject discussed controversially for almost 70 years. The interactions between the uranyl(VI) ion, U(VI), and citric acid, H₃Cit, were examined using a multi-method approach comprising nuclear magnetic resonance (NMR), ultraviolet-visible (UV-vis), attenuated total reflectance Fourier-transform infrared (ATR FT-IR), and extended X-ray absorption fine-structure (EXAFS) spectroscopies as well as density functional theory (DFT) calculations. Combining ¹⁷O NMR spectroscopy and DFT calculation provided an unambiguous decision on complex configurations, evidencing for the first time that the dimeric complex, (UO₂)₂(HCit_-H)₂^2-, exists as two diastereomers with the syn-isomer in aqueous solution strongly favored over the anti-isomer. Both isomers interconvert mutually with exchange rates of ∼30 s^-1 at -6 °C and ∼249 s^-1 at 60 °C in acidic solution corresponding to an activation barrier of about 24 kJ mol^-1. Upon increasing the pH value, ternary dimeric mono- and bis-hydroxo as well as trimeric complexes form, that is, (UO₂)₂(HCit_-H)₂(OH)^3-, (UO₂)₂(HCit_-H)₂(OH)₂^4-, (UO₂)₃(O)(Cit_-H)₃^8-, and (UO₂)₃(O)(OH)(Cit_-H)₂^5-, respectively. Stability constants were determined for all dimeric and trimeric species, with log β° = -(8.6 ± 0.2) for the 3:3 species being unprecedented. Additionally, in the 6:6 sandwich complex, formed from two units of 3:3 species, the ¹⁷O NMR resonance of the trinuclear uranyl(VI) core bridging μ₃-O is shown for the first time. Species distribution calculations suggest that the characterized polynuclear U(VI)-citrate species do not significantly increase uranium(VI) mobility in the environment. Furthermore, we revise the misconceptions in the aqueous U(VI)-citric acid solution chemistry, that is, structures proposed and repeatedly taken up, and outline generalized isostructural considerations to provide a basis for future U(VI) complexation studies.

Halogen bonding in uranyl and neptunyl trichloroacetates with alkali metals and improved crystal chemical formulae for coordination compounds

The structures of the single crystals of compounds K₂UO₂(tca)₄(tcaH)₂ (I), K₄NpO₂(tca)₆(tcaH)(H₂O)₃ (II), Rb₄UO₂(tca)₆(tcaH)(H₂O)₃ (III), and Cs₃UO₂(tca)₅(tcaH)₂·H₂O (IV), where tca is the trichloroacetate ion, were established by X-ray diffraction analysis. The crystals of II-IV have a framework structure, whereas in the layered crystals of I, neighboring layers are connected to each other via halogen bonds. In this regard, the crystals of I possess perfect cleavage along the (001) plane: the crystals are easily cut into stacks of very thin layers. Halogen bonds in the structures of all title compounds were characterized using the method of molecular Voronoi-Dirichlet polyhedra. The donor-acceptor halogen bond synthon, where the same halogen atom is both the donor towards one halogen atom and the acceptor from the second halogen atom, is recognized for its usefulness in the crystal design. The description of the ligand coordination modes and crystal chemical formulae of complexes is adapted for cases when ligands have chemically non-equivalent and unobvious donor atoms (for example, oxygen and halogen atoms in halogen-substituted carboxylate anions).

Trimeric uranyl(vi)-citrate forms Na+, Ca2+, and La3+ sandwich complexes in aqueous solution

M. Basile, et al., Chem. Commun., 2015, 51, 5306-5309, showed that a sodium ion is sandwiched by uranyl(vi) oxygen atoms of two 3 : 3 uranyl(vi)-citrate complex molecules in single-crystals. By means of NMR spectroscopy supported by DFT calculations we provide unambiguous evidence for this complex to persist in aqueous solution above a critical concentration of 3 mM uranyl citrate. Unprecedented Ca²⁺ and La³⁺ coordination by a bis-(η³-uranyl(vi)-oxo) motif advances the understanding of uranium's aqueous chemistry. As determined from ¹⁷O NMR, Ca²⁺ and more distinctly La³⁺ cause strong O[double bond, length as m-dash]U[double bond, length as m-dash]O polarization, which opens up new ways for uranyl(vi)-oxygen activation and functionalization.

Uranyl Tricarballylate Triperiodic and Nanotubular Species. Counterion Control of Nanotube Diameter

Tricarballylic acid (propane-1,2,3-tricarboxylic acid, H₃ tca) was reacted with uranyl nitrate hexahydrate under solvo-hydrothermal conditions and in the presence of different additional cations, yielding four complexes which have been crystallographically characterized. [(UO₂)₂Ba(tca)₂(H₂O)₄] (1), isomorphous to the Pb^II analogue previously reported, crystallizes as a triperiodic framework in which diperiodic uranyl-tca^3- subunits with the hcb (honeycomb) topology are linked by carboxylate-bound Ba^II cations. Triperiodic polymerization is also found in [(UO₂)₂(tca)₂Ni(cyclam)] (2) and [(UO₂)₂(tca)₂Cu(R,S-Me₆cyclam)] (3), but here the diperiodic uranyl-tca^3- subunits have the sql (square lattice) topology, and the frameworks formed through bridging by Ni^II or Cu^II cations have different topologies, tcs in 2 and xww in 3. [Co(en)₃][UO₂(tca)]₃·2H₂O (4) crystallizes as a monoperiodic coordination polymer with the hcb topology and a nanotubular geometry. In contrast to the square-section nanotubules previously found in [NH₄][(UO₂)₂Pb(tca)₂(NO₃)(bipy)] (bipy = 2,2'-bipyridine), those in 4 have a hexagonal section with a width of ∼7 Å. The structure-directing role of the hydrogen bonded counterions in these nanotubular species, either NH₄⁺ located within the nanotubule cavity or [Co(en)₃]³⁺ located outside, is discussed. Emission spectra in the solid state display the usual vibronic fine structure for 1 and 4, while uranyl emission is quenched in 3.

