Comparison of Americium(III) and Neodymium(III) Monothiophosphate Complexes.

Mixed-donor ligands, such as those containing a combination of O/N or O/S, have been studied extensively for the selective extraction of trivalent actinides, especially Am3+ and Cm3+, from lanthanides during the recycling of used nuclear fuel. Oxygen/sulfur donor ligand combinations also result from the hydrolytic and/or radiolytic degradation of dithiophosphates, such as the Cyanex class of extractants, which are initially converted to monothiophosphates. To understand potential differences between the binding of such degraded ligands to Nd3+ and Am3+, the monothiophosphate complexes [M(OPS(OEt)2)5(H2O)2]2- (M3+ = Nd3+, Am3+) were prepared and characterized by single-crystal X-ray diffraction and optical spectroscopy and studied as a function of pressure up to ca. 14 GPa using diamond-anvil techniques. Although Nd3+ and Am3+ have nearly identical eight-coordinated ionic radii, these structures reveal that while the M-O bond distances in these complexes are almost equal, the M-S distances are statistically different. Moreover, for [Nd(OPS(OEt)2)5(H2O)2]2-, the hypersensitive 4I9/2 → 4G5/2 transition shifts as a function of pressure by -11 cm-1/GPa. Whereas for [Am(OPS(OEt)2)5(H2O)2]2-, the 7F0 → 7F6 transition shows a slightly stronger pressure dependence with a shift of -13 cm-1/GPa and also exhibits broadening of the 5f → 5f transitions at high pressures. These data likely indicate an increased involvement of the 5f orbitals in bonding with Am3+ relative to that of Nd3+ in these complexes.

Chemical Kinetics for the Oxidation of Californium(III) Ions with Select Radiation-Induced Inorganic Radicals (Cl2•- and SO4•-).

Despite the availability of transuranic elements increasing in recent years, our understanding of their most basic and inherent radiation chemistry is limited and yet essential for the accurate interpretation of their physical and chemical properties. Here, we explore the transient interactions between trivalent californium ions (Cf3+) and select inorganic radicals arising from the radiolytic decomposition of common anions and functional group constituents, specifically the dichlorine (Cl2•-) and sulfate (SO4•-) radical anions. Chemical kinetics, as measured using integrated electron pulse radiolysis and transient absorption spectroscopy techniques, are presented for the reactions of these two oxidizing radicals with Cf3+ ions. The derived and ionic strength-corrected second-order rate coefficients (k) for these radiation-induced processes are k(Cf3+ + Cl2•-) = (8.28 ± 0.61) × 105 M-1 s-1 and k(Cf3+ + SO4•-) = (9.50 ± 0.43) × 108 M-1 s-1 under ambient temperature conditions (22 ± 1 °C).

Co-Crystallization of Plutonium(III) and Plutonium(IV) Diglycolamides with Pu(III) and Pu(IV) Hexanitrato Anions: A Route to Redox Variants of [PuIII,IV(DGA)3][PuIII,IV(NO3)6]x.

N,N,N',N'-tetramethyl diglycolamide (TMDGA), a methylated variant of the diglycolamide extractants being proposed as curium holdback reagents in advanced used nuclear fuel reprocessing technologies, has been crystallized with plutonium, a transuranic actinide that has multiple accessible oxidation states. Two plutonium TMDGA complexes, [PuIII(TMDGA)3][PuIII(NO3)6] and[PuIV(TMDGA)3][PuIV(NO3)6]2·0.75MeOH, were crystallized through solvent diffusion of a reaction mixture containing plutonium(III) nitrate and TMDGA. The sample was then partially oxidized by air to yield [PuIV(TMDGA)3][PuIV(NO3)6]2·0.75MeOH. Single-crystal X-ray diffraction reveals that the multinuclear systems crystallize with hexanitrato anionic species, providing insight into the first solid-state isolation of the elusive trivalent plutonium hexanitrato species. Crystallography data show a change in geometry around the TMDGA metal center from Pu3+ to Pu4+, with the symmetry increasing approximately from C4v to D3h. These complexes provide a rare opportunity to investigate the bond metrics of plutonium in two different oxidation states with similar coordination environments. Further, these new structures provide insight into the potential chemical and structural differences arising from the radiation-induced formation of transient tetravalent curium oxidation states in used nuclear fuel reprocessing streams.

