Cation-cation Complex Research Articles

Synthesis and Characterization of Water Stable Uranyl(V) Complexes.

The importance of uranyl(V) (UO2 + ) species associated with environmental and geologic applications is becoming increasingly evident, but the tendency of the uranyl(V) cation to disproportionate in water has prevented the isolation of stable complexes. Here we demonstrate that in the presence of the tridentate complexing dipicolinate (dpa2- ), a ligand highly abundant in soil, the uranyl(V) species can be stabilized and isolated in anoxic basic water. Stable uranyl(V) dipicolinate complexes are readily formed from the reduction of the uranyl(VI) analogue both in organic solution and in basic water, and their solution and solid-state structure were determined. A bis-dpa UV O2 + complex was obtained from water at pH 10, while at higher pH values, a trinuclear mono-dpa cation-cation complex was isolated. These results present the second ever isolated water stable uranyl(V) complex. Moreover, we demonstrate that dipicolinate complexes of UVI O2 2+ , UV O2 + and UIV are strongly luminescent with a signature characteristic of each oxidation state. This provides unique examples of luminescent UV and UIV compounds.

Photolysis of Neptunium Ions in Acetic Acid Solutions

Photochemical reactions involving Np(VI) and Np(V) in 0.5–17.3 M CH3COOH (HAc) solutions have been studied. It was found that irradiation with UV light causes reduction of Np(VI) to Np(V). The initial rate of Np(VI) reduction is at a maximum in a 0.5–1 M solution of HAc and decreases with increasing HAc concentration, which is due to the formation of higher acetic acid complexes with Np(VI). After a partial reduction of Np(VI) as a result of irradiation, its concentration continues to decrease in the dark reaction with products formed in the oxidation of HAc. The reduction of Np(V) occurs with an induction period, after which the reaction becomes faster. This is due to the appearance and accumulation of Np(IV), which forms a cation-cation complex with Np(V). The complex absorbs light, which leads to the reaction of Np(V) with HAc. Np(VI) is oxidized in acetic acid solutions by oxygen to Np(V).

Cation–cation interactions between NpO2+ and UO22+ at different temperatures and ionic strengths

Cation-cation interactions between NpO(2)(+) and UO(2)(2+) were studied at different temperatures (283.15 K to 358.15 K) and different ionic strengths (3-4.5 mol dm(-3)) by spectrophotometry and microcalorimetry. The cation-cation complex between NpO(2)(+) and UO(2)(2+) was weak and became stronger as the temperature was increased from 283.15 K to 358.15 K. The molar enthalpy of complexation was directly determined for the first time by microcalorimetry to be (4.2 ± 1.6) kJ mol(-1) at 298.15 K, in good agreement with the trend in the stability constant at different temperatures. The small and positive enthalpy and entropy of complexation support the argument that the cation-cation complex between NpO(2)(+) and UO(2)(2+) is of inner-sphere type. At each temperature, the stability constants of the cation-cation complex were found to increase as the ionic strength was increased. The specific ion interaction theory (SIT) was used to obtain the stability constants at infinite dilution and variable temperatures.

The mechanism of the reduction of [AnO2]2+ (An = U, Np, Pu) in aqueous solution, and by Fe(ii) containing proteins and mineral surfaces, probed by DFT calculations

The fate of actinyl species in the environment is closely linked to oxidation state, since the reduction of An(VI) to An(IV) greatly decreases their mobility due to the precipitation of the relatively insoluble An(IV) species. Here we study the mechanism of the reduction of [AnO(2)](2+) (An = U, Np, Pu) both in aqueous solution and by Fe(II) containing proteins and mineral surfaces, using density functional theory calculations. We find a disproportionation mechanism involving a An(V)-An(V) cation-cation complex, and we have investigated how these complexes are formed in the different environments. We find that the behaviour of U and Pu complexes are similar, but the reduction of Np(V) to Np(IV) would seems to be more difficult, in line with the experimental finding that Np(V) is generally more stable than U(V) or Pu(V). Although the models we have used are somewhat idealised, our calculations suggest that there are strong similarities between the biotic and abiotic reduction pathways.

