Isolation and characterization of a californium metallocene.

Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin-orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table1-3, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry4-12, and to a smaller extent americium (Am)13, transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C5Me4H)2Cl2K(OEt2)]n from two milligrams of 249Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U)14,15, contains the first crystallographically characterized Cf-C bond. Analysis suggests the Cf-C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C5Me4H)2Cl2K(OEt2)]n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds16-22.

Advancing Understanding of the +4 Metal Extractant Thenoyltrifluoroacetonate (TTA-); Synthesis and Structure of MIVTTA4 (MIV = Zr, Hf, Ce, Th, U, Np, Pu) and MIII(TTA)4- (MIII = Ce, Nd, Sm, Yb).

Thenoyltrifluoroacetone (HTTA)-based extractions represent popular methods for separating microscopic amounts of transuranic actinides (i.e., Np and Pu) from macroscopic actinide matrixes (e.g. bulk uranium). It is well-established that this procedure enables +4 actinides to be selectively removed from +3, + 5, and +6 f-elements. However, even highly skilled and well-trained researchers find this process complicated and (at times) unpredictable. It is difficult to improve the HTTA extraction-or find alternatives-because little is understood about why this separation works. Even the identities of the extracted species are unknown. In addressing this knowledge gap, we report here advances in fundamental understanding of the HTTA-based extraction. This effort included comparatively evaluating HTTA complexation with +4 and +3 metals (MIV = Zr, Hf, Ce, Th, U, Np, and Pu vs MIII = Ce, Nd, Sm, and Yb). We observed +4 metals formed neutral complexes of the general formula MIV(TTA)4. Meanwhile, +3 metals formed anionic MIII(TTA)4- species. Characterization of these M(TTA)4x- ( x = 0, 1) compounds by UV-vis-NIR, IR, 1H and 19F NMR, single-crystal X-ray diffraction, and X-ray absorption spectroscopy (both near-edge and extended fine structure) was critical for determining that NpIV(TTA)4 and PuIV(TTA)4 were the primary species extracted by HTTA. Furthermore, this information lays the foundation to begin developing and understanding of why the HTTA extraction works so well. The data suggest that the solubility differences between MIV(TTA)4 and MIII(TTA)4- are likely a major contributor to the selectivity of HTTA extractions for +4 cations over +3 metals. Moreover, these results will enable future studies focused on explaining HTTA extractions preference for +4 cations, which increases from Np IV to PuIV, HfIV, and ZrIV.

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.

Covalency in Americium(III) Hexachloride.

Developing a better understanding of covalency (or orbital mixing) is of fundamental importance. Covalency occupies a central role in directing chemical and physical properties for almost any given compound or material. Hence, the concept of covalency has potential to generate broad and substantial scientific advances, ranging from biological applications to condensed matter physics. Given the importance of orbital mixing combined with the difficultly in measuring covalency, estimating or inferring covalency often leads to fiery debate. Consider the 60-year controversy sparked by Seaborg and co-workers ( Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. J. Am. Chem. Soc. 1954 , 76 , 1461 ) when it was proposed that covalency from 5f-orbitals contributed to the unique behavior of americium in chloride matrixes. Herein, we describe the use of ligand K-edge X-ray absorption spectroscopy (XAS) and electronic structure calculations to quantify the extent of covalent bonding in-arguably-one of the most difficult systems to study, the Am-Cl interaction within AmCl63-. We observed both 5f- and 6d-orbital mixing with the Cl-3p orbitals; however, contributions from the 6d-orbitals were more substantial. Comparisons with the isoelectronic EuCl63- indicated that the amount of Cl 3p-mixing with EuIII 5d-orbitals was similar to that observed with the AmIII 6d-orbitals. Meanwhile, the results confirmed Seaborg's 1954 hypothesis that AmIII 5f-orbital covalency was more substantial than 4f-orbital mixing for EuIII.

Nuclear Magnetic Resonance Measurements and Electronic Structure of Pu(IV) in [(Me)4N]2PuCl6.

