Lanthanide Squarate Complexes Containing Mono- and Trivalent Thallium.

The synthesis and structural characterization of 16 new thallium lanthanide squarate complexes and 1 new cerium squarate oxalate complex are presented. These new complexes─Tl[Ln(C4O4)(H2O)5]·C4O4 (Ln = La-Nd) (1), Tl3[Ln3(C4O4)6(H2O)6]·8H2O (Ln = Sm-Lu, Y) (2), Tl[Ce(C4O4)2(H2O)6]·C4O4 (3), and [Ce2(C4O4)2(C2O4)(H2O)8]·2H2O (4)─all contain the squarate ligand bound to the trivalent lanthanides with varying coordination modes and denticities. Of the four new groups of complexes prepared in this work, two groupings contain monovalent thallium and trivalent lanthanides, the most common oxidation states for these metals. One complex (3), however, contains trivalent thallium, which is an unusual and challenging oxidation state to stabilize. The Tl3+ cation is formed from in situ oxidation by way of tetravalent cerium (Ce4+/Ce3+, E° = 1.72 V; Tl3+/Tl+ = 1.252 V), leading to the formation of a Tl3+-Ce3+-squarate complex. Additionally, one complex (4) is unique in this work in that it contains both the squarate and oxalate ligands, the latter of which was formed in situ from squarate. Single-crystal X-ray diffraction analysis reveals that 1 and 2 have a 2D structure constructed from either LnO4(H2O)5 monocapped square antiprismatic (CN = 9) metal centers (for 1) or LnO4(H2O)4 square antiprismatic (CN = 8) metal centers (for 2), 3 is a 1D chain structure constructed from CeO3(H2O)6 monocapped square antiprismatic (CN = 9) cerium centers, and 4 is a 3D framework structure constructed from CeO5(H2O)4 monocapped square antiprismatic (CN = 9) cerium centers. 2 and 4 display rare coordination modes for the squarate ligand. Herein, the synthesis, characterization, and structural descriptions of these new complexes are presented.

Effect of Reaction Time on Lanthanide Borate Perrhenate Complexes.

Six new trivalent lanthanide borate perrhenate structures─the isostructural series Ln[B8O11(OH)4(H2O)(ReO4)] (Ln = Ce-Nd, Sm, Eu; 1) and La[B6O9(OH)2(H2O)(ReO4)] (2)─have been prepared and structurally characterized. Single-crystal X-ray diffraction analysis reveals that both structures crystallize in the P21/n space group, contain 10-coordinated trivalent lanthanides in a capped triangular cupola geometry, are 3D borate framework materials, and contain either terminal (1) or bridging (2) perrhenate moieties. The presence or lack of a bridging perrhenate, along with the identity of the basal ligands, dictates how the layers are tethered together, ultimately leading to the different structures. Furthermore, the formation of 1 is sensitive to the reaction time employed. Herein, the synthesis, structural descriptions, and spectroscopy of these trivalent lanthanide perrhenate borate complexes are presented.

Trivalent f-Element Squarates, Squarate-Oxalates, and Cationic Materials, and the Determination of the Nine-Coordinate Ionic Radius of Cf(III).

The synthesis, structure, and solid-state UV-vis-NIR spectroscopy of four new f-element squarates, M2(C4O4)3(H2O)4 (M = Eu, Am, Cf) and Sm(C4O4)(C4O3OH)(H2O)2·0.5H2O, four new cationic lanthanide squarate chlorides, [M4(C4O4)5(H2O)12]Cl2·5H2O (M = Eu, Dy, Ho Er), and two new actinide squarate oxalates, M2(C4O4)2(C2O4)(H2O)4 (M = Am, Cf), are presented. All of the metal centers are trivalent. Single-crystal X-ray diffraction analysis reveals that M2(C4O4)3(H2O)4 and Sm(C4O4)(C4O3OH)(H2O)2·0.5H2O have a two-dimensional sheet structure constructed from MO7(H2O)2 monocapped square-antiprismatic (coordination number (CN) = 9) metal centers and SmO6(H2O)2 square-antiprismatic (CN = 8) metal centers, respectively, whereas M2(C4O4)2(C2O4)(H2O)4 have a three-dimensional (3D) structure constructed from MO7(H2O)2 monocapped square-antiprismatic (CN = 9) metal centers. Additionally, the cationic framework materials [M4(C4O4)5(H2O)12]Cl2·5H2O have a 3D structure constructed from two crystallographically unique MO5(H2O)3 square-antiprismatic (CN = 8) metal centers. In these structures, the squarate ligands bind to the metal centers with varying coordination modes and denticities. The results of this study provide another example of the nonparallel chemistry between the lanthanides and transplutonium elements. From the crystallographic data for the isotypic series M2(C4O4)3(H2O)4 (M = La-Nd, Sm, Eu) and the linear regression fit to a plot of the unit cell volume as a function of the cube of the ionic radius, the nine-coordinate ionic radius of Cf 3+ was determined to be 1.127 ± 0.003 Å. Finally, computational analysis of the americium and californium complexes M2(C4O4)3(H2O)4 and M2(C4O4)2(C2O4)(H2O)4 reveals three important attributes: (i) the 5f orbitals are nonbonding in all cases, with the bonding differences occurring with the empty 6d orbitals; (ii) the Cf complexes exhibit more covalent character than their Am counterparts; and (iii) there is more covalent character in the squarate-oxalate complexes than in the squarate complexes.