Disproportionation of Pu(V) in aqueous HCOOH solutions

The behavior of Pu(VI), Pu(V), and Pu(IV) in the HCOOH-H2O system was studied by spectrophotometry. The Pu(VI) absorption spectrum in solutions containing less than 1 mM HClO4 changes on adding HCOOH to a concentration of 0.53 M. Along with a decrease in the intensity of the absorption maximum at 830.6 nm, corresponding to an f-f transition in the Pu 2 2+ aqua ion, a new band arises with the maximum shifted to 834.5 nm. These transformations are due to formation of a Pu(VI) formate complex (1: 1). The Pu(IV) absorption spectra in HCOOH solutions vary insignificantly in going from 3.0 to 9.0 M HCOOH and are similar to the spectrum of Pu(IV) in a 0.88 M HCOOH + 0.41 M NaHCOO + 0.88 M NaClO4 solution, which indicates that the composition of the Pu(IV) formate complexes is constant. Pu(V) is unstable in HCOOH solutions and disproportionates to form Pu(VI) and Pu(IV). The reaction rate is approximately proportional to [Pu(V)]2 and grows with an increase in [HCOOH]. The reaction products affect the reaction rate: Pu(IV) accelerates the process, and Pu(VI) decelerates the consumption of Pu(V) by binding Pu(V) in a cationcation complex. The disproportionation occurs via formation of a Pu(V)-Pu(V) cation-cation complex whose thermal excitation yields an activated complex with its subsequent decomposition to Pu(VI) and Pu(IV).

Equatorial ligand substitution by hydroxide in uranyl Pacman complexes of a Schiff-base pyrrole macrocycle

The synthesis of the mono-uranyl complex [UO(2)(THF)(H(2)L(Me))] of a ditopic Schiff-base pyrrole macrocycle is described and is shown to adopt a Pacman wedge-shaped structure in which the uranyl dication is desymmetrised and sits solely in one N(4)-donor compartment to leave the other vacant. While investigating the mechanism of the previously reported, base-initiated, reductive silylation chemistry of [UO(2)(THF)(H(2)L(Me))], we found that uranyl hydroxide complexes could be isolated. As such, the reaction between [UO(2)(THF)(H(2)L(Me))] and KH in THF generated the dimeric cation-cation hydroxide [{UO(2)(OH)K(C(6)H(6))(H(2)L(Me))}(2)] when crystallised from C(6)H(6), or alternatively, when crystallised from THF, the monomeric THF-adducted cation-cation complex [UO(2)(OH)K(THF)(2)(H(2)L(Me))] was isolated. These compounds result formally from the substitution of the equatorial THF molecule by hydroxide, and it was also shown that the reaction between dry KOH and [UO(2)(THF)(H(2)L(Me))] generated [{UO(2)(OH)K(C(6)H(6))(H(2)L(Me))}(2)].

Disproportionation of Pu(V) in the CH3COOH-H2O system

The behavior of Pu(VI) and Pu(V) in CH3COOH (HAc)-H2O solutions was studied by spectrophotometry. The absorption spectrum of Pu(VI) does not change on adding HAc to a concentration of 5 M in the presence of 0.5–1.0 M HClO4, but in solutions containing less than 0.001 M mineral acid, changes in the spectrum are observed at HAc concentration of 0.6 M.he major absorption band of PuO 2 2+ ions, caused by an f-f transition, with increasing [HAc] is shifted from 830.6 to 836 nm, with a simultaneous decrease in the absorption intensity, which is due to formation of 1: 1 complexes of Pu(VI) with Ac− ions. In anhydrous HAc, the peak intensity increases again, owing to total change in the composition of the solvation shell. Pu(V) is unstable in 1–17 M HAc solutions and disproportionates to form Pu(VI) and Pu(IV). The Pu(V) loss follows a second-order rate law with respect to [Pu(V)] and accelerates with increasing HAc concentration. The reaction products exert opposite effects on the reaction rate: Pu(IV) accelerates the consumption of Pu(V), whereas Pu(VI) does not affect the process in dilute HAc solutions but decelerates the disproportionation in concentrated solutions owing to formation of a cation-cation complex with Pu(V).

A Theoretical Study of the Inner-Sphere Disproportionation Reaction Mechanism of the Pentavalent Actinyl Ions

The inner-sphere mechanisms of the disproportionation reactions of U(V), Np(V), and Pu(V) ions have been studied using a quantum mechanical approach. The U(V) disproportionation proceeds via the formation of a dimer (a cation-cation complex) followed by two successive protonations at the axial oxygens of the donor uranyl ion. Bond lengths and spin multiplicities indicate that electron transfer occurs after the first protonation. A solvent water molecule then breaks the complex into solvated U(OH)2(2+) and UO2(2+) ions. Pu(V) behaves similarly, but Np(V) appears not to follow this path. The observations from quantum modeling are consistent with existing experimental data on actinyl(V) disproportionation reactions.