The synthesis, electronic structure, and characterization via single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and magnetic susceptibility of (Me4N)2PuCl6 are reported. NMR measurements were performed to both search for the direct (239)Pu resonance and to obtain local magnetic and electronic information at the Cl site through (35)Cl and (37)Cl spectra. No signature of (239)Pu NMR was observed. The temperature dependence of the Cl spectra was simulated by diagonalizing the Zeeman and quadrupolar Hamiltonians for (35)Cl, (37)Cl, and (14)N isotopes. Electronic structure calculations predict a magnetic Γ5 triplet ground state of Pu(IV) in the crystalline electric field of the undistorted PuCl6 octahedron. A tetragonal distortion would result in a very small splitting (∼20 cm(-1)) of the triplet ground state into a nonmagnetic singlet and a doublet state. The Cl shifts have an inflection point at T ≈ 15 K, differing from the bulk susceptibility, indicating a nonmagnetic crystal field ground state. The Cl spin-lattice relaxation time is constant to T = 15 K, below which it rapidly increases, also supporting the nonmagnetic crystal field ground state.

Comparing the 2,2'-Biphenylenedithiophosphinate Binding of Americium with Neodymium and Europium.

Advancing our understanding of the minor actinides (Am, Cm) versus lanthanides is key for developing advanced nuclear-fuel cycles. Herein, we describe the preparation of (NBu4 )Am[S2 P((t) Bu2 C12 H6 )]4 and two isomorphous lanthanide complexes, namely one with a similar ionic radius (i.e., Nd(III) ) and an isoelectronic one (Eu(III) ). The results include the first measurement of an Am-S bond length, with a mean value of 2.921(9) Å, by single-crystal X-ray diffraction. Comparison with the Eu(III) and Nd(III) complexes revealed subtle electronic differences between the complexes of Am(III) and the lanthanides.

Ionothermal synthesis of tetranuclear borate clusters containing f- and p-block metals.

The reactions of simple oxides or halides of trivalent lanthanides and actinides or bismuth with boric acid in the ionic liquid 1-butyl-3-methylimidazolium chloride at 150 °C result in the formation and crystallization of a series of isomorphous tetranuclear borate clusters with the general formula M4B22O36(OH)6(H2O)13 (M = La, Ce, Pr, Nd, Sm, Eu, Gd, Pu, and Bi). These clusters do not assemble with trivalent cations smaller than Gd(3+), suggesting that the formation of the clusters is dictated by the size of the metal ion. The cations are found in cavities along the periphery of a cage assembled from the corner- and edge-sharing interactions of BO3 triangles and BO4 tetrahedra, yielding a complex chiral cluster. Both enantiomers cocrystallize. The metal ions are nonacoordinate, and their geometries are best described as distorted tridiminished icosahedra. This coordination environment is new for both Pu(3+) and Bi(3+). In addition to detailed structural information, UV/vis-NIR absorption and photoluminescence spectra are also provided.

Why is uranyl formohydroxamate red?

The complexation of UO2(2+) by formohydroxamate (FHA(-)) creates solutions with dark red coloration. The inherent redox activity of formohydroxamate leads to the possibility that these solutions contain U(V) complexes, which are often red. We demonstrate that the reaction of U(VI) with formohydroxamate does not result in reduction, but rather in formation of the putative cis-aquo UO2(FHA)2(H2O)2, whose polymeric solid-state structure, UO2(FHA)2, contains an unusually bent UO2(2+) unit and a highly distorted coordination environment around a U(VI) cation in general. The bending of the uranyl cation results from unusually strong π donation from the FHA(-) ligands into the 6d and 5f orbitals of the U(VI) cation. The alteration of the bonding in the uranyl unit drastically changes its electronic and vibrational features.

Chirality and polarity in the f-block borates M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, and Cf).

The reactions of trivalent lanthanides and actinides with molten boric acid in high chloride concentrations result in the formation of M4[B16O26(OH)4(H2O)3Cl4] (M = Sm, Eu, Gd, Pu, Am, Cm, Cf). This cubic structure type is remarkably complex and displays both chirality and polarity. The polymeric borate network forms helical features that are linked via two different types of nine-coordinate f-element environments. The f-f transitions are unusually intense and result in dark coloration of these compounds with actinides.

LnV3Te3O15(OH)3·nH2O (Ln = Ce, Pr, Nd, Sm, Eu, Gd; n = 1-2): a new series of semiconductors with mixed-valent tellurium (IV,VI) oxoanions.

