Insight into Speciation and Electrochemistry of Uranyl Ions in Deep Eutectic Solvents.

Understanding the speciation of metal ions in heterogeneous hydrogen-bonded deep eutectic solvents (DES) has immense importance for their wide range of applications in green technology, environmental remediation, and nuclear industry. Unfortunately, the fundamental nature of the interaction between DES and actinide ions is almost completely unknown. In the present work, we outline the speciation, solvation mechanism, and redox chemistry of uranyl ion (UO22+) in DES consisting of choline chloride (ChCl) and urea as the hydrogen-bond donor. Electrochemical and spectroscopic techniques along with molecular dynamics (MD) simulations have provided a microscopic insight into the solvation and speciation of the UO22+ ion in DES and also on associated changes in physical composition of the DES. The hydrogen-bonded structure of DES plays an important role in the redox behavior of the UO22+ ion because of its strong complexation with DES components. X-ray absorption spectroscopy and MD simulations showed strong covalent interactions of uranyl ions with the constituents of DES, which led to rearrangement of the hydrogen-bonding network in it without formation of any clusters or aggregations. This, in turn, stabilizes the most unstable pentavalent uranium (UO2+) in the DES. MD analysis also highlights the fact that the number of H-bonds is reduced in the presence of uranyl nitrate irrespective of the presence of water with respect to pristine reline, which suggests high stability of the formed complexed species. The effect of added water up to 20 v/v % on speciation is insignificant for DES, but the presence of water influences the redox chemistry of UO22+ ions considerably. The fundamental findings of the present work would have far reaching consequences on understanding DES, particularly for application in the field of nuclear fuel reprocessing.

Unusual intramolecular CHO hydrogen bonding interaction between a sterically bulky amide and uranyl oxygen.

The selective separation of toxic heavy metals such as uranyl can be accomplished using ligands with stereognostic hydrogen bonding interactions to the uranyl oxo group, as proposed by Raymond and co-workers (T. S. Franczyk, K. R. Czerwinski and K. N. Raymond, J. Am. Chem. Soc., 1992, 114, 8138-8146). Recently, several ligands possessing this weak interaction have been proposed involving the hydrogen bonding of NH and OH based moieties with uranyl oxygen. We herein report the structurally and spectroscopically characterized CHO hydrogen bonding using a sterically bulky amide based ligand. In conjunction with experiments, electronic structure calculations are carried out to understand the structure, binding and the strength of the CHO hydrogen bonding interactions. This weak interaction is mainly due to the steric effect caused by a bulky substituent around the donor group which has direct relevance in designing novel ligands in nuclear waste management processes. Although the kinetics are very slow, the ligand is also highly selective to uranyl in the presence of other interfering ions such as lanthanides.

Extraction and structural studies of an unexplored monoamide, N,N'-dioctyl, α-hydroxy acetamide with lanthanide(III) and actinide(III) ions.

A monoamide, N,N'-dioctyl, α-hydroxy acetamide, shows unusual extraction properties towards trivalent lanthanide and actinide ions above 3 M HNO3. The extracted ions could be quantitatively back extracted using 0.5 M HNO3. This amide shows negligible extraction towards Sr(II) and Ru(III) ions, making it advantageous over other reported extractants. The structures of Sm(III) and Eu(III) nitrate compounds show that the metal ion is surrounded by three of the ligands, one nitrate and one water molecule. The ligand acts as a neutral bidentate ligand and bonds through the amido and hydroxyl oxygen atoms.

Steric effects on uranyl complexation: synthetic, structural, and theoretical studies of carbamoyl pyrazole compounds of the uranyl(VI) ion.

New bifunctional pyrazole based ligands of the type [C(3)HR(2)N(2)CONR'] (where R = H or CH(3); R' = CH(3), C(2)H(5), or (i)C(3)H(7)) were prepared and characterized. The coordination chemistry of these ligands with uranyl nitrate and uranyl bis(dibenzoyl methanate) was studied with infrared (IR), (1)H NMR, electrospray-mass spectrometry (ES-MS), elemental analysis, and single crystal X-ray diffraction methods. The structure of compound [UO(2)(NO(3))(2)(C(3)H(3)N(2)CON{C(2)H(5)}(2))] (2) shows that the uranium(VI) ion is surrounded by one nitrogen atom and seven oxygen atoms in a hexagonal bipyramidal geometry with the ligand acting as a bidentate chelating ligand and bonds through both the carbamoyl oxygen and pyrazolyl nitrogen atoms. In the structure of [UO(2)(NO(3))(2)(H(2)O)(2)(C(5)H(7)N(2)CON {C(2)H(5)}(2))(2)], (5) the pyrazole ligand acts as a second sphere ligand and hydrogen bonds to the water molecules through carbamoyl oxygen and pyrazolyl nitrogen atoms. The structure of [UO(2)(DBM)(2)C(3)H(3)N(2)CON{C(2)H(5)}(2)] (8) (where DBM = C(6)H(5)COCHCOC(6)H(5)) shows that the pyrazole ligand acts as a monodentate ligand and bonds through the carbamoyl oxygen to the uranyl group. The ES-MS spectra of 2 and 8 show that the ligand is similarly bonded to the metal ion in solution. Ab initio quantum chemical studies show that the steric effect plays the key role in complexation behavior.

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.

Fluoride abstraction and reversible photochemical reduction of cationic uranyl(VI) phosphine oxide complexes.