Formation-decomposition equilibrium of the NpO 2 + ·UO 2 2+ complex in chloride melts

The absorption spectra of the NpO 2 + (5f 2) ion were examined in the region of the 3H 5 ← 3 H 4 magnetic dipole transition (1530–1760 nm) for series of melts with the UO 2 2+ concentration varied in the opposite directions: (1) NaCl-2CsCl eutectic melt with growing additions of the Cs2UO2Cl4 complex salt and (2) Cs2UO2Cl4 melt with growing additions of the NaCl-2CsCl mixture. Measurements of the integrated intensities of the bands belonging to the NpO 2 + ·UO 2 2+ complex and unbound NpO 2 + throughout the UO 2 2+ concentration range examined (up to 4.4 M in neat Cs2UO2Cl4 melt) and processing of the data obtained in terms of the mass action law showed that the formation-decomposition reaction of the cation-cation complex can be described adequately only using the equation of reaction in the form NpO2Cl 4 3− + UO2Cl 4 2− ⇆ {Cl4ONpO⋯UO2Cl3}4− = Cl− (with the equilibrium constant of 1.3±0.1). Thus, the formation of the cationcation complex should be treated as replacement of chloride ion in the equatorial plane of uranyl(VI) by neptunyl(V), rather than as simple addition of UO 2 2+ to NpO 2 + . The reverse reaction, decomposition of the cation-cation complex, consists essentially in replacement of neptunyl(V) by chloride ion.

Isolation of a Tetrameric Cation−Cation Complex of Pentavalent Uranyl

A polymetallic assembly containing mutually coordinated highly reactive UO2+ groups was isolated in the presence of dibenzoylmethanate. NMR studies showed unambiguously the presence of the cation-cation complex in pyridine solution while more polar solvents lead to the disruption of the UO2+/UO2+ interaction and increased stability.

NpO 2 + -UO 2 2+ cation-cation interaction in NaCl-KCl-CsCl and LiCl-KCl-CsCl eutectic melts

Absorption spectra of the NpO 2 + ion in UO 2 2+ -containing NaCl-KCl-CsCl (0.300:0.245:0.455) and LiCl-KCl-CsCl (0.575:0.165:0.260) eutectic melts in the region of the 5f-5f transitions, 700–2000 nm, were studied at 650 and 500°C, respectively. Processing of the spectra allowed the previously discovered phenomenon, cation-cation interaction of actinides in melts, to be interpreted for the first time in terms of the mass action law. It was shown that, at the concentrations of the interacting components studied (CNp = 0.2 M and CU = 0.1–2 M), a 1: 1 NpO 2 + ·UO 2 2+ cation-cation complex is formed. Its concentration stability constant was estimated at 0.06(6) and 0.11 1 mol−1 for the NaCl-KCl-CsCl and LiCl-KCl-CsCl systems, respectively.

First Observation of the Cation-Cation Interaction of Actinides in Melts: NpO 2 + -UO 2 2+ System

Study of the absorption spectra of the NpO2+ ion in a CsCl melt containing 2 M UO22+, as well as in K2UO2Cl4 and Cs2UO2Cl4 melts (T = 700°C), allowed the first detection of the cation-cation interaction of actinides in melts. In all the molten salts studied, the cation-cation interaction of NpO2+ and UO22+ is manifested most clearly (via appearance of an additional band) in the 3H5 ← 3H4 magnetic-dipole transition of the 5f2 configuration of the NpO2+ ion (D4h symmetry) near 1.6 μm: Along with a sharp peak of unbound neptunyl(V) at ∼1660 nm, there is an equally sharp peak at 1633 nm belonging to the cation-cation complex. The intensity ratio for the peaks at 1633 and 1660 nm increases in the series of melts (UO22+ in CsCl) < K2UO2Cl4 < Cs2UO2Cl4 by two orders of magnitude.

Cation-cation complexes of PuO2+and NpO2+with Th4+and UO22+

Cation-cation complexes of NpO2+and PuO2+with UO22+and Th4+were studied using laser-induced photoacoustic spectroscopy (LIPAS). An equilibrium constant of 2.4±0.2 M-1was measured at an ionic strength of 6.0 M for the NpO2+· UO22+complex which is in good agreement with previous measurements using conventional absorption spectroscopy and Raman spectroscopy. Equilibrium constants of 1.8±0.9 and 2.2±1.5 M-1were measured for NpO2+· Th4+and PuO2+· UO22+complexes at an ionic strength of 6.0 M, respectively. This is the first time that LIPAS has been used to study cation-cation complexes and the first time that the PuO2+· UO22+cation-cation complex has been reported.

A refined analysis of oxygen potential of M yU 1− yO 2+ x (M = M 3+and M2+) by lattice statistics based on the grand partition function and the Flory methods

Partial molar thermodynamic quantities ( Δ G O 2 , Δ S O 2 and Δ H O 2 ) of solid solutions M y U 1− y O 2+ x ( M = M 3+ and M 2+, x⪌0 ) were calculated using a cation-cation complex model in combination with the lattice statistics based on the grand partition function method or the Flory method. With these refined methods, configurational entropies were obtained without any discrepancies between the numbers of lattice sites and the sum of ions and complexes. The cation-cation complex model assumes the formation of (M 3+U 5+) complex for M y 3U 1− y O 2+ x while two kinds of complexes, i.e. (M 2+U 5+) and M 2+2U 5+ for M y 2+U 1− y O 2+ x . The experimental results that the steepest change of Δ G O 2 arises at x < 0 for M y 2+ U 1−y O 2+x were well explained by the relation of this model, i.e. x = −(1 − α/2) y with 1 < α < 2. The calculated Δ G O 2 values were in good agreement with the experimental results for lanthanum, magnesium and europium solid solutions.