Six new lanthanide tellurium vanadates with the general formula LnV3Te3O15(OH)3·nH2O (LnVTeO) (Ln = Ce, Pr, Nd, Sm, Eu, and Gd; n = 2 for Ce and Pr; n = 1 for Nd, Sm, Eu, and Gd) have been prepared hydrothermally via the reactions of lanthanide nitrates, TeO2, and V2O5 at 230 °C. LnVTeO adopts a three-dimensional (3D) channel structure with a space group of P63/mmc. Surprisingly, two types of oxoanions: Te(IV)O3(2-) trigonal pyramids and Te(VI)O6(6-) octahedra, coexist in these compounds. Solid-state UV-vis-NIR absorption spectra for LnVTeO show approximate band gaps on the order of 1.9 eV, suggesting the wide band gap semiconducting nature of these materials. No magnetic phase transition was observed in any of the analogues, but a clear increase in the strength of short-range antiferromagnetic correlations was found with the shortening of distances between magnetically coupled Ln(3+) ions in LnVTeO.

Expansion of the rich structures and magnetic properties of neptunium selenites: soft ferromagnetism in Np(SeO3)2.

Two new neptunium selenites with different oxidation states of the metal centers, Np(IV)(SeO3)2 and Np(VI)O2(SeO3), have been synthesized under mild hydrothermal conditions at 200 °C from the reactions of NpO2 and SeO2. Np(SeO3)2 crystallizes as brown prisms (space group P21/n, a = 7.0089(5) Å, b = 10.5827(8) Å, c = 7.3316(5) Å, ß = 106.953(1)°); whereas NpO2(SeO3) crystals are garnet-colored with an acicular habit (space group P21/m, a = 4.2501(3) Å, b = 9.2223(7) Å, c = 5.3840(4) Å, ß = 90.043(2)°). Single-crystal X-ray diffraction studies reveal that the structure of Np(SeO3)2 features a three-dimensional (3D) framework consisting of edge-sharing NpO8 units that form chains that are linked via SeO3 units to create a 3D framework. NpO2(SeO3) possesses a lamellar structure in which each layer is composed of NpO8 hexagonal bipyramids bridged via SeO3(2-) anions. Bond-valence sum calculations and UV-vis-NIR absorption spectra support the assignment of tetravalent and hexavalent states of neptunium in Np(SeO3)2 and NpO2(SeO3), respectively. Magnetic susceptibility data for Np(SeO3)2 deviates substantially from typical Curie-Weiss behavior, which can be explained by large temperature-independent paramagnetic (TIP) effects. The Np(IV) selenite shows weak ferromagnetic ordering at 3.1(1) K with no detectable hysteresis, suggesting soft ferromagnetic behavior.

Synthesis and spectroscopy of new plutonium(III) and -(IV) molybdates: comparisons of electronic characteristics.

Synthesis of a plutonium(III) molybdate bromide, PuMoO4Br(H2O), has been accomplished using hydrothermal techniques in an inert-atmosphere glovebox. The compound is green in color, which is in stark contrast to the typical blue color of plutonium(III) complexes. The unusual color arises from the broad charge transfer (CT) spanning from approximately 300 to 500 nm in the UV-vis-near-IR spectra. Repeating the synthesis with an increase in the reaction temperature results in the formation of a plutonium(IV) molybdate, Pu3Mo6O24(H2O)2, which also has a broad CT band and red-shifted f-f transitions. Performing an analogous reaction with neodymium produced a completely different product, [Nd(H2O)3][NdMo12O42]·2H2O, which is built of Silverton-type polyoxometallate clusters.

Further evidence for the stabilization of U(V) within a tetraoxo core.

Two complex layered uranyl borates, K10[(UO2)16(B2O5)2(BO3)6O8]·7H2O (1) and K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5]·H2O (2), were isolated from supercritical water reactions. Within these compounds, borate exists only as BO3 units and is found as either isolated BO3 triangles or B2O5 dimers, the latter being formed from corner sharing of two BO3 units. These anions, along with oxide and hydroxide, bridge between uranyl centers to create the complex layers in these compounds. U(VI) cations are found within uranyl, UO2(2+) units, that are bound by four or five oxygen atoms to create tetragonal and pentagonal bipyramidal environments. The most striking feature in this system is found in 2, where a [UO4(OH)2] unit exists that contains U(V) within a tetraoxo core with trans hydroxide anions; therefore, this compound is a mixed-valent U(VI)/U(V) borate. The presence of a 5f(1) uranium site within 2 leads to unusual optical properties.

