Iain May

Polyoxometal cations within polyoxometalate anions. Seven-coordinate uranium and zirconium heteroatom groups in [(UO2)12(μ3-O)4(μ2-H2O)12(P2W15O56)4]32− and [Zr4(μ3-O)2(μ2-OH)2(H2O)4 (P2W16O59)2]14−

Abstract Two new composite polyoxotungstate anions with unprecedented structural features, [(UO 2 ) 12 (μ 3 -O) 4 (μ 2 -H 2 O) 12 (P 2 W 15 O 56 ) 4 ] 32− ( 1 ) and [Zr 4 (μ 3 -O) 2 (μ 2 -OH) 2 (H 2 O) 4 (P 2 W 16 O 59 ) 2 ] 14− ( 2 ) contain polyoxo-uranium and -zirconium clusters as bridging units. The anions are synthesized by reaction of Na 12 [P 2 W 15 O 56 ] with solutions of UO 2 (NO 3 ) 2 and ZrCl 4 . The structure of 1 in the sodium salt contains four [P 2 W 15 O 56 ] 12− anions assembled into an overall tetrahedral cluster by means of trigonal bridging groups formed by three equatorial-edge-shared UO 7 pentagonal bipyramids. The structure of anion 2 consists of a centrosymmetric assembly of two [P 2 W 16 O 59 ] 12− anions linked by a {Zr 4 O 2 (OH) 2 (H 2 O) 4 } 10+ cluster. Both complexes in solution yield the expected two-line 31 P-NMR spectra with chemical shifts of −2.95, −13.58 and −6.45, −13.69 ppm, respectively.

A rare structural characterisation of the phosphomolybdate lacunary anion, [PMo11O39]7−. Crystal structures of the Ln(III) complexes, (NH4)11[Ln(PMo11O39)2]·16H2O (Ln = CeIII, SmIII, DyIII or LuIII)

The syntheses and the crystal structures of (NH4)11[LnIII(PMo11O39)2]·16H2O (LnIII = Ce (1), Sm (2), Dy (3) or Lu (4)) are reported in which an LnIII cation is sandwiched between two ‘lacunary’ [PMo11O39]7− anions to give a complex with eight oxygen atoms coordinated to the lanthanide centre in a twisted square antiprismatic geometry. This analogous series of complexes represents only the second time that the [PMo11O39]7− anion has been fully crystallographically characterised. All four compounds are synthesised in high yield and characterised by a range of physical and spectroscopic techniques. 31P NMR, Raman and UV/Vis/nIR spectroscopies give a clear indication that the [LnIII(PMo11O39)2]11− anion is also stable in solution. As LnIII contracts across the lanthanide series the Ln–O bond distances decrease and the splitting of the νP–O vibrational mode within the [PMo11O39]7− unit increases. The relative stability of this species in solution may have importance in elucidating the speciation of Ln(III) and An(III) cations in nuclear waste solutions where phosphopolymolybdate anions are known to form.

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.

Tetravalent Metal Complexation by Keggin and Lacunary Phosphomolybdate Anions

We report the synthesis, spectroscopic and structural characterization, and computational analysis of a series of phosphomolybdate complexes with tetravalent metal cations. The reaction between Ce (IV) and Th (IV) with phosphomolybdate at the optimum pH for the stabilization of the lacunary heteropolyoxometalate anion, [PMo 11O 39] (7-), results in the formation of compounds containing the anions [Ce(PMo 11O 39) 2] (10-) and [Th(PMo 11O 39) 2] (10-), respectively. Single crystal X-ray diffraction analysis was performed on salts of both species, Cs 10[Ce(PMo 11O 39) 2].20H 2O and (NH 4) 10[Th(PMo 11O 39) 2].22H 2O. In both anionic complexes the f-block metal cation is coordinated to the four unsaturated terminal lacunary site oxygens of each [PMo 11O 39] (7-) anion, yielding 8 coordinate sandwich complexes, analogous to previously prepared related complexes. Spectroscopic characterization points to the stability of these complexes in solution over a reasonably wide pH range. Density functional analysis suggests that the Ce-O bond strength in [Ce(PMo 11O 39) 2] (10-) is greater than the Th-O bond strength in [Th(PMo 11O 39) 2] (10-), with the dominant bonding interaction being ionic in both cases. In contrast, under similar reaction conditions, the dominant solid state Zr (IV) and Hf (IV) complexes formed contain the anions [Zr(PMo 12O 40)(PMo 11O 39)] (6-) and [Hf(PMo 12O 40)(PMo 11O 39)] (6-), respectively. In these complexes the central Group 4 d-block metal cations are coordinated to the four unsaturated terminal lacunary site oxygens of the [PMo 11O 39] (7-) ligand and to four bridging oxygens of a plenary Keggin anion, [PMo 12O 40] (3-). In addition, (NH 4) 5{Hf[PMo 12O 40][(NH 4)PMo 11O 39]}.23.5H 2O can be crystallized as a minor product. The structure of the anion, {Hf[PMo 12O 40][(NH 4)PMo 11O 39]} (5-), reveals coordination of the central Hf (IV) cation via four bridging oxygens on both the coordinated [PMo 11O 39] (7-) and [PMo 12O 40] (3-) anions. Unusually, the highly charged lacunary site remains uncoordinated to the Hf metal center but instead interacts with an ammonium cation. (31)P NMR indicates that complexation of the Keggin anion, [PMo 12O 40] (3-), to Hf (IV) and Zr (IV) will stabilize the Keggin anion to a much higher pH than usually observed.

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.

In Situ Spectroscopy and Spectroelectrochemistry of Uranium in High-Temperature Alkali Chloride Molten Salts.

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO 2Cl 4] (2-). Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO 2Cl 4] (2-). The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts.

Group 15 quaternary alkyl bistriflimides: ionic liquids with potential application in electropositive metal deposition and as supporting electrolytes

We report the electrochemical properties of Group 15 quaternary alkyl bistriflimide salts, which have very wide electrochemical windows (between +2.6 and −3.4 V vs. Fc+/Fc for [(Me)4As][N(SO2CF3)2]) when used as supporting electrolytes in MeCN and which can be used for the electrodeposition of very electropositive metals, including Eu, in the molten state.

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.

Neptunium(IV) and uranium(VI) complexation by hydroxamic acids

Hydroxamic acids complex more readily with Ac(IV) than Ac(VI). Formohydroxamic acid (FHA) is a hydrophilic organic ligand which can readily complex with Np(IV), as indicated by near infrared spectroscopy. Distribution experiments have also shown that FHA can strip Np(IV) from 30% TBP/OK into aqueous nitric acid. In contrast U(VI) does not complex as strongly with FHA and the reaction is greatly inhibited by nitric acid, as observed by UV-Vis spectroscopy. After stripping Np(IV) it has also been proven, by 13C NMR spectroscopy, that FHA can be decomposed to gaseous products in concentrated nitric acid leaving no organic waste products in solution. This experimental evidence proves that FHA can be used to selectively strip Np(IV) from a 30% TBP/OK solution of Np(IV) and U(VI) into aqueous nitric acid and is thus a viable new reagent for inclusion in an advanced PUREX process.

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.