Origins of the odd optical observables in plutonium and americium tungstates.

A series of trivalent f-block tungstates, MW2O7(OH)(H2O) (M = La, Ce, Pr, Nd, and Pu) and AmWO4(OH), have been prepared in crystalline form using hydrothermal methods. Both structure types take the form of 3D networks where MW2O7(OH)(H2O) is assembled from infinite chains of distorted tungstate octahedra linked by isolated MO8 bicapped trigonal prisms; whereas AmWO4(OH) is constructed from edge-sharing AmO8 square antiprisms connected by distorted tungstate trigonal bipyramids. PuW2O7(OH)(H2O) crystallizes as red plates; an atypical color for a Pu(iii) compound. Optical absorption spectra acquired from single crystals show strong, broadband absorption in the visible region. A similar feature is observed for CeW2O7(OH)(H2O), but not for AmWO4(OH). Here we demonstrate that these significantly different optical properties do not stem directly from the 5f electrons, as in both systems the valence band has mostly O-2p character and the conduction band has mostly W-5d character. Furthermore, the quasi-particle gap is essentially unaffected by the 5f degrees of freedom. Despite this, our analysis demonstrates that the f-electron covalency effects are quite important and substantially different energetically in PuW2O7(OH)(H2O) and AmWO4(OH), indicating that the optical gap alone cannot be used to infer conclusions concerning the f electron contribution to the chemical bond in these systems.

[Ag2M(Te2O5)2]SO4 (M = CeIV or ThIV): A New Purely Inorganic d/f-Heterometallic Cationic Material.

Two new isotypic d/f-heterometallic purely inorganic cationic materials, [Ag2M(Te2O5)2]SO4 (M = CeIV or ThIV), were synthesized using the metal oxides (MO2 and TeO2), silver nitrate, and sulfuric acid under mild hydrothermal conditions. The prepared materials were characterized via single-crystal X-ray diffraction, which revealed that the materials possess a 3D framework of corner-sharing Te2O52- units. The tellurite framework creates four unique pores, three of which are occupied by the MIV and AgI metal centers. The tellurite network, metal coordination, and total charge yield a cationic framework, which is charge-balanced by electrostatically bound sulfate anions residing in the largest of the four framework pores. These materials also possess AgI in a ligand-imposed linear geometry.

Incipient class II mixed valency in a plutonium solid-state compound.

Electron transfer in mixed-valent transition-metal complexes, clusters and materials is ubiquitous in both natural and synthetic systems. The degree to which intervalence charge transfer (IVCT) occurs, dependent on the degree of delocalization, places these within class II or III of the Robin-Day system. In contrast to the d-block, compounds of f-block elements typically exhibit class I behaviour (no IVCT) because of localization of the valence electrons and poor spatial overlap between metal and ligand orbitals. Here, we report experimental and computational evidence for delocalization of 5f electrons in the mixed-valent PuIII/PuIV solid-state compound, Pu3(DPA)5(H2O)2 (DPA = 2,6-pyridinedicarboxylate). The properties of this compound are benchmarked by the pure PuIII and PuIV dipicolinate complexes, [PuIII(DPA)(H2O)4]Br and PuIV(DPA)2(H2O)3·3H2O, as well as by a second mixed-valent compound, PuIII[PuIV(DPA)3H0.5]2, that falls into class I instead. Metal-to-ligand charge transfer is involved in both the formation of Pu3(DPA)5(H2O)2 and in the IVCT.

Characterization of berkelium(III) dipicolinate and borate compounds in solution and the solid state.

Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f(7) configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium's only available isotope, (249)Bk, has hindered in-depth studies of the element's coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium's complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium's multiconfigurational ground state.

Emergence of californium as the second transitional element in the actinide series.