Straightforward reductive routes to air-stable uranium(III) and neptunium(III) materials.

Studies of trivalent uranium (U(3+)) and neptunium (Np(3+)) are restricted by the tendency of these ions to oxidize in the presence of air and water, requiring manipulations to be carried out in inert conditions to produce trivalent products. While the organometallic and high-temperature reduction chemistry of U(3+) and, to a much smaller extent, Np(3+) has been explored, the study of the oxoanion chemistry of these species has been limited despite their interesting optical and magnetic properties. We report the synthesis of U(3+) and Np(3+) sulfates by utilizing zinc amalgam as an in situ reductant with absolutely no regard to the exclusion of O2 or water. By employing this method we have developed a family of alkali metal U(3+) and Np(3+) sulfates that are air and water stable. The structures, electronic spectra, and magnetic behavior are reported.

From yellow to black: dramatic changes between cerium(IV) and plutonium(IV) molybdates.

Hydrothermal reactions of CeCl(3) and PuCl(3) with MoO(3) and Cs(2)CO(3) yield surprisingly different results. Ce(3)Mo(6)O(24)(H(2)O)(4) crystallizes as bright yellow plates (space group C2/c, a = 12.7337(7) Å, b = 22.1309(16) Å, c = 7.8392(4) Å, ß = 96.591(4)°, V = 2194.6(2) Å(3)), whereas CsPu(3)Mo(6)O(24)(H(2)O) crystallizes as semiconducting black-red plates (space group C2/c, a = 12.633(5) Å, b = 21.770(8) Å, c = 7.743(7) Å, ß = 96.218(2)°, V = 2117(2) Å(3)). The topologies of the two compounds are similar, with channel structures built from disordered Mo(VI) square pyramids and (RE)O(8) square antiprisms (RE = Ce(IV), Pu(IV)). However, the Pu(IV) compound contains Cs(+) in its channels, while the channels in Ce(3)Mo(6)O(24)(H(2)O)(4) contain water molecules. Disorder and an ambiguous oxidation state of Mo lead to the formula CsPu(3)Mo(6)O(24)(H(2)O), where one Mo site is Mo(V) and the rest are Mo(VI). X-ray absorption near-edge structure (XANES) experiments were performed to investigate the source of the black color of CsPu(3)Mo(6)O(24)(H(2)O). These experiments revealed Pu to be tetravalent, while the strong pre-edge absorption from the distorted molybdate anions leaves the oxidation state ambiguous between Mo(V) and Mo(VI).

Thermochromism, the Alexandrite effect, and dynamic Jahn-Teller distortions in Ho2Cu(TeO3)2(SO4)2.

A 3d-4f heterobimetallic material with mixed anions, Ho2Cu(TeO3)2(SO4)2, has been prepared under hydrothermal conditions. Ho2Cu(TeO3)2(SO4)2 exhibits both thermochromism and the Alexandrite effect. Variable temperature single crystal X-ray diffraction and UV-vis-NIR spectroscopy reveal that changes in the Cu(II) coordination geometry result in negative thermal expansion of axial Cu-O bonds that plays a role in the thermochromic transition of Ho2Cu(TeO3)2(SO4)2. Magnetic studies reveal an effective magnetic moment of 14.97 µB. which has a good agreement with the calculated value of 15.09 µB.

Comparisons of plutonium, thorium, and cerium tellurite sulfates.

The hydrothermal reaction of PuCl3 or CeCl3 with TeO2 in the presence of sulfuric acid under the comparable conditions results in the crystallization of Pu(TeO3)(SO4) or Ce2(Te2O5)(SO4)2, respectively. Pu(TeO3)(SO4) and its isotypic compound Th(TeO3)(SO4) are characterized by a neutral layer structure with no interlamellar charge-balancing ions. However, Ce2(Te2O5)(SO4)2 possesses a completely different dense three-dimensional framework. Bond valence calculation and UV-vis-NIR spectra indicate that the Ce compound is trivalent whereas the Pu and Th compounds are tetravalent leading to the formation of significantly different compounds. Pu(TeO3)(SO4), Th(TeO3)(SO4), and Ce2(Te2O5)(SO4)2 represent the first plutonium/thorium/cerium tellurite sulfate compounds. Our study strongly suggests that the chemistries of Pu and Ce are not the same, and this is another example of the failure of Ce as a surrogate.

