Andrew J. Gaunt

Covalency in the f element-chalcogen bond. Computational studies of M N(EPR(2))(2) (3) (M = La, Ce, Pr, Pm, Eu, U, Np, Pu, Am, Cm; E = O, S, Se, Te; R = H, (i)Pr, Ph)

The geometric and electronic structures of the title complexes have been studied using scalar relativistic, gradient-corrected density functional theory. Extension of our previous work on six-coordinate M[N(EPH 2) 2] 3 (M = La, Ce, U, Pu; E = O, S, Se, Te), models for the experimentally characterized M[N(EP (i)Pr 2) 2] 3, yields converged geometries for all of the other 4f and 5f metals studied and for all four group 16 elements. By contrast, converged geometries for nine-coordinate M[N(EPPh 2) 2] 3 are obtained only for E = S and Se. Comparison of the electronic structures of six- and nine-coordinate M[N(EPH 2) 2] 3 suggests that coordination of the N atoms produces only minor changes in the metal-chalcogen interactions. Six-coordinate Eu[N(EPH 2) 2] 3 and Am[N(EPH 2) 2] 3 with the heavier group 16 donors display geometric and electronic properties rather different from those of the other members of the 4f and 5f series, in particular, longer than expected Eu-E and Am-E bond lengths, smaller reductions in charge difference between M and E down group 16, and larger f populations. The latter are interpreted not as evidence of f-based metal-ligand covalency but rather as being indicative of ionic metal centers closer to M (II) than M (III). The Cm complexes are found to be very ionic, with very metal-localized f orbitals and Cm (III) centers. The implications of the results for the separation of the minor actinides from nuclear wastes are discussed, as is the validity of using La (III)/U (III) comparisons as models for minor actinide/Eu systems.

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.

Uncovering f-element bonding differences and electronic structure in a series of 1 : 3 and 1 : 4 complexes with a diselenophosphinate ligand

Understanding the bonding trends within, and the differences between, the 4f and 5f element series with soft donor atom ligands will aid elucidation of the fundamental origins of actinide (An) versus lanthanide (Ln) selectivity that is integral to many advanced nuclear fuel cycle separation concepts. One of the principal obstacles to acquiring such knowledge is the dearth of well characterized transuranic molecules that prevents the necessary comparison of 4f versus 5f coordination chemistry, electronic structure, and bonding. Reported herein is new chemistry of selenium analogues of dithiophosphinate actinide extractants. LnIII and AnIII/IV complexes with the diselenophosphinate [Se2PPh2]− anion have been synthesized, structurally and spectroscopically characterized, and quantum chemical calculations performed on model compounds in which the phenyl rings have been replaced by methyl groups. The complexes [LnIII(Se2PPh2)3(THF)2] (Ln = La (1), Ce (2), Nd (3)), [LaIII(Se2PPh2)3(MeCN)2] (4), [PuIII(Se2PPh2)3(THF)2] (5), [Et4N][MIII(Se2PPh2)4] (M = Ce (6), Pu (7)), and [AnIV(Se2PPh2)4] (An = U (8), Np (9)), represent the first f-element diselenophosphinates. In conjunction with the calculated models, complexes 1–9 were utilized to examine two important factors: firstly, bonding trends/differences between trivalent 4f and 5f cations of near identical ionic radii; secondly, bonding trend differences across the 5f series within the AnIV oxidation state. Analysis of both experimental and computational data supports the conclusion of enhanced covalent bonding contributions in PuIII–Se versus CeIII–Se bonding, while differences between UIV–Se and NpIV–Se bonding is satisfactorily accounted for by changes in the strength of ionic interactions as a result of the increased positive charge density on NpIV compared to UIV ions. These findings improve understanding of soft donor ligand binding to the f-elements, and are of relevance to the design and manipulation of f-element extraction processes.

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.

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.

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.

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.

Identification of the Formal +2 Oxidation State of Plutonium: Synthesis and Characterization of {PuII[C5H3(SiMe3)2]3}−

Over 70 years of chemical investigations have shown that plutonium exhibits some of the most complicated chemistry in the periodic table. Six Pu oxidation states have been unambiguously confirmed (0 and +3 to +7), and four different oxidation states can exist simultaneously in solution. We report a new formal oxidation state for plutonium, namely Pu2+ in [K(2.2.2-cryptand)][PuIICp″3], Cp″ = C5H3(SiMe3)2. The synthetic precursor PuIIICp″3 is also reported, comprising the first structural characterization of a Pu-C bond. Absorption spectroscopy and DFT calculations indicate that the Pu2+ ion has predominantly a 5f6 electron configuration with some 6d mixing.