Crystal structure of a neptunyl cation-cation complex (NpO 2+) with mellitic acid: Na 4(NpO 2) 2Cl 12O 12·8H 2O

The crystal structure of the compound formed by the neptunyl(V) ion ( 237NpO 2 + ) and mellitic acid was determined. The stoichiometric formula of the compound was Na 4(NpO 2) 2C 12O 12·8H 2O with the following crystal data: Z = 4; V c = 2486.7 A ̊ 3 ; D calc = 2.96 g cm −3; a = 12.53(1) A ̊ ; b = 11.58(2) A ̊ ; c = 17.81(2) A ̊ ; β = 105.79°; space group, C2/ c. The structure was obtained using classical methods with R = 0.044 and R w = 0.056 for 2372 independent reflections. The most interesting result was the evidence for neptunyl dimers in the lattice, possibly as a result of cation-cation interactions. The two neptunium atoms in such dimers are 3.4824(8) Å apart. The neptunyl dinier is associated with four mellitate ligands. The sodium ions and the water molecules serve as bridging species to increase the crystalline cohesion.

The quantum chemical interpretation for the cation-cation interactions

The quantum chemical interpretation for the cation-cation interactions

Crystal structures and Mössbauer studies of NpO +2 complexes with polycarboxylic acids

Complexes of neptunyl ( 237NpO + 2 ) with pyromellitic (benzene tetracarboxylic) and mellitic (benzene hexacarboxylic acids were prepared; their formula is: (Na 3NpO 2C 10O 8H 2) 2·11H 2O and Na 4(NpO 2) 2C 12O 12·8H 2O respectively. These solids were studied by X-ray diffraction, Mössbauer spectroscopy ( 237Np) and magnetic susceptibility measurements. ▪ ▪ ▪Fig. 3. Cation-cation complex of NhO 2 + The crystal structure of the Na 3NpO 2 pyromellitate was refined to R = 0,027 and R w = 0.040 for 6793 independent reflexions. Crystal data are: P 1 , a = 14.46(1) Å, b = 9.404(8) Å, c = 6.768(4) Å, α = 95.08°, β ] 103.23°, γ = 87.47°, Z = 1. In this structure (Fig.1), the NpO + 2 ions are bonded to four pyromellitate anions whose carboxyl oxygens form the base of a pentagonal bipyramid surrounding the Np atom. The Mössbauer spectrum recorded at 4.2 K and shown in Fig. 2, exhibited a magnetic hyperfine splitting attributed to paramagnetic relaxation phenomena [1, 2]. The crystal structure of the Na 4(NpO 2) 2 mellitate was solved by classical methods. R = 0.044 and R w = 0.056 for 2372 independent reflexions. a = 12.53(1) Å, b = 11.58(2) Å, c = 17.81(2) Å, β = 105.79°, Z = 4, space group C 2/c. Two NpO + 2 ions are connected to each other and coordinated to four mellitate anions. Figure 3 shows the ‘cationcation’ complex of NpO + 2 . The χ curve for this complex follows the Curie-Weiss law. A magnetic transition occurs at about 10 K. The Mössbauer spectrum ( 237Np) recorded at 4.2 K and then fitted [3] leads to the following hyperfine parameters: H eff = 5490(10) k0e, e 2qQ = 97(2) mm s −1. The isomer shift (δ = −19.2(5) mm s −1 ref. NpAl 2 [4]) corresponds to that of Np(V). Two Np atoms are at a distance shorter than 3.5 Å in this complex and this could explain the magnetic transition observed.

Spectrophotometric identification of a mixed-valence cation-cation complex between aquadioxovanadium(V) and aquaoxovanadium(IV) ions in perchloric, sulfuric, and hydrochloric acid media

Spectrophotometric identification of a mixed-valence cation-cation complex between aquadioxovanadium(V) and aquaoxovanadium(IV) ions in perchloric, sulfuric, and hydrochloric acid media

Electron transfer and aqua cation-cation complex formation in the excited state of the uranyl ion

Excited-state mechanistic aspects of electron transfer between two aqua-cationic species are investigated and discussed in the case of excited UO2−2(aq) as acceptor and Ag−(aq) as donor. It is shown that excited-state electron transfer leads to an inner-sphere non-radiative complex of appreciable binding energy.