Functionalized Aromatic Dicarboxylate Ligands in Uranyl-Organic Assemblies: The Cases of Carboxycinnamate and 1,2-/1,3-Phenylenedioxydiacetate

2-Carboxycinnamic acid (ccnH₂) and the isomeric 1,2- and 1,3-phenylenedioxydiacetic acids (1,2- and 1,3-pddaH₂) have been used to synthesize eight uranyl ion complexes under solvo-hydrothermal conditions. In the four complexes [PPh₄]₂[UO₂(ccn)(NO₃)]₂ (1), [PPh₄]₂[UO₂(ccn)(dibf)]₂ (2), [UO₂(ccn)(bipy)]₂ (3), and [Ni(R,S-Me₆cyclam)][UO₂(ccn)(HCOO)]₂ (4), the ccn^2- dianion retains a nearly planar geometry, which favors the formation of the centrosymmetric [UO₂(ccn)]₂ dimeric unit. Additional terminal ligands, either neutral (bipy = 2,2'-bipyridine) or anionic (nitrate, dibf^- = 1,3-dihydro-3-oxo-1-isobenzofuranacetate, and formate, the two latter formed in situ), complete the uranyl coordination sphere, leading in all cases to discrete, dinuclear species. Sodium(I) bonding to the carboxylate/ether O₄ site of the 1,2-pdda^2- dianion in the two complexes [UO₂Na(1,2-pdda)(OH)] (5) and [(UO₂)₂Na₂(1,2-pdda)₂(C₂O₄)] (6) results in this ligand being planar. Further lateral coordination to uranyl and sodium bonding to a uranyl oxo group allow formation of heterometallic diperiodic networks containing monoperiodic uranyl-only subunits. In the absence of Na⁺ cations, 1,2-pdda^2- adopts a conformation in which one carboxylate group is tilted out of the ligand plane in [UO₂(1,2-pdda)₂Ni(cyclam)] (7) and diaxial carboxylato bonding to nickel(II) unites uranyl-only monoperiodic subunits into a diperiodic network. The 1,3-pdda^2- ligand in [UO₂(1,3-pdda)(H₂O)] (8) is also nonplanar with one tilted carboxylate group, and the bridging bidentate nature of both carboxylate groups allows formation of a triperiodic framework in which both metal and ligand are four-coordinated nodes. While the emission spectra of complexes 1 and 5 display the vibronic progression considered typical of uranyl ion, those of complexes 2, 4, and 8 show broad emission bands which in the case of complex 4 completely replace the uranyl emission and which appear to be ligand-centered. The low energy of these broad bands can be rationalized in terms of the close association of certain ligand pairs within the structures.

Chiral Discrete and Polymeric Uranyl Ion Complexes with (1 R,3 S)-(+)-Camphorate Ligands: Counterion-Dependent Formation of a Hexanuclear Cage

Reaction of (1 R,3 S)-(+)-camphoric acid (H₂cam) with uranyl ions under solvo-hydrothermal conditions and in the presence of bulky countercations gave five chiral complexes of varying dimensionality. [Cu( R,S-Me₆cyclam)][UO₂(Hcam)₂(HCOO)₂] (1) and [Ni( R,S-Me₆cyclam)][UO₂(cam)(HCOO)₂] (2), in which the formate coligand is formed in situ, involve very similar countercations, but 1 is a discrete, mononuclear complex, whereas 2 crystallizes as a one-dimensional (1D) coordination polymer, and NH-bond donation by the macrocyclic ligand of the countercation complexes is present in both. [Co(en)₃][(UO₂)₄(cam)( R,R-tart)₂(OH)]·3H₂O (3), in which en is ethylenediamine and H₄ R,R-tart is R,R-tartaric acid, contains three enantiomerically pure chiral species, and it displays a two-dimensional (2D) arrangement, with the countercation again involved in NH-bond donation. While [PPh₄][UO₂(cam)(NO₃)] (4) is a 1D polymer, [PPh₃Me]₃[NH₄]₃[(UO₂)₆(cam)₉] (5) is a discrete, homochiral, and homoleptic hexanuclear cage with C₃ point symmetry and a trigonal prismatic arrangement of the uranium atoms. This cage differs from the octanuclear, pseudocubic uranyl camphorate species previously described, thus providing an example of modulation of the cage size through variation of the structure-directing counterions. The cage in 5 is closely associated with three PPh₃Me⁺ cations, two of them outside and with their methyl group directed toward the prism basis center, and one inside the cage cavity. While complex 5 is nonluminescent, complexes 1 and 4 have emission spectra in the solid state typical of equatorially hexacoordinated uranyl complexes. Solid-state photoluminescence quantum yields of 2 and 23% have been measured for complexes 1 and 4, respectively.