The syntheses, structural and spectroscopic characterization, fluoride abstraction reactions, and photochemical reactivity of cationic uranyl(VI) phosphine oxide complexes are described. [UO2(OPPh3)4][X]2 (1a, X = OTf; 1b, X = BF4) and [UO2(dppmo)2(OPPh3)][X]2 (2a, X = OTf; 2b, X = BF(4)) are prepared from the corresponding uranyl(VI) chloride precursor and 2 equiv each of AgX and phosphine oxide. The BF4- compounds 1b and 2b are prone to fluoride abstraction reactions in methanol, leading to dinuclear fluoride-bridged uranyl(VI) complexes. Fluoride abstraction of 2b in methanol generates two structural isomers of the fluoride-bridged uranyl(VI) dimer [(UO2(dppmo)2)2(mu-F)][BF4]3 (4), both of which have been structurally characterized. In the major isomer 4C, the four dppmo ligands are all chelating, while in the minor isomer 4B, two of the dppmo ligands bridge adjacent uranyl(VI) centers. Photolysis of 2b in methanol proceeds through 4 to form the uranium(IV) fluoride complex [UO2F2(dppmo)3][BF4]2 (5), involving another fluoride abstraction step. X-ray crystallography shows 5 to be a rare example of a structurally characterized uranium(IV) complex possessing terminal U-F bonds. Complex 5 reverts to 4 in solution upon exposure to air.

Anhydrous photochemical uranyl(VI) reduction: unprecedented retention of equatorial coordination accompanying reversible axial oxo/alkoxide exchange.

In a dramatic reversal of the normal trend of observed reactivity in uranyl(VI) coordination chemistry, an unprecedented retention of the normally labile equatorial coordination plane accompanies facile and reversible axial oxo/alkoxide exchange during both the photochemical reduction of cationic uranyl(VI) phosphine-oxide complexes with organic substrates and subsequent hydrolysis of the uranium(IV) alkoxide complexes to regenerate the uranyl(VI) starting complex.

Versatile new uranyl(VI) dihalide complexes supported by tunable organic amide ligands.

A series of uranyl(VI) dihalide complexes UO2X2L2 (X = Cl, Br) supported by organic amide ligands (L = R'C(O)NR2; R' = i-Pr; R = i-Pr, i-Bu, s-Bu) offers the versatile combination of facile synthesis using benchtop methods, air-stable crystalline solids obtained in high yield, high solubility in common organic solvents and tunable steric/electronic properties.

Synthesis and structural characterization of a uranyl(VI) complex possessing unsupported unidentate thiolate ligands.

The synthesis and structural characterization of the first example of a uranyl(VI) complex possessing unsupported unidentate thiolate ligands, UO2(S-2,6-Cl2C6H3)2L2 (2b, L=N,N-diisobutylisopropylamide), are reported. Isolation of 2b as a stable mononuclear complex is provided by the alkyl substituents of the organic amide ligands, which offer enhanced solubility, electron-releasing properties, and steric protection to help saturate the uranyl(VI) coordination sphere.

Extraction and coordination studies of the unexplored bifunctional ligand carbamoyl methyl sulfoxide (CMSO) with uranium(VI) and cerium(III) nitrates. Synthesis and structures of [UO2(NO3)2(PhSOCH2CON(i)Bu2)] and [Ce(NO3)3(PhSOCH2CONBu2)2].

The bifunctional carbamoyl methyl sulfoxide ligands, PhCH(2)SOCH(2)CONHPh (L1), PhCH(2)SOCH(2)CONHCH(2)Ph (L2), PhSOCH(2)CON(i)Pr(2)(L3), PhSOCH(2)CONBu(2)(L4), PhSOCH(2)CON(i)Bu(2)(L5) and PhSOCH(2)CON(C(8)H(17))(2)(L6) have been synthesized and characterized by spectroscopic methods. The selected coordination chemistry of L1, L3, and L5with [UO(2)(NO(3))(2)] and [Ce(NO(3))(3)] has been evaluated. The structures of the compounds [UO(2)(NO(3))(2)(PhSOCH(2)CON(i)Bu(2))](10) and [Ce(NO(3))(3)(PhSOCH(2)CONBu(2))(2)](12) have been determined by single crystal X-ray diffraction methods. Preliminary extraction studies of ligand L6 with U(VI), Pu(IV) and Am(III) in tracer level showed an appreciable extraction for U(VI) and Pu(IV) in up to 10 M HNO(3) but not for Am(III). Thermal studies on compounds 8 and 10 in air revealed that the ligands can be destroyed completely on incineration. The electron spray mass spectra of compounds 8 and 10 in acetone show that extensive ligand distribution reactions occur in solution to give a mixture of products with ligand to metal ratios of 1: 1 and 2 :1. However, 10 retains its solid state structure in CH(2)Cl(2).

Bis(trimethylphosphine)silver(I) hexafluorophosphate.

In the title two-coordinate silver compound, [Ag(C(3)H(9)P)(2)]PF(6), the cation has crystallographically imposed mirror symmetry, and approximates very closely to 3m (D(3d)) symmetry with fully staggered methyl groups in the solid state. The Ag atom has a nearly linear coordination geometry, with a P[bond]Ag[bond]P angle of 178.70 (4) degrees. The Ag[bond]P bond lengths are 2.3746 (12) and 2.3783 (12) A, which are significantly longer than the Au[bond]P bond length of 2.304 (1) A in the analogous two-coordinate gold cation. The lack of intramolecular steric effects within the present cations containing trimethylphosphine (cone angle 118 degrees), compared with those in known cations containing trimesitylphosphine (cone angle 212 degrees), provides a better comparison of M[bond]P distances and thus more conclusive evidence that Au really is smaller than Ag.