Sean D. Reilly

Bonding Trends Traversing the Tetravalent Actinide Series: Synthesis, Structural, and Computational Analysis of AnIV(Aracnac)4 Complexes (An = Th, U, Np, Pu; Aracnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-tBu2C6H3)

A series of tetravalent An(IV) complexes with a bis-phenyl β-ketoiminate N,O donor ligand has been synthesized with the aim of identifying bonding trends and changes across the actinide series. The neutral molecules are homoleptic with the formula An((Ar)acnac)(4) (An = Th (1), U (2), Np (3), Pu (4); (Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)) and were synthesized through salt metathesis reactions with actinide chloride precursors. NMR and electronic absorption spectroscopy confirm the purity of all four new compounds and demonstrate stability in both solution and the solid state. The Th, U, and Pu complexes were structurally elucidated by single-crystal X-ray diffraction and shown to be isostructural in space group C2/c. Analysis of the bond lengths reveals shortening of the An-O and An-N distances arising from the actinide contraction upon moving from 1 to 2. The shortening is more pronounced upon moving from 2 to 4, and the steric constraints of the tetrakis complexes appear to prevent the enhanced U-O versus Pu-O orbital interactions previously observed in the comparison of UI(2)((Ar)acnac)(2) and PuI(2)((Ar)acnac)(2) bis-complexes. Computational analysis of models for 1, 2, and 4 (1a, 2a, and 4a, respectively) concludes that both the An-O and the An-N bonds are predominantly ionic for all three molecules, with the An-O bonds being slightly more covalent. Molecular orbital energy level diagrams indicate the largest 5f-ligand orbital mixing for 4a (Pu), but spatial overlap considerations do not lead to the conclusion that this implies significantly greater covalency in the Pu-ligand bonding. QTAIM bond critical point data suggest that both U-O/U-N and Pu-O/Pu-N are marginally more covalent than the Th analogues.

Plutonium(IV) complexation by diglycolamide ligands—coordination chemistry insight into TODGA-based actinide separations

Complexation of Pu(IV) with TMDGA, TEDGA, and TODGA diglycolamide ligands was followed by vis-NIR spectroscopy. A crystal structure determination reveals that TMDGA forms a 1 : 3 homoleptic Pu(IV) complex with the nitrate anions forced into the outer coordination sphere.

Synthesis and Structure of (Ph4P)2MCl6 (M = Ti, Zr, Hf, Th, U, Np, Pu)

High-purity syntheses are reported for a series of first, second, and third row transition metal and actinide hexahalide compounds with equivalent, noncoordinating countercations: (Ph(4)P)(2)TiF(6) (1) and (Ph(4)P)(2)MCl(6) (M = Ti, Zr, Hf, Th, U, Np, Pu; 2-8). While a reaction between MCl(4) (M = Zr, Hf, U) and 2 equiv of Ph(4)PCl provided 3, 4, and 6, syntheses for 1, 2, 5, 7, and 8 required multistep procedures. For example, a cation exchange reaction with Ph(4)PCl and (NH(4))(2)TiF(6) produced 1, which was used in a subsequent anion exchange reaction with Me(3)SiCl to synthesize 2. For 5, 7, and 8, synthetic routes starting with aqueous actinide precursors were developed that circumvented any need for anhydrous Th, Np, or Pu starting materials. The solid-state geometries, bond distances and angles for isolated ThCl(6)(2-), NpCl(6)(2-), and PuCl(6)(2-) anions with noncoordinating counter cations were determined for the first time in the X-ray crystal structures of 5, 7, and 8. Solution phase and solid-state diffuse reflectance spectra were also used to characterize 7 and 8. Transition metal MCl(6)(2-) anions showed the anticipated increase in M-Cl bond distances when changing from M = Ti to Zr, and then a decrease from Zr to Hf. The M-Cl bond distances also decreased from M = Th to U, Np, and Pu. Ionic radii can be used to predict average M-Cl bond distances with reasonable accuracy, which supports a principally ionic model of bonding for each of the (Ph(4)P)(2)MCl(6) complexes.

Structural and spectroscopic characterization of plutonyl(VI) nitrate under acidic conditions.

The plutonyl(VI) dinitrate complex [PuO(2)(NO(3))(2)(H(2)O)(2)]·H(2)O (1) has been structurally characterized by single-crystal X-ray diffraction and spectroscopically characterized by solid-state vis-NIR and Raman spectroscopies. Aqueous solution spectroscopic studies indicate only weak plutonyl(VI) nitrate complexation, with the mononitrate complex dominating and negligible dinitrate formation, even in concentrated nitric acid.

Differences in actinide metal–ligand orbital interactions: comparison of U(IV) and Pu(IV) β-ketoiminate N,O donor complexes

Syntheses and characterization of UCl(2)((Ar)acnac)(2), UI(2)((Ar)acnac)(2), and PuI(2)((Ar)acnac)(2) are reported ((Ar)acnac denotes a bis-phenyl β-ketoiminate ligand where Ar = 3,5-(t)Bu(2)C(6)H(3)). Structural analyses and computations show significant metal-ligand orbital interaction differences in U(IV) vs. Pu(IV) bonding.

Low-valent molecular plutonium halide complexes.