Radium Revisited: Revitalization of the Coordination Chemistry of Nature's Largest +2 Cation.

The crystallization, single crystal structure, and Raman spectroscopy of Ra(NO3)2 have been investigated by experimentation and theory, which represent the first pure radium compound characterized by single crystal X-ray diffraction. The Ra2+ centers are bound by six chelating nitrate anions to form an anticuboctahedral geometry. The Raman spectrum acquired from a single crystal of Ra(NO3)2 generally occurs at a lower frequency than found in Ba(NO3)2, as expected. Computational studies on Ra(NO3)2 provide an estimation of the bond orders via Wiberg bond indices and indicate that Ra-O interactions are weak with values of 0.025 and 0.026 for Ra-O bonds. Inspection of natural bond orbitals and natural localized molecular orbitals suggest negligible orbital mixing. However, second-order perturbation interactions show that donation from the lone pairs of the nitrate oxygen atoms to the 7s orbitals of Ra2+ stabilizes each Ra-O interaction by ca. 5 kcal mol-1.

Isolation of a californium(II) crown-ether complex.

The actinides, from californium to nobelium (Z = 98-102), are known to have an accessible +2 oxidation state. Understanding the origin of this chemical behaviour requires characterizing CfII materials, but investigations are hampered by the fact that they have remained difficult to isolate. This partly arises from the intrinsic challenges of manipulating this unstable element, as well as a lack of suitable reductants that do not reduce CfIII to Cf°. Here we show that a CfII crown-ether complex, Cf(18-crown-6)I2, can be prepared using an Al/Hg amalgam as a reductant. Spectroscopic evidence shows that CfIII can be quantitatively reduced to CfII, and rapid radiolytic re-oxidation in solution yields co-crystallized mixtures of CfII and CfIII complexes without the Al/Hg amalgam. Quantum-chemical calculations show that the Cf‒ligand interactions are highly ionic and that 5f/6d mixing is absent, resulting in weak 5f→5f transitions and an absorption spectrum dominated by 5f→6d transitions.

CO2 capture from ambient air via crystallization with tetraalkylammonium hydroxides.

Aqueous solutions of a series of short carbon chain tetra(n-alkyl)ammonium hydroxides, [Nnnnn][OH] with n = 2: n-ethyl, 3: n-propyl, 4: n-butyl, have been serendipitously found to be potential candidates for direct air carbon capture (DAC) when being used as reagents in more complicated reactions. Aqueous solutions of [N3333][OH], [N2222][OH], or [N3333][OH] with UO2SO4·3H2O and 1,4-diamidoximylbenzene, and [N4444][OH] with cytosine (HCyt) directly absorb CO2 from the atmosphere upon mild heating in the open atmosphere crystallizing in complexes reaching up to 2 : 1 CO2/[Nnnnn]OH ratio. [N2222][HCO3]·3H2O (1), [N2222]2[H(HCO3)3]·5H2O (2), [N3333][HCO3]·0.5H2O (3), [N3333][H(HCO3)2] (4), [N3333]2[(tpa)(H2CO3)2] (5; tpa = terephthalate), [N4444][H(Cyt)(HCO3)]·H2O (6) and [N4444][H2(Cyt)2(HCO3)]·H2O (7) have been isolated in crystalline form and structurally characterized by single crystal X-ray diffraction. The compounds are characterized by complex polyanionic formations from bicarbonate dimers ([(HCO3)2·(H2O)]24-) or chains ([H(HCO3)2]nn- or [H2(tpa)(HCO3)2]n2n-) to water-bicarbonate associates ([(HCO3)2·6H2O]2- and [(H2CO3·(HCO3)2)2·6H2O·2H2O]2-) and three-component anionic layers ([H(Cyt)(HCO3)·H2O]nn- and [H2(Cyt)2(HCO3)·H2O]nn-) frequently showing proton sharing. While some hydroxides themselves can maintain a high CO2/[Nnnnn][OH] ratio, particularly 2 and 4, the presence of secondary hydrogen bond donors/acceptors may increase the sorption efficiency through decreased solubility and enhanced crystallization.