A break in periodicity occurs in the actinide series between plutonium and americium as the result of the localization of 5f electrons. The subsequent chemistry of later actinides is thought to closely parallel lanthanides in that bonding is expected to be ionic and complexation should not substantially alter the electronic structure of the metal ions. Here we demonstrate that ligation of californium(III) by a pyridine derivative results in significant deviations in the properties of the resultant complex with respect to that predicted for the free ion. We expand on this by characterizing the americium and curium analogues for comparison, and show that these pronounced effects result from a second transition in periodicity in the actinide series that occurs, in part, because of the stabilization of the divalent oxidation state. The metastability of californium(II) is responsible for many of the unusual properties of californium including the green photoluminescence.

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.

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.

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.

Unusual structure, bonding and properties in a californium borate.

The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations.

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.

Novel fundamental building blocks and site dependent isomorphism in the first actinide borophosphates.

Three novel uranyl borophosphates, Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]H2O (AgNBPU-1), Ag(2-x)(NH4)3[(UO2)2{B2P5O(20-x)(OH)x}] (x = 1.26) (AgNBPU-2), and Ag(2-x)(NH4)3[(UO2)2{B2P(5-y)AsyO(20-x)(OH)x}] (x = 1.43, y = 2.24) (AgNBPU-3), have been prepared by the H3BO3-NH4H2PO4/NH4H2AsO4 flux method. The structure of AgNBPU-1 has an unprecedented fundamental building block (FBB), composed of three BO4 and six PO4 tetrahedra which can be written as 9□:[Φ] □<3□>□|□<3□>□|□<3□>□|. Two Ag atoms are linearly coordinated; the coordination of a third one is T-shaped. AgNBPU-2 and AgNBPU-3 are isostructural and possess a FBB of two BO4 and five TO4 (T = P, As) tetrahedra (7□:□<4□>□|□). AgNBPU-3 is a solid solution with some PO4 tetrahedra of the AgNBPU-2 end-member being substituted by AsO4. Only two out of the three independent P positions are partially occupied by As, resulting in site dependent isomorphism. The three compounds represent the first actinide borophosphates.

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%.

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.

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).

Incorporation of neptunium(VI) into a uranyl selenite.

The incorporation of neptunium(VI) into the layered uranyl selenite Cs[(UO(2))(HSeO(3))(SeO(3))] has yielded the highest level of neptunium uptake in a uranyl compound to date with an average of 12(±3)% substitution of Np(VI) for U(VI). Furthermore, this is the first case in nearly 2 decades of dedicated incorporation studies in which the oxidation state of neptunium has been determined spectroscopically in a doped uranyl compound and also the first time in which neptunium incorporation has resulted in a structural transformation.

Effect of pH and reaction time on the structures of early lanthanide(III) borate perchlorates.

Reactions of LnCl(3)·6H(2)O (Ln = La-Nd, Sm, Eu), concentrated (11 M) perchloric acid, and molten boric acid result in the formation of four different compounds. These compounds are Ln[B(8)O(10)(OH)(6)(H(2)O)(ClO(4))]·0.5H(2)O (Ln = La-Nd, Sm), Pr[B(8)O(11)(OH)(4)(H(2)O)(ClO(4))], Ln[B(7)O(11)(OH)(H(2)O)(2)(ClO(4))] (Ln = Pr, Nd, Sm, and Eu), and Ce[B(8)O(11)(OH)(4)(H(2)O)(ClO(4))]. All Ln(III) cations are ten-coordinate with a capped triangular cupola geometry and contain an inner-sphere, monodentate perchlorate moiety. This geometry is obtained because of the coordination of the oxygen donors within the polyborate sheet which create triangular holes and provide residence for the lanthanide metal centers. Aside from Ln[B(8)O(10)(OH)(6)(H(2)O)(ClO(4))]·0.5H(2)O (Ln = La-Nd, Sm), which are two-dimensional sheet structures, all other compounds are three-dimensional frameworks in which the layers are tethered together by BO(3) units found roughly perpendicular to the sheets. Furthermore, a change in product is observed depending on the reaction duration while holding all other synthetic variables constant. This report also demonstrates that lanthanide borates can be prepared in extreme acidic conditions.

Unusual coordination for plutonium(IV), cerium(IV), and zirconium(IV) in the cationic layered materials [M2Te4O11]X2 (M = Pu, Ce, Zr; X = Cl, Br).

Four isotypic cationic layered materials, [Pu(2)Te(4)O(11)]Cl(2), [Ce(2)Te(4)O(11)]Cl(2), [Zr(2)Te(4)O(11)]Cl(2), and [Zr(2)Te(4)O(11)]Br(2), have been prepared under hydrothermal conditions. Single crystal diffraction studies reveal that these materials possess cationic Pu/Ce/Zr tellurite layers with halides as interlamellar charge-balancing anions. The Pu(IV), Ce(IV), and Zr(IV) centers of the cationic layers exhibit a quite rare pentagonal bipyramid coordination environment.