Treatment of plutonium metal with 1.5 equiv of bromine in tetrahydrofuran (thf) led to isolation of PuBr3(thf)4 (1), which is a new versatile synthon for exploration of non-aqueous Pu(III) chemistry. Adventitious water in the system resulted in structural characterization of the eight-coordinate complex [PuBr2(H2O)6][Br] (2). The crystal structure of PuI3(thf)4 (3) has been determined for the first time and is isostructural with UI3(thf)4. Attempts to form a bis(imido) plutonyl(VI) moiety ([Pu(NR)2](2+)) by oxidation of PuI3(py)4 with iodine and (t)BuNH2 resulted in crystallization of the Pu(III) complex [PuI2(thf)4(py)][I3] (4). Dissolution of a Pu(IV) carbonate with a HCl/Et2O solution in thf gave the mixed valent (III/IV) complex salt [PuCl2(thf)5][PuCl5(thf)] (5) as the only tractable product. Oxidation of Pu[N(SiMe3)2]3 with TeCl4 afforded the Pu(IV) complex Pu[N(SiMe3)2]3Cl (6), which may prove to be a useful entry route for investigation of organometallic/non-aqueous tetravalent plutonium chemistry.

SPECTROSCOPIC INVESTIGATION OF THE FORMATION OF PUO2CL+ AND PUO2CL2 IN NACL SOLUTIONS AND APPLICATION FOR NATURAL BRINE SOLUTIONS

The chloride complexation of the PuO22+ ion has been studied in acidic NaCl solutions with electrolyte concentrations as high as 5 mol kg−1 at 23°C by using conventional absorption spectrophotometry. Plutonyl and its complexes have ionic strength-dependent molar absorptivities that were determined in NaClO4, the first essential step in the quantitative analysis of chloride complexation. The distributions of species for the Pu complexes, PuO22+, PuO2Cl+, and PuO2Cl2o, formed under the conditions investigated, were determined by peak-fitting of optical absorption spectra. The apparent stability constants of the Pu(VI) chloro complexes were calculated at each NaCl concentration. Specific ion-interaction theory parameters were determined for the plutonyl chloro complexes and the electrolyte constituents, then compared with the literature data. The calculated values for log β° were determined to be 0.23 ± 0.03 and −1.7 ± 0.2 for the mono and bis chloro complexes, respectively. Spectra of Pu(VI) in brines representative of waters at the Waste Isolation Pilot Plant, the licensed nuclear waste repository in a salt formation at Carlsbad, NM, USA, were measured and modeled by using the thermodynamic data and ion interaction parameters were determined. In these brines, less than 10% of the total Pu(VI) concentration exists as the Pu(VI) aquo ion, whereas about 90% is present as Pu(VI) chloro complexes.

[N(n-Bu)4]2[Pu(NO3)6] and [N(n-Bu)4]2[PuCl6]: starting materials to facilitate nonaqueous plutonium(IV) chemistry.

The reaction of plutonium(IV) in aqueous nitric acid with tetra-n-butylammonium nitrate leads to the immediate precipitation of [N(n-Bu)(4)](2)[Pu(NO(3))(6)] (1) in high yield. The analogous reaction in HCl with tetra-n-butylammonium chloride gives [N(n-Bu)(4)](2)[PuCl(6)] (2). Both 1 and 2 are soluble in a range of organic solvents and have been characterized by single-crystal X-ray diffraction, IR spectroscopy, and solid- and solution-phase vis-near-IR spectroscopy. 1 and 2 provide facile synthetic entry routes to study plutonium(IV) ligand complexation reactions in organic solvent media under both air/moisture-stable and -sensitive conditions.

A Linear trans-Bis(imido) Neptunium(V) Actinyl Analog: NpV(NDipp)2(tBu2bipy)2Cl (Dipp = 2,6-iPr2C6H3)

The discovery that imido analogs of actinyl dioxo cations can be extended beyond uranium into the transuranic elements is presented. Synthesis of the Np(V) complex, Np(NDipp)2((t)Bu2bipy)2Cl (1), is achieved through treatment of a Np(IV) precursor with a bipyridine coligand and lithium-amide reagent. Complex 1 has been structurally characterized, analyzed by (1)H NMR and UV-vis-NIR spectroscopies, and the electronic structure evaluated by DFT calculations.

Synthesis and Structural Characterization of Molecular Dy(III) and Er(III) Tetra-Carbonates

Single crystal structures of lanthanide carbonate and hydroxy-carbonate compounds have been previously reported in the literature, with the majority of these compounds being extended one- to three-dimensional compounds. Very few lanthanide compounds have been isolated that contain molecular moieties, and none have been reported for either erbium or dysprosium. Single crystals of the tetra-carbonate complexes, [C(NH(2))(3)](5)[Er(CO(3))(4)].11H(2)O (I) and [C(NH(2))(3)](4)[Dy(CO(3))(4)(H(2)O)](H(3)O).13H(2)O (II), were isolated from concentrated guanidinium carbonate solutions and characterized by single crystal X-ray diffraction studies. Compounds I and II are the first reported molecular carbonate structures for Er and Dy to be characterized via single crystal X-ray diffraction studies. Crystallographic data for I: monoclinic, space group P21/n, a = 8.8160(6) A, b = 21.0121(14) A, c = 19.6496(14) A, Z = 4. Data for II: tetragonal, space group P4/n, a = b = 15.3199(11) A, c = 7.5129(11) A, Z = 2.