A new family of lanthanide borate halides with unusual coordination and a new neodymium-containing cationic framework.

The reactions of Ln(2)O(3)/CeO(2)/Pr(6)O(11) (Ln = La-Nd, Sm), molten boric acid, and concentrated HBr or HI result in the formation of La[B(7)O(10)(OH)(3)(H(2)O)Br], Ln[B(6)O(9)(OH)(2)(H(2)O)(2)Br]·0.5H(2)O (Ln = Ce, Pr), Nd(2)[B(12)O(17.5)(OH)(5)(H(2)O)(4)Br(1.5)]Br(0.5)·H(2)O (NdBOBr), Sm(4)[B(18)O(25)(OH)(13)Br(3)], and Ln[B(7)O(11)(OH)(H(2)O)(3)I] (Ln = La-Nd, Sm). The lanthanide(III) centers in these compounds are found with 9-coordinate hula hoop or 10-coordinate capped triangular cupola geometries, where there are six approximately coplanar oxygen donors provided by the polyborate sheet. The sheets are formed into three-dimensional frameworks via BO(3) triangles that are roughly perpendicular to the layers. Additionally, a new cationic framework, NdBOBr, has been isolated. NdBOBr is unusual in that not only is it a cationic framework, but it is also the first trivalent f-element borate to have terminal halides bound exclusively to the base site of the hula hoop. The Ln[B(7)O(11)(OH)(H(2)O)(3)I] (Ln = La-Nd, Sm) structures require two corner-shared BO(3) units in order to tether the layers together because of the large size of the capping iodine atom.

Synthesis of divalent europium borate via in situ reductive techniques.

A new divalent europium borate, Eu[B8O11(OH)4], was synthesized by two different in situ reductive methodologies starting with a trivalent europium starting material in a molten boric acid flux. The two in situ reductive techniques employed were the use of HI as a source of H2 gas and the use of a Zn amalgam as a reductive, reactive surface. While both of these are known reductive techniques, the title compound was synthesized in both air and water which demonstrates that strict anaerobic conditions need not be employed in conjunction with these reductive methodologies. Herein, we report on the structure, spectroscopy, and synthetic methodologies relevant to Eu[B8O11(OH)4]. We also report on a europium doping study of the isostructural compound Sr[B8O11(OH)4] where the amount of doped Eu(2+) ranges from 2.5 to 11%.

Differentiating between trivalent lanthanides and actinides.

The reactions of LnCl(3) with molten boric acid result in the formation of Ln[B(4)O(6)(OH)(2)Cl] (Ln = La-Nd), Ln(4)[B(18)O(25)(OH)(13)Cl(3)] (Ln = Sm, Eu), or Ln[B(6)O(9)(OH)(3)] (Ln = Y, Eu-Lu). The reactions of AnCl(3) (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu[B(4)O(6)(OH)(2)Cl] and Pu(2)[B(13)O(19)(OH)(5)Cl(2)(H(2)O)(3)], Am[B(9)O(13)(OH)(4)]·H(2)O, or Cm(2)[B(14)O(20)(OH)(7)(H(2)O)(2)Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO(3) and/or BO(4) groups. There is a shift from 10-coordinate Ln(3+) and An(3+) cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm(3+) is anomalous in that it contains both 9- and 10-coordinate metal ions. Despite these materials being synthesized under identical conditions, the two series do not parallel one another. Electronic structure calculations with multireference, CASSCF, and density functional theory (DFT) methods reveal the An 5f orbitals to be localized and predominately uninvolved in bonding. For the Pu(III) borates, a Pu 6p orbital is observed with delocalized electron density on basal oxygen atoms contrasting the Am(III) and Cm(III) borates, where a basal O 2p orbital delocalizes to the An 6d orbital. The electronic structure of the Ce(III) borate is similar to the Pu(III) complexes in that the Ce 4f orbital is localized and noninteracting, but the Ce 5p orbital shows no interaction with the coordinating ligands. Natural bond orbital and natural population analyses at the DFT level illustrate distinctive larger Pu 5f atomic occupancy relative to Am and Cm 5f, as well as unique involvement and occupancy of the An 6d orbitals.